What is an HRV, how does it work, and why should I care?

WHAT IS AN HRV?

An HRV, or heat recovery ventilator, is a balanced mechanical ventilation unit that captures up to 90% of thermal energy (heat or cool) from the stale air exhausted from a building and recycles it back into fresh intake air. In non-techno speak, an HRV is a a fresh air system that preserves indoor warmth (or cool) inside a building while providing a 24/7 supply of fresh, filtered air to building occupants. ERVs, or energy recovery ventilators, are closely related to HRVs, but in addition to transferring sensible heat they can also transfer water vapor and, in our Pacific NW climate, can reduce over-drying of indoor air during winter.

Photo courtesy of Skylar Swinford

WHY SHOULD I CARE?

The old way of “ventilating” our buildings through random air leaks is not only inefficient and drafty, but also unhealthy. Poor indoor air quality is a chronic problem in conventionally-built structures. Buildings with no ventilation system depend on weather to exchange air through leaks and flaws in the building enclosure. If there’s no wind or inadequate difference between indoor and outdoor temperatures then there’s no force to drive air changes in the building. The result is stale, often unhealthy, air.

Even on those days that the weather does cooperate, the source quality of incoming air is poor when we rely on the old way of ventilating. A recent Washington State University study showed that forty percent of all indoor air in existing homes originates in crawlspaces and unconditioned basements. Trying to dilute indoor air pollutants with dirty crawlspace air doesn’t work.

Recent building code changes attempt to replace our buildings’ dependence on weather-based pressure and temperature differences with a dependence on mechanical pressure differences: a fan (in the bathroom, laundry room, or elsewhere) is required to run intermittently to either pressurize or, as is more common in the NW, depressurize the building. But this unbalanced ventilation approach still results in poor source quality, with indoor air being sucked through crawlspaces, basements, and leaks in walls. And even when “ports” are placed in windows or walls to create a hole for air movement, there’s no guarantee that the pattern of pressurization will actually draw air through them.

For these reasons, virtually every building built would benefit from a balanced, mechanical, fresh air HRV or ERV system – and the more airtight the building, the better that system will perform. We know where incoming air is coming from: a clean, filtered intake leading directly through the HRV or ERV, delivering healthy fresh air, comfortable interior temperatures, and a smaller carbon footprint.

HOW DO HRVs WORK?

Fresh intake air is (1) drawn in from outside, (2) passes through the HRV’s (or ERV’s) heat exchanger where up to 90% of thermal energy from exhaust air is transferred into the incoming air, and (3) is delivered to bedrooms and living areas. Exhaust air is (1) drawn from kitchen and bathrooms where odors and pollutants collect, (2) passes through the heat exchanger where it shares its thermal energy with intake air, and (3) is exhausted to the outside.

Diagram courtesy of Skylar Swinford

The two air streams – fresh intake air and stale exhaust air – never mix. In the highly efficient units we use, the two streams pass through a honeycomb-like structure of thin-walled passages inside the heat exchanger that provide a very large surface area for the transfer of energy between adjacent intake and exhaust air streams.

Image courtesy of Paul Wärmerückgewinnung GmbH

BUT WHAT IF THE POWER GOES OUT?

To be clear, it’s not a big deal if the HRV cuts out during a power outage, as even the most “airtight” buildings are far from hermetically sealed. No one’s going to suffocate! Granted, it might get a little stuffy, though. The solution? Crack a window or two until power is restored. The good news is that the passive qualities of high performance buildings extend the “open window season” anyway, so you’ll still be more comfortable during that power outage than your neighbors in their old drafty buildings.

P.S.
Check out these videos about the HRV at Karuna House:

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