Heat Recovery Ventilation (HRV): ΔT, Airflow, Defrost, Metering
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Heat Recovery Ventilation (HRV) is best evaluated and debugged with measurable evidence: multi-point ΔT coherence, supply/exhaust airflow balance, verifiable frost/defrost triggers, and trustworthy energy metering. This page shows where to measure, how to isolate root causes fast, and which first fixes restore stable recovery, quiet operation, and reliable efficiency.
H2-1 — System Boundary & What “Good HRV” Means
This page is strictly scoped to HRV sensible-heat recovery evidence chains: ΔT (delta-T), balanced airflow, frost/defrost behavior, and energy metering. ERV latent/humidity exchange is out of scope (link-only, not explained here).
Boundary (HRV vs ERV)
3 measurable KPIs
System blocks & roles
A. One-sentence boundary (HRV vs ERV)
- HRV focuses on temperature (sensible) exchange; success is proven by ΔT, recovered heat trend, and frost/defrost impact on airflow.
- ERV additionally targets moisture/enthalpy exchange; humidity-transfer materials and psychrometric modeling are not covered on this page.
B. “Good HRV” = three KPIs that can be verified
KPI-1: Target ΔT by condition
- Definition: ΔT must be interpreted with operating condition (outdoor/indoor temps, airflow/level, bypass state).
- Evidence: use four temperature points (in/out on both supply and exhaust paths) to avoid “fake ΔT”.
- Common traps: wall-contact sensor, thermal short-circuit, condensate wetting, or bypass leakage can make ΔT look normal while recovery is poor.
KPI-2: Supply/Exhaust balance error (%)
- Definition: balance error is the % difference between supply airflow and exhaust airflow under steady condition.
- Evidence: airflow (or dP+curve) + fan feedback (tach/Hall) + damper state, tied to an event log.
- Common traps: filter loading or duct resistance shifts the fan curve; “same RPM” does not guarantee same airflow.
KPI-3: Frost trigger thresholds (T & duration)
- Definition: defrost triggers must include temperature and time duration (to reject transients).
- Evidence: ΔT trend + airflow/dP degradation + core-side temperature plateau signals.
- Common traps: sensor drift/noise causes false triggers; every defrost must record a reason code for field diagnosis.
C. Canonical HRV blocks (and what each proves)
- Heat-exchanger core: defines theoretical recovery and frost sensitivity; evidence is ΔT and recovery trend over airflow.
- Dual fans (supply + exhaust): enforce balance and static-pressure adaptation; evidence is airflow parity and stable feedback loops.
- Bypass damper: enables defrost/seasonal bypass; evidence is ΔT and airflow recovery time after switching.
- Sensors: temperature and airflow/dP must be positioned to resist condensate/thermal shortcuts; evidence is cross-consistency.
- Controller + event log: turns symptoms into root cause; evidence is “why it happened” (defrost/brownout/sensor fault codes).
- Energy metering: turns power into a diagnostic signal; evidence is abnormal kWh/W trend vs airflow and filter loading.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F1 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F1 — URL — Accessed 2026-01-16
H2-2 — HRV Hardware Block Diagram (Signal / Power / Control)
The diagram below is a measurement-first map: it shows what must be measured (ΔT and airflow evidence), what must be controlled (dual-fan balance and bypass), what must be protected (brownout/EMC events), and what must be recorded (defrost and fault reason codes). Each block exists to support the KPIs in H2-1.
A. Signal chain (prove recovery, not just “read temperature”)
- Temperature points (T1–T4): supply/exhaust in/out points enable ΔT and consistency checks.
- Airflow or dP evidence: airflow sensors or differential pressure + curve provide balance evidence.
- State markers: bypass position and defrost state must be logged to avoid misinterpreting ΔT.
B. Control chain (balance first, then pressure and noise)
- Outer loop: balanced ventilation (supply ≈ exhaust) or static-pressure target depending on installation.
- Inner loop: fan speed/current limits for stability, efficiency, and protection.
- Bypass damper: acts as an execution lever for defrost and recovery, not a cosmetic feature.
C. Power, protection, and logging (diagnosability is a design feature)
- Power rails: sensors/MCU rails must stay stable through fan start and line transients.
- Protection events: UVLO/brownout, watchdog reset, and EMC/ESD disturbances must leave a trace.
- Event log: defrost start/stop, trigger reason, airflow degradation flags, and sensor invalid flags.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F2 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F2 — URL — Accessed 2026-01-16
H2-3 — ΔT Sensing: Where to Measure & How to Avoid “Fake ΔT”
ΔT is only meaningful when it is backed by correct placement, cross-consistency, and state awareness (bypass/defrost). A single temperature point can look “stable” while recovery is actually poor.
T1–T4 placement rules
Error fingerprints
NTC vs RTD (why)
Calibration + drift checks
Quick discriminator
A. Minimum sensing set: 4 points (T1–T4) and why
- T1/T2 (supply path in/out): prove the recovered temperature rise on the outdoor-to-supply path.
- T4/T3 (exhaust path in/out): prove the temperature drop on the return-to-exhaust path.
- Consistency requirement: when bypass is closed and airflow is steady, both paths must show a coherent heat-transfer story. If only one side is measured, bypass leakage, wall conduction, or wet sensors can masquerade as “good ΔT”.
B. Placement rules that prevent “fake ΔT”
RuleAvoid wall conduction
Keep sensors in the air stream, not on the enclosure
- Place probes away from metal walls and core frame contact points to avoid temperature being dominated by enclosure conduction.
- Use a small standoff and airflow-centered insertion so the measured point represents air, not structure.
RuleAvoid thermal shortcuts
Block “thermal short” paths near the core and mixing regions
- Do not mount directly at mixing zones where supply and exhaust streams can locally short-circuit.
- Route sensor leads so they do not act as a heat wick from a warmer surface into the sensing bead.
RuleCondensate resilience
Design around wetting and freeze-thaw cycles
- Keep sensors out of condensate drip lines; water film can bias readings and slow response dramatically.
- Prefer sealed probes or coated beads when repeated wetting is expected during frost/defrost cycles.
RuleResponse lag
Match thermal time constants to airflow dynamics
- High airflow reduces local boundary layer thickness; low airflow increases lag. Plan for different response times across operating levels.
- Use step-change tests (fan level changes) to confirm each point reaches steady state without excessive delay.
C. Error fingerprints (what “bad placement” looks like in data)
- Wall conduction: temperature drifts with enclosure temperature and correlates poorly with airflow changes.
- Thermal short: ΔT looks “too stable” even when bypass/damper state changes; supply and exhaust trends stop mirroring.
- Condensate wetting: slow recovery after defrost; apparent offset that persists until the probe dries.
- Air-speed lag: different points show inconsistent time constants during fan steps; one point “leads” while another “lags” abnormally.
