What is an HRV and how does it work?
Heat Recovery Ventilation (HRV) systems are becoming increasingly common in cold climate construction and are almost indispensible in today’s super-insulated, airtight homes. As older homes are being retrofit to be more energy efficient, they often face the same indoor air quality issues you find in new construction. In a cold climate, HRVs are the best option to improve the indoor air quality of your home and conserve energy.
The main job of the HRV is to supply fresh outdoor air to the house while expelling stale indoor air—which contains unwanted things like moisture, animal dander and gases from combustion appliances and carpets. This is especially important in a home that is too tight to rely on passive air exchange. At the heart of the HRV is a heat exchanger (often called a core) that transfers heat from outgoing stale air to incoming fresh air without the airstreams mixing. Most HRVs recover between 70-90 percent of the heat from exhaust air, making them much more efficient than a standard exhaust fan that blows warm air directly outside.
The ducting of an HRV system typically supplies fresh air to bedrooms and living areas while exhausting humid air from bathrooms, kitchens, laundry rooms and crawlspaces. The HRV does not eliminate the need for a cooking fan, so a range hood should still be the main outlet for grease and smoke above the cook stove.
Balancing the HRV
The HRV is designed to be balanced, meaning it takes in as much air as it exhausts, maintaining close to neutral pressure inside the home. It should not create a negative pressure in the home, like an unregulated exhaust fan might, because that could cause appliances to backdraft (suck in air from an exhaust flue and bring dangerous combustion gases into the home). It’s also important to remember that HRVs are not intended to supply air to combustion appliances, just to the living environment.
Energy Use & Maintenance
Residential-sized HRVs use about as much power as a 60-watt light bulb when running, and are getting more efficient all the time. As with any appliance, an HRV requires some maintenance, such as checking the built-in filters every fall to see if they need to be cleaned or replaced.
Recirculation mode
One of the often-debated topics of HRVs is “recirculation mode.” In recirculation mode, the unit closes the connection to the outside and brings exhaust air back into the rooms. That means there is no cold, fresh air coming in from outside and no warm air being exhausted, so your house is using less energy. It can also make the house more humid as the moisture generated inside accumulates.
Recirculation mode is not a common feature in Europe because it is believed that the health risks outweigh the energy benefits. Recirculating indoor air means moisture and indoor pollutants are no longer being flushed out of the home and can eventually reach harmful levels. Recirculation can also spread unwanted smells from more to less polluted areas, such as from the bathroom to the living room.
In order to balance health and energy efficiency, many HRVs offer settings that recirculate air for part of each hour. For example, in a 20/40 mode the HRV will bring in fresh air for 20 minutes and then recirculate it for 40 minutes (likewise for 30/30 mode). Other “smart” modes usually require some kind of sensor (based on humidity or carbon dioxide) to decide when to ventilate and when to recirculate, based on which measurements the HRV controller deems more relevant at any given time.
The major advantage of recirculation mode is that it redistributes heat throughout the house, particularly helpful if you have a localized heat source like a woodstove that only heats one area. On the flip side, it can potentially transfer pollution from one room to another rather than dumping it outside. While Smart Mode seeks a happy medium between the two, there are still times when recirculation mode should not be used at all—for example, during times of high occupancy or if someone is cooking or smoking inside. One way to override Smart Mode during these situations is with a push-button timer, a common feature that temporarily ventilates the HRV.
If you do use recirculation mode, here are some tips to maintain good air quality:
- High quality filters (High Efficiency Particulate Filters, HEPA, in combination with activated carbon filters) should be added to the supply duct to mitigate odor or pollution from spreading
- Constant recirculation should only be used when the building is unoccupied
- If recirculation is used during occupied periods, make sure the HRV is exchanging indoor and outdoor air for at least part of every hour
While recirculation offers some advantages, such as distributing heat evenly throughout a home, relying on it too much can undermine the benefit of having an HRV.
How efficient is your HRV?
Because HRVs recycle the heat contained in exhaust air, your home loses much less energy than if you were simply dumping this heated air outside. How well an HRV is able to pre-heat the supply air depends on several factors – the temperature of the indoor air, the temperature of the outside air, the flow rate through the HRV, and the unit’s efficiency. More efficient HRVs can recover more heat from the exhaust air. When comparing the efficiencies of HRV models, you may see different measures of efficiency that are calculated differently. Additionally, these efficiencies vary depending on the outdoor temperatures at your site and the flow of air through the HRV.
The Sensible Recovery Efficiency (SRE) quantifies the amount of heat recovered by the incoming fresh air from the exhaust air. It is reported as a percentage of the total heat that is available for recovery. For instance, if an HRV has an SRE of 55% for a given flow rate and temperature difference, this means that the HRV core transfers 55% of the heat available in the outgoing air to the incoming airstream. The measurement and calculations for the SRE correct for a number of events that add heat to the incoming air in a realistic situation, such as heat that might enter the airstream through the HRV case, airflow imbalances between the supply and exhaust airstreams, heat gains from circulating fans, and energy that might be used to defrost the HRV core. The SRE is often used to compare the heating season performances of HRVs because it calculates the efficiency of an HRV independent of different circumstances (such as flow rates, temperatures, and heat gains) in installed locations.
The Apparent Sensible Effectiveness (ASE) is another efficiency often listed by manufacturers. Unlike the SRE, the ASE includes the heat transfer from inefficient fans to the airflow, heat leaking in or out of the HRV case, and any leaks between the airstreams when they cross through the HRV core. It is also expressed as a percentage and is equal to the temperature rise of the incoming outdoor air divided by the temperature difference between indoor and outdoor air. Typically, it will be higher than the SRE for a given HRV, air flow, and temperature difference because it includes heat gained by the incoming air from sources other than the exhaust air. Ultimately, the ASE provides the final delivered supply air temperature at a given flow rate for an HRV – and thus is used to predict the final delivered air temperature at a given flow rate.
With energy recovery ventilators, or ERVs, manufacturers may also report a third efficiency, the total recovery efficiency (TRE). This efficiency gives the net energy that is recovered by the ERV, which includes the sensible (temperature) heat and latent (moisture) heat that is recovered. It is similar to the SRE, except that it also includes the moisture recovered from the exhaust air.
When comparing different HRVs, first ensure that reported efficiencies were measured in an independent lab. Second, take care to compare equivalent efficiencies. For instance, the SRE of one HRV at a given air flow and temperature should be compared to the SRE of another HRV at the same air flow and outdoor temperature, not to the ASE or TRE. This will ensure you are comparing HRVs measured in equivalent circumstances. If you would like to explore the reported efficiencies for a wide variety of HRVs, visit the website of the Home Ventilating Institute, a non-profit institution that provides performance certification of residential ventilation products. Their website contains a directory of certified HRVs and their efficiency ratings.