How an ERV Works vs an HRV — Heat-Only vs Heat-and-Moisture Recovery

What it is

Both an HRV (heat recovery ventilator) and an ERV (energy recovery ventilator) are balanced ventilators: they push outdoor air in and pull stale indoor air out in equal amounts, through one box, while a core inside lets the two airstreams trade energy without mixing. The point is to recover most of the heating or cooling you already paid for, instead of throwing it out the exhaust and dragging raw outdoor air inside.

The difference between the two is what the core transfers. An HRV moves heat only (sensible energy). An ERV moves heat AND moisture (sensible plus latent — "total energy"). That one difference is the entire decision, and it's driven by climate and the moisture situation in the house.

How it works

Inside the box, the incoming fresh stream and the outgoing stale stream run past each other through a core, separated by thin walls so they never actually mix. Heat conducts across those walls from the warmer stream to the cooler one. In winter, the warm outgoing house air pre-heats the cold incoming fresh air. In summer, the cool outgoing house air pre-cools the hot incoming air. Either way you recover a big chunk of the energy and hand your furnace or AC a much smaller load.

That's an HRV — pure heat transfer through a sensible core (often aluminum or a sensible-only plastic core).

An ERV adds moisture transfer. Its core is built from a vapor-permeable material (an enthalpy core — treated paper, membrane, or a desiccant-coated wheel) that lets water vapor move across along with the heat, driven by the humidity difference between the two streams. In a humid summer, the ERV lets the drier exhaust air pull moisture OUT of the incoming humid fresh air before it enters the house, so you're not dumping a latent load on the AC. In a dry winter, it works the other way: it holds some of the indoor humidity in instead of exhausting it, so the house doesn't dry out to a nosebleed.

The mechanical recovery is the same balanced airflow in both. The ERV just adds the latent (moisture) leg through a permeable core.

In the field

• Pick the box by climate first. Cold/dry climate → HRV is the traditional pick (you want to dump moisture, not keep it). Hot/humid climate → ERV (keep summer humidity from riding the fresh air inside). Mixed climate → an ERV is usually the safer default because it handles both summer humidity and winter dryness. Many techs now default to ERV across most of the U.S. for that reason, but the climate and the house's moisture loads decide.
• Watch the indoor moisture situation, not just the map. A very tight, occupant-heavy house in a cold climate can actually run TOO humid in winter (window sweat). There an ERV that retains less moisture, or an HRV, can be better. Conversely a dry house in a humid climate might still want an ERV. Read the actual humidity, not just the zip code.
• Confirm core type before you assume. Some units take both sensible and enthalpy cores. Don't guess — an HRV core in an ERV application (or vice versa) will fight the climate.
• Both still need balancing and commissioning. The recovery only happens if both streams are actually flowing at the rated, balanced CFM. Measure and balance them (see the sizing article).
• Neither one dehumidifies or filters like a dedicated appliance. An ERV reduces the latent load of the incoming air; it is not a dehumidifier and won't dry out an already-humid house on its own. Set expectations accordingly.

Normal values & targets

• Recovery efficiency: good cores recover roughly 60–85% of the energy that would otherwise leave with the exhaust. HRVs report a sensible recovery efficiency; ERVs report total (sensible + latent) recovery, plus a moisture transfer rate.
• HRV = sensible only: transfers heat, not moisture. Best where you want to expel indoor humidity (cold, dry winters).
• ERV = total energy: transfers heat and a fraction of the moisture. Best where you want to keep summer humidity out and retain some winter humidity.
• Moisture transfer is partial, not total. An ERV doesn't move all the water vapor — it moves a fraction set by the core. It moderates the latent load; it doesn't eliminate it.
• Both run balanced: supply CFM ≈ exhaust CFM. An out-of-balance unit pressurizes or depressurizes the house and loses efficiency.

Common faults & what they mean

• House too humid in summer with an HRV installed: wrong device for a humid climate. The HRV recovers heat but lets all the incoming moisture through. An ERV would have knocked down the latent load.
• House bone-dry and staticky in winter with an HRV: the HRV is exhausting indoor moisture with no latent recovery. In a tight cold-climate house an ERV would retain some humidity.
• Window sweat / high RH in winter with an ERV in a very tight, crowded house: the ERV may be retaining more moisture than the house can shed. Sometimes the answer is more ventilation, sometimes a different core, sometimes managing the moisture sources.
• "Recovery seems poor": plugged filters or a fouled core (kills heat transfer), unbalanced airflow, or a frosted core in a cold-climate HRV that isn't defrosting. Check airflow and core condition before condemning the unit.

Tech tips & gotchas

• The core is the difference — heat only (HRV) vs heat + moisture (ERV). Memorize that and the rest follows.
• ERV is not a dehumidifier. Customers and even some installers expect an ERV to "dry out the house." It only reduces the moisture coming in on the fresh-air stream. A humid house with internal moisture problems still needs a dehumidifier or AC runtime to fix it.
• Climate AND the house decide. The map gives you the default; the actual indoor humidity, occupancy, and tightness can flip the call. Measure RH in the worst season before committing.
• Cold-climate HRVs need a defrost strategy. When it's very cold, the moist exhaust air can frost the core. HRVs handle this with a defrost cycle (recirculation or a damper). If a cold-climate unit has lost recovery in deep winter, check whether defrost is working.
• Don't cross-connect an ERV to a bathroom as its only exhaust source. Pulling shower moisture through an enthalpy core can hand some of that moisture right back to the supply side. Follow the manufacturer's guidance on what rooms the stale-air side should pull from.

Safety / code notes

• Whole-house balanced ventilation rates follow ASHRAE 62.2 (referenced by IRC/IMC). The ERV/HRV must be sized and balanced to that method.
• Equipment selection, duct connections, and condensate handling (ERVs and defrosting HRVs can produce condensate) follow the manufacturer's instructions and applicable IMC provisions. Route any condensate properly.
• Outdoor intake location and clearances to contaminant sources (dryer vents, flue terminations, driveways) follow IMC intake provisions — don't draw fresh air from a dirty spot.