D. NTC divider vs RTD excitation (selection by engineering reality)
- NTC divider: low cost, simple interface; be mindful of high node impedance and noise pickup on long leads. Best when wiring is short and environment is stable.
- RTD excitation: better interchangeability and linearity; sensitive to lead resistance and requires controlled excitation/measurement strategy. Best when calibration stability and traceability matter.
- Practical takeaway: in HRV, mounting and wetting resilience often dominate over theoretical sensor accuracy. Electrical choice must match the installation reality.
E. Calibration & drift detection (make drift observable)
- Single-point calibration: aligns offset near a dominant operating band; appropriate when absolute accuracy requirements are moderate.
- Two-point calibration: constrains both slope and offset; appropriate when recovery estimation needs stable mapping over a wide temperature span.
- Cross-consistency drift check: when bypass is closed and airflow is steady, temperature relationships should remain within a narrow band. Persistent deviation flags sensor bias, wetting, or mounting change.
- Power-on self-check: record a short stable window at startup; compare against expected pairwise relations to flag early sensor anomalies.
Quick Discriminator: ΔT is small — sensor placement, airflow, or bypass?
Step 1State first
Verify bypass/defrost state before judging ΔT
- If bypass is open or leaking, a small ΔT is expected. Use state markers (damper position / defrost flag) as a prerequisite.
Step 2Airflow evidence
Check balance and airflow magnitude
- Airflow too high (or strongly imbalanced) reduces contact time and can “dilute” ΔT. Use airflow/dP + fan feedback evidence (H2-4).
Step 3Consistency check
Validate T1–T4 coherence
- If T1–T4 relationships are not self-consistent under steady condition, prioritize sensor mounting, wetting, or thermal shortcuts over core performance conclusions.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F3 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F3 — URL — Accessed 2026-01-16
H2-4 — Airflow / Static Pressure Sensing (Balanced Ventilation Evidence)
Balanced ventilation is not proven by “same RPM” or “same duty”. Airflow evidence is required to interpret ΔT correctly and to diagnose odor/backdraft and door-gap pressure symptoms.
Two inference paths
Field symptoms → evidence
Two-step measurement
Evidence → Isolate → First fix
A. Why airflow evidence is mandatory
- Same RPM ≠ same airflow: duct resistance, filter loading, frost, and damper position shift the operating point.
- Same duty ≠ same static pressure: motor/driver limits and supply voltage sag can distort the fan curve.
- Practical rule: any balance claim must cite air-side evidence (airflow or dP+curve), not only drive commands.
B. Two airflow evidence paths (within this page scope)
Path 1dP + flow curve
Differential pressure + calibrated flow curve
- Strength: directly tracks duct resistance changes (filter loading, damper movement, frosting restriction).
- Requirement: stable sensing ports and a curve tied to the specific duct/orifice implementation.
- Failure mode: incorrect port placement or leakage collapses the relationship between dP and true flow.
Path 2Fan inference
Fan RPM/power inference (requires baseline calibration)
- Strength: minimal additional hardware when tach/Hall and power telemetry already exist.
- Requirement: a calibrated mapping across operating levels; update or re-learn if filters/ducts change.
- Failure mode: power increases can reflect higher resistance rather than higher airflow; inference must not be treated as ground truth without periodic checks.
C. Field symptoms → measurable evidence (keep it diagnosable)
- Door-gap pressure / drafts: persistent supply–exhaust mismatch shows up as a balance error and/or dP bias.
- Backdraft / odor complaints: often indicates exhaust under-delivery or supply over-delivery; correlate with damper position and filter state.
- “ΔT looks OK but comfort is bad”: check whether airflow is actually reaching target or being throttled by resistance and limits.
D. Two-step measurement (fast isolate)
Step 1Air-side
Capture air-side evidence and states
- Measure airflow or dP (plus valve/damper position and filter state marker).
- Look for restriction signatures: airflow drop with rising dP, or dP rise at constant command.
Step 2Drive-side
Capture drive-side evidence
- Record tach/Hall RPM and command (duty) plus power/current.
- Interpretation: “drive saturating” vs “drive not demanding” distinguishes duct restriction from control/limit issues.
Evidence → Isolate → First fix (stacked, mobile-friendly)
EvidenceAirflow↓ + dP↑
Restriction or frosting signature
- Isolate: filter loading, frost restriction, damper not fully open, condensate obstruction.
- First fix: service filter and verify damper travel; confirm defrost state and recovery trend before re-tuning control.
EvidenceAirflow mismatch + RPM not saturated
Control target / gain / limit mismatch
- Isolate: balance loop target wrong, asymmetric limits, or incorrect valve position mapping.
- First fix: enforce a balance “guard band” and re-check at multiple levels; log reason codes when limits are hit.
EvidenceRPM↑ + Power↑ but Airflow flat
Higher resistance, not higher delivery
- Isolate: duct resistance shift, partial blockage, filter saturation, or leakage between sensing ports.
- First fix: verify dP ports and leakage; use a baseline flow check after maintenance to keep inference mapping valid.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F4 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F4 — URL — Accessed 2026-01-16
H2-5 — Dual-Fan Drive & Control (Match, Balance, Noise)
Dual-fan HRV performance depends on selecting the correct top-level target (balance vs static pressure), then separating problems cleanly into an air-side outer loop and a drive-side inner loop.
Balance vs static-pressure priority
Outer / inner loop evidence
Saturation discriminator
Noise & surge root causes
First-fix priority queue
A. Choose the control priority by installation reality
PriorityBalance first
Best when indoor pressure/odor sensitivity dominates
- Goal: minimize supply–exhaust mismatch (%) to avoid backdraft/odor and door-gap pressure issues.
- Condition: duct resistance is relatively stable across seasons and filter service intervals.
- Constraint: enforce a minimum delivery guard band (do not let airflow collapse while balancing).
PriorityStatic pressure first
Best when resistance variation is large or delivery must be guaranteed
- Goal: maintain target dP (or equivalent) so airflow delivery holds under filter loading, dampers, and long ducts.
- Condition: installations with long runs, many bends, or frequent state changes (valves/dampers).
- Constraint: keep balance error bounded while chasing dP, and log limit hits when the drive saturates.
B. Typical closed-loop structure: outer loop vs inner loop
Outer loopAir-side
Balances airflow or regulates static pressure
- Inputs (evidence): airflow/dP, balance error, damper state, filter loading marker.
- Output: target fan levels (requested RPM / duty) for supply and exhaust.
- Common failure: instability when resistance changes quickly (filter/damper/frost), especially without filtering and gain scheduling.
Inner loopDrive-side
Enforces speed/current limits (protection + noise control)
- Inputs (evidence): tach/Hall RPM, current/power, supply voltage status (UVLO), limit flags.
- Output: actual motor PWM/FOC command with safe ramps and limit handling.
