Green Building Library
Ventilation

Heat Recovery Ventilators

(aka Air to Air Heat Exchangers)

New homes, additions and even remodeling projects are built far more airtight than they used to be. Building a tight home to today's standards can cut the overall heat loss by 25 to 50 percent. This is progress; a tight house is more comfortable, because it is less drafty, and less expensive to heat, because the heat you pay for stays in the house longer. However, a tightly constructed home needs mechanical ventilation to keep the air inside fresh and prevent the build-up of indoor air pollutants such as excess moisture, carbon dioxide, formaldehyde and various volatile organic compounds found in building materials, paints, furnishings, cleaning products and smoke. The heat recovery ventilator (HRV) is designed to provide continuous or timed ventilation throughout a home, and recover the heat carried in the exhausted stale air.

The ventilation rate of a traditionally leaky, drafty home is determined by the weather; the outdoor temperature dives stack effect in the home, and the wind just plain blows right through! The colder and windier it gets, the better ventilated your house is. When you build a tight home with a high-quality ventilation system, you control the ventilation rate. Complaints you may have heard about stuffiness and moisture problems in tight houses come from houses where the builder did not install a ventilation system commensurate with the quality of the house, or the system is not being controlled properly. HRVs can be retrofit in most existing homes, but a blower door test (check with your electric utility or heating contractor) should be performed first. HRVs are generally only used in homes which require mechanical ventilation, usually homes with a natural ventilation rate of less than .35 ACH.

The HRV provides several unique benefits as a mechanical ventilation system. First, it is a balanced-type ventilation system, meaning it removes and replaces equal volumes of air from the home. No pressure imbalances occur in the house because of the HRV operating, which improves energy efficiency, comfort and safety. The HRV alone will not cause combustion appliance backdrafting, increased vapor transmission into the structure and insulation of the house, or cold drafts. Second, the HRV recovers 60 to 75% of the heat in the exhaust air, and returns it to the home. Some HRVs (properly called Energy Recovery Ventilators, or ERVs) will recover moisture from the exhaust stream as well, helping to maintain indoor humidity in cold climates. Third, the HRV is usually located in a closet or utility room, making for a quiet ventilation system. You won't even notice a properly installed HRV operating in most of the house. Fourth, the HRV replaces several bath and utility room fans with a single high-quality, long-lived system, which may run continuously or intermittently. Fifth, an HRV allows a tight, well-insulated home which will only have a 2-4 degree F inside temperature difference between the floor and ceiling, for exceptional comfort.

HRVs typically use about 100 to 200 Watts per hour of electrical energy. Compare this usage to the 2000 to 4000 Watts per hour of heating energy typically saved by building a tight home instead of a leaky, drafty home. Controls include timers for scheduled ventilation, demand switches for high-speed ventilation of bathrooms, utility rooms and kitchens (although a range hood is still needed), humidistats to control moisture levels in the home and various gas sensors. HRVs require their own duct system, except for some installations where the forced air system and HRV share some ducts. HRV ducts are usually 6" to 8", and require sealing and insulation (like any good duct system) when outside the thermal envelope.

Remember: When you install an HRV, YOU control the ventilation in your home, not the weather!

Copyright Alan Van Zuuk

 Three Types of Ventilation Systems

Balanced Ventilation Systems

Balanced Ventilation Systems

  • Two fans allow precise home pressure adjustment
  • Allows heat recovery from ventilation air stream
  • Choose ventilated areas, rates and temperature

Exhaust Only Ventilation Systems

Exhaust Only Ventilation Systems

  • Ventilation air is cold: causes drafts, call for heat
  • Depressurization of home may cause life-threatening combustion appliance backdrafting
  • May draw in outdoor air pollution such as: exhaust, radon, crawlspace miasma

Supply Only Ventilation Systems

Supply Only Ventilation Systems

  • Ventilation air reduces temperature of supply air delivered by heating system
  • Pressurization of home causes structural moisture problems
  • Forced air blower must be used for ventilation

(click drawing for larger version)