A heat recovery ventilator (HRV) is a balanced mechanical ventilation system that exhausts stale indoor air and supplies fresh outdoor air while transferring heat between the two streams through a heat exchanger core. An energy recovery ventilator (ERV) works the same way but also transfers moisture, which helps keep indoor humidity stable.

Airtight construction has made this equipment hard to avoid. Once a building envelope drops below roughly 1 air change per hour, opening a window or relying on leaks no longer delivers reliable fresh air, and exhaust-only fans throw conditioned air straight outside. An HRV or ERV solves both problems at once: it ventilates continuously and recovers most of the energy you already paid to heat or cool. This article explains how the technology works, where HRV and ERV differ, and how to pick and detail the right unit for a project.

How Does a Heat Recovery Ventilator Work?

An HRV moves two separate airstreams through one box. A fan pulls stale air from kitchens, bathrooms and laundry rooms and pushes it outside. A second fan draws fresh outdoor air in and delivers it to bedrooms and living spaces. Inside the unit, both streams pass through a counterflow heat exchanger, a stack of thin plates that lets heat migrate from the warm stream to the cold one without the two airflows ever mixing.

In winter, outgoing indoor air preheats the incoming cold air. In summer the process reverses, and cooler exhaust air tempers the hot supply air. The U.S. Department of Energy's Building Science Education resource on energy recovery ventilation describes this as a controlled way of ventilating a home while minimizing energy loss, since the heating and cooling plant no longer has to condition ventilation air from scratch.

Because supply and exhaust flows are balanced, the building stays at neutral pressure. That matters more than it sounds. Depressurized houses pull in radon, soil gases and combustion byproducts; pressurized ones push humid indoor air into wall cavities. Systems that hold the building at neutral pressure avoid both failure modes, which is one reason they outperform the simpler options covered in our comparison of natural vs mechanical ventilation.

What Makes an ERV Different?

An ERV recovers heat and moisture. Its core uses a vapor-permeable membrane or an enthalpy wheel, so water vapor moves from the more humid airstream to the drier one along with sensible heat. The hardware otherwise looks identical: same fans, same filters, same duct connections.

The practical consequences show up at the extremes of the year. In a humid summer climate, an ERV transfers part of the moisture in the incoming air to the outgoing exhaust before it reaches your cooling coil, cutting the latent load on the air conditioner. In a cold, dry winter, the same membrane keeps some indoor humidity in the building instead of exhausting it, so occupants suffer less from static, dry skin and cracking wood floors. An HRV does none of this; it simply trades heat.

Neither device dehumidifies on its own. An ERV moderates how much outdoor moisture enters, but in muggy climates you still need cooling or dedicated dehumidification to actually remove water from the air. Treating the ERV as part of a larger conditioning strategy is the same logic behind pairing ventilation with efficient cooling plant, a topic we cover in our guide to eco friendly ventilation and AC systems.

HRV vs ERV: Side-by-Side Comparison

The two systems share a footprint, a duct layout and a maintenance routine, so the decision usually comes down to climate and interior moisture loads.

Comparison of HRV vs ERV

The following table summarizes the key differences:

Performance targets for both types are well documented. The strictest widely used benchmark comes from the Passive House Institute, whose component certification criteria for ventilation units with heat recovery set hard limits on both thermal and electrical performance.

Which One Should You Choose for Your Climate?

Choose an HRV where winters are long and indoor moisture is the enemy. In cold climates, occupant activity generates plenty of humidity, and the drying effect of an HRV protects windows and wall assemblies from condensation. Small, tightly sealed homes with many occupants fall into this category almost automatically.

Choose an ERV where summers are humid, because rejecting outdoor moisture before it reaches the cooling system saves real energy. ERVs also suit cold but very dry regions, where winter indoor humidity would otherwise drop to uncomfortable levels. Mixed climates in most of the United States now default to ERVs for this reason, and guidance from the Building America Solution Center, run by Pacific Northwest National Laboratory, treats the two as interchangeable options within one balanced ventilation checklist.

Sizing is the second decision. Building America's checklist targets continuous rates of 50 cfm for homes up to 1,500 square feet, 70 cfm between 1,501 and 2,500 square feet, and 100 cfm above that. Formal compliance in North America runs through ASHRAE Standard 62.2, the residential ventilation and indoor air quality standard, which calculates a required rate from floor area and bedroom count. An HRV or ERV combined with kitchen and bath exhaust meets the standard comfortably.

Design and Installation Details That Decide Performance

A certified core does not guarantee a working system. The rated 75 to 90 percent recovery figures assume balanced airflows, sealed ducts and clean filters, and each of those depends on design decisions made long before commissioning.

Plan dedicated ductwork where the budget allows. Fully ducted systems deliver fresh air directly to bedrooms and pull exhaust from wet rooms, giving predictable distribution. Coupling the unit to a forced-air furnace loop is cheaper but forces the air handler to run whenever ventilation is needed, which erodes the electrical savings. Seal every joint with mastic or UL 181 tape, and insulate any duct running outside the thermal envelope to at least R-8, both requirements in the Building America checklist cited above.

Frost control deserves attention in cold regions. When outdoor air drops well below freezing, condensate inside an HRV core can turn to ice, so units defend themselves with preheaters, recirculation dampers or timed defrost cycles. Ask for the manufacturer's defrost strategy and its effect on net ventilation rate before specifying, because some budget units simply stop supplying fresh air during defrost.

Finally, remember that recovery ventilation only pays off inside a tight envelope. Leaky construction lets air bypass the heat exchanger entirely, which is why HRVs appear alongside airtight assemblies such as SIP panels in most high performance specifications, and why they anchor the strategies described in our article on air conditioning and sustainable ventilation in architecture.

Technical specifications and ventilation rates should be verified by a licensed mechanical engineer for your specific project and jurisdiction.

Putting It All Together

Bottom Line: An HRV recovers heat, an ERV recovers heat and moisture, and both exist to ventilate airtight buildings without wasting conditioned air. Pick the HRV for cold climates with high indoor moisture, the ERV for humid or very dry ones, then protect the investment with short duct runs, sealed joints, sensible hood placement and a commissioning test that proves the airflows are actually balanced.