- Common failure: saturation: the inner loop hits a limit (speed/current/voltage), making outer-loop corrections ineffective.
Saturation discriminator (fast isolate)
RuleAlways check inner-loop saturation first
If the outer loop cannot reach target, determine whether the drive is capped
- Inner loop saturated: RPM cannot increase, current limit/UVLO flags appear, or power rises with no airflow gain. Prioritize resistance, limits, and power integrity.
- Inner loop not saturated: drive has headroom. Prioritize outer-loop gain/filtering, target selection (balance vs dP), and state-based parameter switching.
C. Noise / surge / stall: common root-cause fingerprints
Noise classPWM tone
Whine concentrated at a specific frequency band
- Fingerprint: narrowband tone that shifts with PWM settings and operating level.
- First fix: move PWM above audible range (while checking thermal margin), add smoother ramping to avoid sudden tonal jumps.
Noise classMechanical resonance
Noise peaks within a narrow RPM band
- Fingerprint: strong noise only at certain RPM; changes with mounting stiffness or duct coupling.
- First fix: apply avoid-band speed limits and soft-start; skip through resonant RPM regions instead of dwelling.
Noise classDuct impedance
Surge/oscillation when filter/damper state changes
- Fingerprint: airflow/dP oscillates with fan command; instability appears after a resistance step (filter loading, damper movement).
- First fix: add filtering and gain scheduling to the outer loop; constrain rate-of-change of targets during state transitions.
Noise classDrive boundary
High load + limit hits cause RPM/power ripple
- Fingerprint: RPM ripple and power surges at high resistance; current limit flags appear; airflow gains flatten.
- First fix: adjust limit curves and ramp rates; verify supply sag and UVLO behavior; avoid operating at boundary with unstable gains.
D. First-fix priority queue (do the cheapest, highest-leverage actions first)
- 1) Avoid-band speed limits: skip resonant RPM regions; cap top-end when the duct is restrictive.
- 2) Soft-start ramps: reduce impulsive noise and power dips; prevent sudden pressure steps.
- 3) Outer-loop gain & filtering: stabilize under resistance changes; use level-based gain scheduling.
- 4) State-aware control switching: use different tuning when dampers change or during defrost phases (local state only).
- 5) Log limit hits: record which limit was active (speed/current/UVLO) to make field diagnosis evidence-based.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F5 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F5 — URL — Accessed 2026-01-16
H2-6 — Frost & Defrost Control (Decision Logic You Can Verify)
Frost control is the highest-value HRV long-tail topic because it drives real field failures: airflow collapse, noise increase, and misleading ΔT readings. A robust strategy is verifiable: triggers, actions, guards, and recovery metrics are all evidence-backed.
Mechanism → evidence
Verifiable triggers (3 signals)
Defrost actions (HRV only)
False-trigger guards
Logs + recovery metrics
A. Frost mechanism (only what affects evidence)
- Low outdoor temperature drives the core surface toward freezing conditions.
- Condensation → icing accumulates on the core, increasing resistance and narrowing passages.
- Primary observable impact: airflow decreases and/or dP rises under the same fan command, then ΔT behavior becomes misleading unless airflow evidence is checked.
B. Verifiable trigger signals (use combinations, not a single threshold)
SignalΔT abnormal trend
Efficiency drops relative to the current airflow level
- Evidence form: ΔT deviates from expected behavior for the current airflow/dP state (not “small ΔT” alone).
- Interpretation: use only after temperature placement/coherence checks pass (avoid fake ΔT).
SignalAirflow↓ / dP↑
Restriction trend indicates passage narrowing
- Evidence form: airflow decreases and/or dP rises while drive command increases or stays constant.
- Interpretation: strongest physical evidence of restriction growth (frost or blockage).
SignalTemperature plateau
Core-related points stall at a “frozen platform” level
- Evidence form: a temperature point becomes unusually slow-changing and recovers slowly after state changes.
- Guard: plateau must be checked against wetting/placement artifacts and sensor coherence rules.
Recommended trigger strategy (verifiable and resistant to false alarms)
- 2-out-of-3 evidence: confirm frost when any two of {ΔT trend, airflow/dP trend, temperature plateau} agree over a minimum duration window.
- Two-stage decision: Suspect on early trend, Confirm only after coherence checks and duration thresholds pass.
- Reason codes: record which evidence triggered the decision to enable field diagnosis and tuning.
C. Defrost actions (HRV local actuators only)
ActionBypass damper
Reduce freezing tendency and allow recovery
- What to verify: damper position change is real (not just a command); airflow/dP and ΔT recovery trend follows.
- Risk: short-term heat recovery reduction; acceptable only when logged as a defrost state.
ActionFan slow / swap
Use fan strategy to change core thermal balance
- What to verify: RPM/power change matches the commanded state; airflow collapse stops and begins to recover.
- Risk: temporary imbalance; manage with bounded windows and explicit state markers.
ActionPreheat request
Interface boundary only (no heat-source topology)
- What to verify: the preheat request is asserted and a measurable improvement occurs (airflow/dP recovery and reduced plateau behavior).
- Boundary: the heat source implementation is outside this page; only request timing and verification conditions belong here.
D. False-trigger guards (prevent “defrost for the wrong reason”)
GuardSensor coherence
Block decisions when temperature evidence is inconsistent
- Reject frost confirmation if T-pair relationships fail coherence checks under steady state (indicates wet probe, wall conduction, or placement artifact).
GuardTransient reject
Require minimum duration and trend confirmation
- Ignore short cold-air bursts and brief disturbances by enforcing duration thresholds and slope/trend rules.
GuardState stable
Do not trigger during recent state transitions
- Apply a short “settling window” after fan-level changes or damper movements before evaluating frost evidence.
E. What to log (to make defrost auditable)
- Reason code: ΔT trend / airflow-dP restriction / temperature plateau / combination.
- Start/End timestamps: duration matters for quality and user experience.
- Actuator states: bypass position, fan levels, any limit flags (current limit / UVLO).
- Recovery metrics: time to regain airflow baseline, dP rollback amount, ΔT stabilization window.
F. Pass/Fail: verify defrost effectiveness by recovery curves
- Airflow recovers: delivery rises back toward baseline under comparable fan command.
- dP relaxes: resistance indicators drop from the restriction peak.
- ΔT returns to a reasonable band: only after airflow evidence indicates normal operating point.
- Recovery time bounded: excessive recovery time indicates unresolved restriction or incorrect trigger/action selection.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F2 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F2 — URL — Accessed 2026-01-16
H2-7 — Energy Metering: What to Measure & How to Trust It
HRV metering is valuable only when it can be trusted under low power and low power-factor conditions, and when it is split into meaningful loads so trends can be converted into field evidence.
