What it is

An ERV or HRV is a balanced ventilator with two airstreams and a recovery core. Like any air-moving appliance with filters and a heat-transfer surface, it fouls up and drifts out of adjustment over time, and then it quietly stops doing its job — the homeowner just notices the house feels stuffy or humid again. Maintenance keeps the airflow, the recovery, and the balance where they belong. Most "my ERV is broken" calls are actually a plugged filter, a fouled core, a frozen core, or a clogged drain — not a dead unit.

This is the service-and-troubleshoot guide. (For what the box does and ERV-vs-HRV selection, see the dedicated articles.)

How it works

Two things foul: the filters at each airstream inlet, and the core itself. Filters catch the dust and pollen before it reaches the core; when they load up, airflow drops on that stream and the unit goes out of balance and loses capacity. The core's job is heat (and, on an ERV, moisture) transfer across thin walls; dust, grease, and on enthalpy cores any damage to the permeable media all cut transfer and airflow.

In cold climates an HRV has a second failure mode: frost. The warm, moist exhaust air gives up its heat to the core, and if the incoming air is cold enough the moisture in the exhaust stream condenses and freezes on the core, choking it. That's why cold-climate units have a defrost strategy — they periodically stop the fresh-air intake and recirculate, or open a damper, to thaw the core. If defrost fails, the core ices up and airflow collapses in deep winter.

ERVs and defrosting units can also produce condensate, which needs a drain and a trap. A clogged drain backs water up into the unit.

In the field

Routine PM (typically every few months for filters, at least annually for the core — follow the manufacturer's interval):

  1. Power down the unit before opening it.
  2. Pull and clean or replace the filters. Vacuum or wash per the manufacturer; replace if they're the disposable type. Loaded filters are the #1 cause of low airflow.
  3. Remove and clean the core. Sensible (HRV) cores are usually washable or vacuumable per instructions. Enthalpy (ERV) cores are more delicate — many are NOT to be soaked or pressure-washed because that damages the permeable media; follow the maker's cleaning method exactly. Inspect for damage.
  4. Clear the condensate drain and check the trap. Flush it; make sure it actually drains and the trap has water.
  5. Wipe the cabinet interior and check the gaskets/seals so the two streams aren't short-circuiting (cross-leaking supply into exhaust).
  6. Check the fans/motors for noise, balance, and that both run.
  7. Verify defrost operation on cold-climate units (you may need to simulate or check the control's defrost mode).
  8. Re-balance the airflow. After service, measure supply and exhaust CFM and rebalance so they're equal and at the target. Cleaning changes the airflow; confirm it.
  9. Confirm intake/exhaust hoods outside are clear of debris, nests, snow, and lint, and that the exhaust hasn't been cross-contaminating the intake.

Normal values & targets

  • Filter interval: commonly every 1–3 months depending on the environment; dusty/pet/construction conditions shorten it.
  • Core cleaning: at least annually, more in dusty or greasy conditions. Method depends on core type — sensible cores often washable, enthalpy cores often vacuum-only/no-soak.
  • Recovery efficiency target: a healthy core recovers roughly 60–85% of the energy. A noticeable drop in recovered temperature across the core points to fouling or imbalance.
  • Balance: supply CFM ≈ exhaust CFM (within the manufacturer's tolerance). Out of balance = pressurizing/depressurizing the house and lost efficiency.
  • Defrost (cold climate): kicks in below roughly the manufacturer's low-temp threshold (often somewhere around the low 20s°F and colder). Below that, expect periodic defrost cycles.

Common faults & what they mean

  • Low or no airflow: loaded filters (most common), fouled or iced core, crushed/disconnected duct, or failed fan. Check filters and core first, then ducting, then the motor.
  • Core iced up in winter (HRV): defrost not working — failed defrost damper/control, or the unit isn't rated for that cold. Restore defrost; in extreme cold the unit may need a different defrost setup.
  • Water leaking from the unit / under it: clogged condensate drain or trap, or a cracked drain pan. Clear/flush the drain, check the trap.
  • Musty or stale odor from the supply air: fouled or moldy core, dirty drain pan, or the exhaust short-circuiting into the supply (bad gasket/seal or hoods too close outside). Clean and check the seals and hood separation.
  • House humid in winter (cold climate): could be an HRV whose recovery is degraded, an out-of-balance unit, or simply under-ventilation — but if it's an ERV retaining moisture in a tight crowded house, that's expected behavior, not a fault.
  • House too dry in winter: an HRV exhausting all the indoor moisture (normal for an HRV) — may indicate the wrong device choice rather than a malfunction.
  • One stream much weaker than the other: imbalance from a dirty filter on one side, a kinked duct, or a balancing damper that drifted. Rebalance.

Tech tips & gotchas

  • Most "dead ERV" calls are filters or the core. Check the cheap, obvious stuff before condemning the unit. A vacuum and a fresh filter fixes a lot of "it stopped working" complaints.
  • Don't wash an enthalpy (ERV) core like a sensible (HRV) core. Soaking or pressure-washing a permeable enthalpy core can wreck the moisture-transfer media. Read the manufacturer's cleaning instructions — the cores are not interchangeable in how you treat them.
  • Always rebalance after service. Cleaning and filter changes shift the airflow. If you don't re-measure and rebalance, you may leave the house net-positive or net-negative.
  • Check the outdoor hoods. Lint, leaves, bug screens packed with debris, snow drifts, and bird nests choke the airflow at the terminations. And if the intake and exhaust hoods are too close, the unit re-breathes its own stale air.
  • Frost is a control problem, not a refrigerant problem. A cold-climate HRV that ices up is a defrost issue — verify the defrost cycle and damper, don't go looking for a refrigerant fault (there isn't one; these have no refrigerant).
  • Verify the drain and trap on anything that makes condensate. A backed-up drain is a quiet, common failure that floods the cabinet and can damage the fan or the floor below.

Safety / code notes

  • Service per the manufacturer's instructions, especially core cleaning method and defrost setup. Equipment requirements and condensate disposal follow IMC provisions — route condensate to an approved point, never just let it run onto the floor or framing.
  • Maintain the whole-house ventilation rate per ASHRAE 62.2 after service — confirm the unit still meets the required CFM once filters/core are cleaned and the streams rebalanced.
  • Keep outdoor intake clearances from contaminant sources (flue terminations, dryer/exhaust vents) per IMC intake provisions when checking or relocating hoods.