Split loads (2 fans + control)
CT vs shunt tradeoffs
Low-power / low-PF drift
Consistency checks
Metering → diagnostics
Log taps
A. What to meter (split by load, not “one number”)
Meter targetSupply fan
Separate fan trends prevent false root causes
- Why split: supply-side resistance and damper states can diverge from exhaust-side behavior in real ducts.
- What to record: instantaneous power (W), energy (Wh), speed level marker (RPM/duty bucket).
Meter targetExhaust fan
Captures restriction growth and imbalance patterns
- Why split: exhaust ducts often see different contamination and backpressure conditions.
- What to record: W/Wh plus limit-flag markers (current limit / UVLO) if available.
Meter targetControl board
Acts as a state witness for strategy switching and resets
- Why split: board power changes can indicate defrost activity, actuator movement, or abnormal reset loops.
- What to record: W/Wh plus reset reason snapshots near events.
B. Sampling chain choice (CT vs shunt) — engineering tradeoffs
OptionCT
Good isolation, but bandwidth/phase behavior matters
- Strength: galvanic isolation and low insertion loss at higher currents.
- Risk: phase/bandwidth limits can distort real-power estimation under distorted waveforms and low PF.
- Best use: higher-power branches where isolation and safety margin dominate.
OptionShunt
Direct sensing, but noise/offset dominate at low power
- Strength: strong fidelity for current shape when coupled with appropriate AFE/ADC.
- Risk: offset/noise and thermal drift become a larger fraction of signal in low-power modes.
- Best use: low-to-mid power branches where precision at small currents is critical.
C. Why low power and low PF drift is the common failure mode
- Low power: signal is small, so offset/noise/quantization becomes visible in the W estimate.
- Low PF: true power is a small difference between voltage and current phase components; phase errors inflate W error.
- Distorted waveforms: fan drive behavior can create non-sinusoidal current; metering depends on bandwidth and sampling alignment.
D. How to trust metering (consistency checks, not hope)
CheckState consistency
Power changes must match local states and events
- During defrost / bypass states, W/Wh should shift in a direction consistent with the command and duration.
- Actuator movement should create a short, explainable board-power signature.
CheckAir-side correlation
Fan power trend should correlate with airflow/dP trends
- If resistance grows, fan power often rises for the same delivery target; if metering shows the opposite, validate drift.
- Split-fan metering helps spot one-duct restriction vs global effects.
CheckBaseline stability
Same mode should have repeatable W/Wh patterns
- For a fixed fan level and stable damper state, W should form a stable baseline band.
- Long-term baseline shift without matching air-side evidence is a metering drift candidate.
E. Evidence chain: metering → diagnose real HRV problems
DiagnosisFilter loading
Power creeps up while delivery target is unchanged
- Meter evidence: fan W/Wh trends upward over days at the same control level bucket.
- Corroborate: airflow decreases and/or dP increases at similar drive command.
- First fix: service filter and verify baseline resets; keep the “before/after” snapshot as proof.
DiagnosisDuct restriction
One fan shows abnormal power rise relative to the other
- Meter evidence: supply and exhaust W diverge significantly under comparable mode.
- Corroborate: imbalance error grows; dP markers become asymmetric if available.
- First fix: isolate which side changed first; inspect dampers and duct segments on the dominant side.
DiagnosisDefrost too frequent
Energy spend shifts toward defrost windows
- Meter evidence: defrost event count rises and the Wh share during defrost windows increases.
- Corroborate: recovery time lengthens (airflow/dP and ΔT stabilize slower after defrost).
- First fix: validate triggers and guards (duration, coherence). Frequent false triggers waste energy and comfort.
F. Logging fields (minimum set for audit and tuning)
- Split power: P_supp, P_exh, P_ctrl (W).
- Split energy: Wh_supp, Wh_exh, Wh_ctrl (Wh).
- Mode tags: fan level bucket, damper state, defrost state.
- Event taps: defrost start/end + reason code; snapshot metering ± a short window around events.
Practical rule: If metering contradicts air-side evidence (airflow/dP) repeatedly at low power, treat W as “untrusted”
until coherence checks pass.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F3 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F3 — URL — Accessed 2026-01-16
H2-8 — Rugged Power Entry & Protections (HRV Specific)
HRV field instability often traces to the power path: fan-start dips, brownout resets, and surge-triggered false alarms. This chapter stays HRV-specific: which rails matter, what evidence to capture, and what protections prevent repeat failures.
HRV rail partition
Fan-start dip
UVLO / brownout
Surge → false alarms
First 2 captures
Reset reason logs
A. Common HRV power domains (and what each one hates)
DomainLogic rail
MCU and memory domain (most sensitive to brownout)
- Failure signature: resets during fan start or during surge events; watchdog/brownout counters rise.
- Evidence: rail dip timing aligns with fan start command or mains transients.
DomainSensor rail
Sensor readout domain (noise and droop create strategy mistakes)
- Failure signature: sudden sensor jumps that trigger defrost or alarms without matching airflow/dP evidence.
- Evidence: sensor reading anomalies coincide with rail noise or ground bounce episodes.
DomainFan drive
Motor drive supply (inrush and limit behavior dominate)
- Failure signature: current limit / UVLO flags around start, audible surges, or unstable speed.
- Evidence: drive flags appear at the same time as rail droop or input sag.
DomainMotor bus
If present, this is the transient injector into low rails
- Failure signature: fan start causes system-wide dip; the board reboots or logs corrupt.
- Evidence: motor-bus step aligns with logic rail dip; mitigation requires inrush control and rail separation.
B. Field problems and their evidence fingerprints
ProblemRandom reboot
Often a brownout/UVLO story, not “software mystery”
- Fingerprint: resets cluster at fan start, high load, or during mains disturbance windows.
- Must log: reset reason + brownout/UVLO count + last timestamp.
ProblemFan-start dip
Start transient pulls rails down
- Fingerprint: logic rail dip appears within milliseconds of motor start; metering snapshots show a sharp W step.
- Must capture: input voltage + logic rail during fan start (triggered capture).
ProblemSurge → false alarms
Alarm without matching air-side evidence
- Fingerprint: alarm/defrost starts, but airflow/dP does not show the expected restriction trend.
- Must capture: surge event marker + sensor-rail noise indicator + reason code history.
C. The first 2 captures (minimum evidence to isolate 80% of cases)
- Capture #1: input + logic rail transient on power-up and on fan-start (compare droop depth and recovery time).
- Capture #2: reset reason and UVLO/brownout/watchdog flags with timestamps (store across reboots).
D. Protections that matter for HRV field reliability
ProtectionInrush / soft-start
Stops fan-start dips from collapsing logic rails
- Evidence of success: smaller droop and fewer UVLO hits during motor start; reboot clusters disappear.
ProtectionUVLO / brownout
Converts “random reboot” into a visible reason code
- Evidence of success: consistent reset reason codes; controlled shutdown behavior rather than corrupted state.
ProtectionSurge clamp
Prevents false alarms and drive damage under transients
- Evidence of success: fewer unexplained alarms; sensor rails remain stable during disturbance.
ProtectionWatchdog + reset logs
Keeps failures diagnosable
- Evidence of success: post-reboot logs show last known state and the exact reason; metering snapshots align with events.
E. Logging fields (tie power events to metering and behavior)
- Power events: uvlo_count, brownout_count, last_uvlo_ts, reset_reason.
- Fan events: fan_start_ts, drive_limit_flag snapshot.
- Metering snapshots: P_supp/P_exh/P_ctrl around fan start and around alarm/defrost events.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F8 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F8 — URL — Accessed 2026-01-16
H2-9 — EMC/ESD/Surge & Safety Interlocks (What to Test Here)
This section is a test-point checklist with pass/fail fingerprints. It avoids standards clause summaries and focuses on HRV-specific coupling paths, safety chains, and the minimum evidence needed to isolate failures.
ESD: panel / harness
EFT/Surge: AC entry coupling
Safety interlocks
Pass/fail symptoms
First 2 captures
Logs & reason codes
A. Test-point checklist (what to stress, where to stress)
ESD — UI / exposed interfaces
Panel keys/knob/touch, service port/connector shells, external terminals.
ESD — long sensor harness
Temperature/pressure/flow harness runs that behave like antennas and inject into sensor rails.
ESD — fan harness
Motor cable and drive harness; common path for coupling into drive UVLO/limit behavior.
EFT/Surge — AC entry
AC inlet, PE path, front-end protection; observe coupling into logic rails and sensor rails.
EFT/Surge — relay/fan line coupling
Switching edges and long runs couple into low rails and signal references.
Safety chain
Door/service switch, over-temp, condensate overflow (if present), stall/jam protection.
B. ESD pass/fail fingerprints (symptom → evidence)
ESD targetPanel UITypical failReboot / freeze
UI hit causes logic-domain collapse or state corruption
- Fail symptoms: reboot, frozen UI, parameter reset, unexpected mode change.
- First 2 captures: logic rail transient (V_logic) + reset_reason (brownout/WDT) stored across reboot.
- Discriminator: if reset_reason is stable and V_logic shows a droop aligned to the strike, treat it as a power-path coupling issue.
ESD targetSensor harnessTypical failFalse defrost/alarm
Sensor-domain injection creates “fake events” without air-side evidence
- Fail symptoms: sudden ΔT jump, sudden restriction indicator, defrost starts without matching airflow/dP trend.
- First 2 captures: sensor rail noise/dip (V_sensor) + sensor_fault_code / defrost_reason_code around the hit.
- Discriminator: if air-side evidence does not support the event, prioritize sensor-rail and reference integrity.
ESD targetFan harnessTypical failDropout / stall
Drive-domain upset shows as speed collapse or limit flag bursts
- Fail symptoms: fan speed dips, short stop-and-recover, imbalance spikes.
- First 2 captures: drive rail transient (V_drive or motor bus) + drive_limit_flag / UVLO flag snapshot.
- Discriminator: if drive flags spike without logic reset, the coupling is likely concentrated in the drive harness and return path.
C. EFT/Surge pass/fail fingerprints (AC entry coupling)
StressEFT / SurgePathAC entry
The key question: did the disturbance reach low rails?
- Fail symptoms: reboot clusters, metering discontinuity, sudden alarms without mechanical/air-side change.
- First 2 captures: input + logic rail transient (triggered) + uvlo/brownout counters with timestamps.
- Discriminator: if V_logic stays clean while symptoms appear, prioritize sensor reference and logging integrity over “power collapse” hypotheses.
StressEFT / SurgePathRelay/fan lines
Coupling often presents as short-latency glitches
- Fail symptoms: short fan dropout, mode flips, actuator mis-step, transient false fault codes.
- First 2 captures: V_sensor + V_logic noise spikes (two-rail view) + reason codes (fault_code / reset_reason).
- Discriminator: if both rails spike but only sensor data jumps, treat it as sensor-path sensitivity rather than firmware logic instability.
D. Safety interlocks (test chain + anti-false-trigger evidence)
InterlockDoor / service switch
Must force a safe state and leave a trace
- Pass: fan drive enters a defined safe state; recovery is deterministic after close.
- Fail symptoms: stuck in fault, unsafe fan restart, missing event record.
- Evidence: event log (door_open/close) + fan command / speed snapshot.
InterlockOver-temp
Verify trigger, hysteresis, and priority order
- Pass: predictable de-rate or shutdown path; clear exit condition; no oscillation.
- Fail symptoms: rapid toggling, late response, or silent shutdown without reason codes.
- Evidence: temp channel trace + overtemp_fault_code + mode transitions.
InterlockCondensate overflow(if present)
Detect real overflow while rejecting splash and transient noise
- Pass: clear alarm condition and safe handling; no chatter.
- Fail symptoms: false alarms during vibration or after ESD/EFT events.
- Evidence: sensor rail noise + condensate_fault_code + correlation with air-side stability.
InterlockStall / jam
Prove protection triggers for real stalls, not for benign transients
- Pass: stall recognized by RPM collapse and/or drive-limit pattern; safe stop and retry policy is consistent.
- Fail symptoms: repeated false stalls after disturbances; missing drive flags.
- Evidence: RPM trace + drive_limit_flag/UVLO + timestamped retry events.
E. Fail symptom → “first 2 captures” (fast isolation)
SymptomReboot after UI ESD
- Capture #1: V_logic droop aligned with strike.
- Capture #2: reset_reason + brownout/UVLO counters.
SymptomDefrost triggers “out of nowhere”
- Capture #1: V_sensor noise/dip around the event.
- Capture #2: defrost_reason_code + sensor_fault_code timeline.
SymptomFan dropouts after EFT/Surge
- Capture #1: V_drive/motor-bus transient.
- Capture #2: drive_limit_flag/UVLO + timestamp.
Pass criterion mindset: A pass is not “no crash once”; it is repeatable behavior with coherent logs and rails that prove why the system stayed stable.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F9 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F9 — URL — Accessed 2026-01-16
H2-10 — Validation Test Plan (Lab → Field)
This validation plan is written as an SOP: setup → run steps → captures → pass/fail → if fail, the first two measurements. It covers baseline performance, freeze cycles, extremes, and a field deployment checklist with mandatory log schema.
SOP format
Baseline KPIs
Freeze step/cycle
Recovery time
Extremes
Mandatory logs
A. Pre-check (before any performance claims)
SetupLogs readySensors sane
Establish a reproducible baseline and auditable logs
- Confirm: timestamp continuity and persistence across reboot; reason codes are readable.
- Confirm: temperature channels are coherent at steady conditions; airflow/dP channels have stable zero behavior.
- Record: initial baseline snapshots for ΔT, airflow balance, noise, split power.
B. Baseline performance (ΔT, balance, noise, power)
TestΔT accuracy
Prove temperature channels and derived ΔT are stable and coherent
- Run: steady-state at multiple fan levels; hold long enough to see drift trends.
- Pass: ΔT settles and remains stable within a narrow band for the same mode.
- If fail: capture #1 temperature channel traces; capture #2 sensor_fault_code and sensor rail noise around anomalies.
TestAirflow balance
Demonstrate supply/exhaust balance and long-hold stability
- Run: hold each mode; compute balance error over time windows (not single points).
- Pass: balance error stays bounded; no monotonic drift with constant settings.
- If fail: capture #1 airflow/dP evidence; capture #2 split fan power trend (P_supp vs P_exh) to isolate which side shifted.
TestNoise
Identify resonance bands and unstable control windows
- Run: sweep fan levels; note any narrow bands with sudden noise jumps.
- Pass: monotonic noise trend; no narrow-band “spikes” tied to control instability.
- If fail: capture #1 RPM/command vs noise window; capture #2 drive flags (limit/UVLO) and rail noise around the band.
TestPower
Verify split metering is coherent under low power and distorted waveforms
- Run: include low-power and low-PF conditions; compare to air-side evidence.
- Pass: W/Wh trends correlate with airflow/dP changes and mode events (defrost, damper).
- If fail: capture #1 rail noise and sensor coherence; capture #2 metering coherence checks + reason codes for event windows.
C. Freeze scenario (step → cycle → recovery time)
StepOutdoor temp change
Trigger logic should be explainable and repeatable
- Run: apply a temperature step; observe time-aligned ΔT, airflow/dP, and defrost state transitions.
- Pass: defrost triggers only when evidence supports restriction/icing patterns.
- If fail: capture #1 sensor coherence around the trigger; capture #2 defrost_reason_code and sensor_fault_code timeline.
CycleFrost → defrost loops
Cycle stability matters more than a single “successful” defrost
- Run: repeat multiple cycles; measure cycle-to-cycle variability.
- Pass: similar trigger points and durations; no runaway frequency increase without cause.
- If fail: capture #1 split fan power + airflow/dP trends; capture #2 defrost event counts and energy share per cycle.
RecoveryTime-to-baseline
Recovery time is the KPI that reveals hidden restrictions
- Run: measure time for airflow/dP and ΔT to return to baseline after defrost exit.
- Pass: recovery time stays within a defined window across cycles.
- If fail: capture #1 airflow/dP stabilization curve; capture #2 metering + mode tags to see if the system stayed in a hidden partial-defrost state.
D. Extremes (filter, duct resistance, condensate disturbance)
ExtremeFilter loading
Expected trend vs bad trend must be separated by evidence
- Expected: fan W rises and airflow falls gradually; imbalance may drift on the dominant side.
- Bad: sudden jumps, oscillation, or defrost triggers without restriction evidence.
- Fail fast: capture #1 split power trend; capture #2 airflow/dP and event log coherence.
ExtremeDuct resistance change
Control should remain stable under resistance steps
- Expected: airflow balance corrects without overshoot; noise does not spike in narrow bands.
- Bad: hunting, oscillation, narrow-band noise spikes, or repeated limit flags.
- Fail fast: capture #1 RPM/command vs airflow/dP; capture #2 drive limit flags and rail noise.
ExtremeCondensate disturbance
Reject transient sensor artifacts while keeping real safety triggers
- Expected: stable strategy; no false alarms during benign disturbance.
- Bad: repeated false interlock triggers or sensor faults after mechanical vibration.
- Fail fast: capture #1 sensor rail stability; capture #2 fault codes + timestamps and correlations to mode tags.
E. Field checklist (deploy → 5-minute baseline → log pull)
- Deploy notes: duct length category, damper initial state, filter condition, power quality notes.
- 5-minute baseline: fixed mode, capture ΔT + airflow/dP + split power + noise impression (repeatable window).
- Log pull: extract reason codes and event markers with timestamps; align with metering snapshots.
F. Mandatory log schema (minimum fields)
RequiredReason codes
- power_on_reason (cold start / resume / brownout recovery)
- reset_reason (WDT / brownout / SW reset)
- defrost_reason_code (restriction evidence tag)
- sensor_fault_code (range / open / short / coherence)
RequiredContext tags
- timestamp, mode, fan_level, damper_state
- P_supp/P_exh/P_ctrl snapshot around key events (defrost, stall, alarms)
SOP principle: a “pass” is repeatable behavior with coherent logs and correlated air-side and power evidence across cycles,
not a single successful run.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F10 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F10 — URL — Accessed 2026-01-16
H2-11 — Field Debug Playbook (Symptom → Evidence → Isolate → Fix)
Use this as a repeatable on-site SOP. Every symptom is handled the same way: First 2 measurements → discriminator (separate root-cause classes) → first fix (lowest-cost, highest-signal action).
Air-side: airflow / dP / balance
Thermal: 4 temps / ΔT coherence
Drive: RPM / command / limit
Power+Logs: split W/Wh + reason codes
No cloud / no standards clauses
Symptom 1 — Heat recovery poor (ΔT small)
Goal: prove whether the small ΔT is real (air-side) or “fake ΔT” (sensor placement / bypass / leakage).
First 2 measurements
- 4-point temperature snapshot: OA/SA/RA/EA (or at least both core in/out pairs).
- Air-side evidence: airflow + dP trend (or balance error) at the same fan level.
Discriminator (separate root causes)
- Temps not self-consistent (lags, jumps, wall-touch bias) → sensor placement / condensation influence.
- Temps consistent but ΔT still small → bypass damper leak/stuck, casing leakage, or airflow too high for the core.
- ΔT small + airflow low + dP rising → restriction/icing trend is likely (check defrost behavior).
First fix (lowest-cost, highest-signal)
- Lock/verify bypass damper state; re-test ΔT in a fixed mode.
- Inspect obvious leakage paths (gaskets, duct joints); re-test with identical fan level.
- Reduce airflow one step; if ΔT improves immediately, prioritize airflow/core matching over sensor recalibration.
Symptom 2 — Airflow imbalance (backdraft / odor)
Goal: identify whether imbalance comes from duct resistance, sensing bias, or control hunting.
First 2 measurements
- Balance evidence: supply vs exhaust airflow (or balance error).
- Drive evidence: P_supp vs P_exh (or RPM_supp vs RPM_exh) at the same command level.
Discriminator
- Power/RPM asymmetry grows while airflow barely changes → duct resistance / filter / damper on the stressed side.
- Airflow imbalance jumps with stable drive evidence → airflow sensing/curve bias (calibration / placement).
- Imbalance oscillates with noise spike → control hunting (gain/filter limits or unstable region).
First fix
- Verify filter and obvious duct restriction first (cheapest, highest probability).
- Freeze damper positions; repeat the same fan level to see if balance stabilizes.
- If oscillation: cap speed ramp rate and avoid the narrow band; re-check stability.
Symptom 3 — Frequent defrost / stuck in defrost
Goal: prove whether defrost is triggered by real restriction evidence or by false signals (sensor/EMC/placement).
First 2 measurements
- Event timeline: defrost_start/end + defrost_reason_code (with timestamps).
- Restriction evidence: airflow/dP trend across each cycle.
Discriminator
- Defrost without restriction trend → false trigger (sensor coherence, harness injection, placement/condensation).
- Defrost triggers and airflow recovers slowly → residual ice / bypass not effective / exit condition too early.
- Defrost correlates with power/drive flags → drive UVLO/limit or power entry disturbance causing state churn.
First fix
- Audit sensor coherence at trigger moments; log sensor_fault_code around events.
- Verify bypass/damper motion and sealing; re-run one controlled freeze cycle.
- Check split power and drive_limit_flag around transitions; eliminate rail dips before retuning logic.
Symptom 4 — Noise loud / resonance spike
Goal: separate duct/mechanical resonance from control instability and from drive limit/UVLO events.
First 2 measurements
- RPM vs command: commanded level and actual RPM around the noise band.
- Air-side load clue: airflow/dP at the same band (to detect sudden resistance shift).
Discriminator
- Narrow band only (specific RPM window) → mechanical/duct resonance likely.
- RPM oscillates or hunts with airflow/dP oscillation → control loop instability (gain/filter margin).
- Noise coincides with limit flags → drive current limit, UVLO, or protection toggling.
First fix
- Define a “no-go” RPM band; cap ramp rate through the resonance window.
- Check duct restriction changes (filter, damper, bends) that shift the resonance band.
- If flags appear: fix rail integrity and limit behavior before retuning control gains.
Symptom 5 — Fan intermittent stop / jitter
Goal: decide whether it is drive protection, power entry sag, or harness/EMC upset.
First 2 measurements
- Drive event evidence: RPM trace + drive_limit_flag/UVLO around the dropout.
- Power evidence: V_drive (or motor bus) transient during the same event window.
Discriminator
- UVLO/limit flags spike with bus dip → power entry sag or drive rail weakness.
- RPM collapses without bus dip → protection thresholds, jam/stall detection, or control command drop.
- Events correlate with ESD/EFT → harness coupling (fan cable return path) is the first suspect.
First fix
- Capture a triggered event window; confirm whether bus dip precedes RPM collapse.
- Stabilize supply/soft-start to the drive domain; then re-check dropout frequency.
- If EMC-correlated: add/verify harness suppression and return path integrity before changing thresholds.
Symptom 6 — Power unusually high
Goal: determine whether high power is caused by restriction, imbalance compensation, or abnormal defrost frequency.
First 2 measurements
- Split power: P_supp + P_exh + P_ctrl (or at least fan power vs controller power).
- Air-side load: airflow/dP (to detect restriction/drag) at the same mode.
Discriminator
- Fan power rises with falling airflow → restriction (filter/duct/core icing) forces higher effort.
- One fan dominates power → imbalance compensation due to asymmetric resistance/damper position.
- Power spikes at defrost transitions → excessive defrost frequency or unstable state transitions.
First fix
- Check filter/core restriction first; compare airflow and dP before/after maintenance.
- Freeze damper state and re-run at one fixed fan level; confirm whether split power symmetry improves.
- Audit defrost_reason_code distribution; eliminate false triggers before changing power targets.
Symptom 7 — Random reboot / freeze
Goal: decide power-path brownout vs watchdog/software lock vs EMC upset.
First 2 measurements
- Reset evidence: reset_reason + brownout/UVLO counters preserved across reboot.
- Rail evidence: V_logic transient (triggered) during fan start, relay switching, or EFT/surge exposure.
Discriminator
- Brownout/UVLO reason + rail dip → power entry/inrush/rail margin.
- WDT reason with clean rail → firmware deadlock (still check ESD/EFT correlation).
- Strong correlation to ESD/EFT → coupling path (UI/sensor harness) first.
First fix
- Reproduce under a controlled trigger (fan start or relay) and capture V_logic + reason code together.
- Fix inrush/soft-start/UVLO margin; then re-test without changing control loops.
- Harden logs: ensure reason codes and timestamps survive the reset for future isolation.
Symptom 8 — Sensor reading jump / drift
Goal: separate true environmental change from sensor placement artifacts, rail noise injection, or open/short faults.
First 2 measurements
- Coherence check: compare related channels (e.g., paired temps) and look for impossible combinations.
- Electrical check: V_sensor stability + sensor_fault_code around the jump.
Discriminator
- One channel jumps while neighbors remain stable → wiring/open/short or local placement issue.
- Multiple channels jump together → sensor rail/reference injection or reset event nearby.
- Jump correlates with defrost/fan transitions → thermal lag/condensation or harness coupling.
First fix
- Re-seat harness and confirm fault codes; then repeat the same mode window.
- Improve sensor placement (avoid wall touch/short circuiting airflow path) before recalibrating.
- Stabilize sensor rail and reference; confirm the jump disappears under identical conditions.
Symptom 9 — Defrost works, but recovery is slow
Goal: isolate residual ice / partial bypass / hidden restriction that keeps airflow depressed after defrost ends.
First 2 measurements
- Recovery curve: airflow/dP vs time from defrost_end to baseline.
- Mode integrity: damper_state + fan_level tags (ensure system truly left defrost).
Discriminator
- Airflow recovers slowly while damper/fan tags are stable → residual ice or mechanical restriction.
- Tags show hidden state churn (rapid mode toggles) → logic exit condition too sensitive / false re-entry.
- Recovery time grows cycle-to-cycle → accumulating restriction (filter/core) or ineffective bypass sealing.
First fix
- Measure recovery time across 3 cycles; treat monotonic growth as a restriction signature.
- Verify bypass sealing and motion; re-run a controlled cycle with fixed damper state.
- If churn: tighten discriminator using airflow/dP evidence, not temperature spikes alone.
Symptom 10 — Metering/logs look inconsistent (data cannot be trusted)
Goal: decide whether the data stream is broken (timestamps/reset gaps) or the measurements disagree with air-side physics.
First 2 measurements
- Log continuity: timestamp continuity + power_on_reason/reset_reason around gaps.
- Physics cross-check: split power trend vs airflow/dP trend (should correlate under stable modes).
Discriminator
- Log gaps align with resets → persistence problem or brownout resets; fix rail/logging first.
- Power trend conflicts with airflow/dP under stable mode → metering chain or scaling error.
- Only one channel drifts (e.g., P_supp) → sensor/AFE path, shunt/CT scaling, or channel wiring.
First fix
- Harden reset logging: preserve reason codes and last-event timestamp across reboot.
- Run a stable-mode window and verify correlation: airflow↓ should not accompany fan power↓.
- Isolate one channel at a time; verify scaling/coherence before using analytics for decisions.
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H2-12 — FAQs (HRV) ×12
Each answer follows a field-debug structure: First 2 measurements → Discriminator → First fix. All content stays within the HRV unit boundary (ΔT/airflow/bypass/defrost/drive/metering/power/EMC evidence).
ΔT evidence
Airflow balance
Defrost logic
Dual-fan control
Metering trust
Power & EMC
1) Airflow looks normal, but ΔT is always small. Check sensor placement or bypass first?
First 2 measurements: capture the four temperature points (OA/SA/RA/EA) and log the bypass damper state (command vs feedback if available).
Discriminator: if the four temperatures are not self-consistent, suspect placement/condensation/thermal shorting; if they are consistent yet ΔT remains small, suspect bypass leakage, short-circuit airflow, or duct leakage around the core.
First fix: lock bypass closed for an A/B test, then re-validate temperature coherence.
2) Defrost becomes frequent after a season change. Threshold too sensitive or sensors drifting?
First 2 measurements: plot defrost_reason_code with timestamps and track airflow/Δp trend leading into each trigger.
Discriminator: if defrost triggers without a restriction signature (Δp rising or airflow falling), treat it as false triggering from sensor drift/noise or poor ΔT coherence; if restriction evidence is present, the threshold may be appropriate and the issue is the operating window.
First fix: validate sensor coherence first, then adjust trigger persistence (time windows) before moving thresholds.
3) After defrost ends, heat recovery recovers very slowly. Residual ice or airflow balance not restored?
First 2 measurements: record the post-defrost recovery curves of airflow/Δp and ΔT, plus mode tags (damper state, fan level) during the first minutes after exit.
Discriminator: if the controller state is stable but airflow/Δp recovers slowly, suspect residual icing or remaining restriction; if state tags oscillate or never fully exit, suspect defrost state machine or balance loop interaction.
First fix: quantify “time-to-baseline” across 3 cycles and debug the outliers with logs.
4) Both fan speeds change, yet balance gets worse. Outer loop or inner loop problem?
First 2 measurements: log balance error over time and capture RPM/command (or split fan power) with the same timestamps.
Discriminator: if RPM/command changes lead the balance error and show hunting, the inner loop/actuation is unstable; if balance error leads and the controller output ramps (integrator windup), the outer loop gain/scheduling is the issue.
First fix: freeze one loop (hold RPM or hold balance target) to isolate the unstable layer before tuning gains.
5) Power rises soon after a filter replacement. Duct resistance, bearing friction, or metering error?
First 2 measurements: compare split power (supply fan vs exhaust fan) and airflow/Δp at the same mode and fan level.
Discriminator: power↑ with Δp↑ and airflow↓ indicates rising resistance; power↑ with little Δp change suggests mechanical friction (bearing/impeller); power trends that do not correlate with airflow/Δp suggest metering chain bias or windowing issues.
First fix: run a fixed-mode A/B test with a known restriction step to verify correlation before blaming hardware.
6) Noise increases in cold weather. PWM/FOC parameters or structural resonance?
First 2 measurements: identify the RPM band where noise spikes and log whether RPM is steady or hunting in that band.
Discriminator: a narrow, repeatable RPM window points to mechanical resonance/duct impedance shifts; wideband noise with oscillating RPM points to control-loop instability, PWM frequency interaction, or limit behaviors.
First fix: add a “no-go RPM band” (speed notch) and soften ramps; then retune control gains or PWM/FOC parameters after stability is proven.
7) A fan stops briefly and recovers. UVLO/power dip or stall protection?
First 2 measurements: capture drive/logic rail dips during the event and read the drive flags (UVLO/limit/jam) plus reset_reason if a reboot occurred.
Discriminator: a rail dip preceding speed collapse indicates UVLO or brownout; stable rails with jam/limit patterns indicate stall detection or protection thresholds.
First fix: reproduce at the same fan level and duct load; if UVLO is confirmed, address startup transient and rail margin before changing stall parameters.
8) After ESD, nothing fails immediately, but days later random alarms appear. Which two event records first?
First 2 records: (1) reset_reason/power_on_reason history with timestamps, and (2) a rolling counter timeline of sensor_fault_code/defrost_reason_code around the first appearance of alarms.
Discriminator: growing counters without resets suggest intermittent harness/contact degradation; resets clustered near alarms suggest power integrity or firmware lockups; a single channel dominating points to a localized coupling path.
First fix: ensure logs persist across reboots and add a “last N events” ring buffer to prove causality.
9) ΔT occasionally jumps. Condensation effect or harness interference? How to prove?
First 2 measurements: check multi-channel coherence (do adjacent temperature channels jump together?) and correlate jump timestamps to switching events (fan step, damper move, defrost transitions).
Discriminator: a single-channel jump tied to wet/cold conditions points to condensation/placement; multi-channel simultaneous jumps or alignment to switching points to harness injection or reference disturbance.
First fix: improve sensor placement and anti-condensation measures first, then validate harness routing/returns if time correlation persists.
10) After bypass damper moves, airflow balance breaks. Feedback issue or no gain scheduling?
First 2 measurements: compare damper command vs feedback (or inferred position) and log balance error plus controller output immediately before and after the movement.
Discriminator: feedback mismatch indicates actuator/sensor issues; correct feedback with balance divergence indicates missing gain scheduling or integrator windup after system impedance changes.
First fix: confirm damper closes/seals with an A/B hold test, then apply mode-dependent gains and integrator limits for bypass transitions.
11) Energy metering does not match an external meter. PF/harmonics or sampling window/calibration?
First 2 measurements: log internal P/V/I/PF (or equivalent) at a steady fixed mode and run a known, repeatable operating point for comparison.
Discriminator: large PF/harmonic variability can create definition differences vs external meters; if PF is stable yet error remains consistent, suspect sampling window alignment, phase error, or calibration point mismatch.
First fix: validate against a stable reference condition first, then adjust windowing/phase compensation before changing hardware.
12) Why can higher airflow reduce heat recovery? Normal physics or abnormal leakage/short-circuit?
First 2 measurements: compare two airflow levels under the same mode and bypass-locked condition, capturing ΔT and airflow/Δp (plus power).
Discriminator: a smooth, predictable ΔT decrease with higher airflow is normal due to shorter exchange time; abrupt or inconsistent behavior (ΔT collapse without matching airflow evidence) suggests leakage, bypass short-circuit, or duct mixing.
First fix: lock bypass, verify sealing/leak paths, then set airflow targets that balance comfort vs recovery performance.
Cite this figure
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F12 — URL — Accessed 2026-01-16
ICNavigator — Heat Recovery Ventilation (HRV) — Figure F12 — URL — Accessed 2026-01-16
