Ventilators. HRV and ERV’s… What they do and how they work.
An HRV’s fans pull fresh air into a building while simultaneously exhausting stale air from the building. Both the fresh air stream and the stale air stream flow through the HRV. The core of the appliance allows some of the heat from the warmer air stream (the stale air in winter, the fresh air in summer) to be transferred to the cooler air stream. In winter, in other words, the appliance recovers some of the heat that would have otherwise been exhausted. This heat transfer occurs without any mixing of the two air streams.
An ERV does everything that an HRV does. In addition, an ERV allows some of the moisture in the more humid air stream (usually the stale air in winter and the fresh air in summer) to be transferred to the air stream which is dryer. This transfer of moisture called enthalpy transfer occurs with very little mixing of the two air streams.
Why ventilate a building?
Before we can clarify the choice between an HRV and an ERV, we have to consider the question: “Why should a building be ventilated?” As it turns out, the question has several answers, including:
- To provide enough fresh air to keep the occupants healthy;
- To remove odours;
- To dilute indoor pollutants; and
- To lower the indoor relative humidity.
Most of these goals are easy to understand. (Even so, establishing an optimal ventilation rate to achieve these goals is a contentious issue.) However, using ventilation to achieve the last of these four goals ó lowering the indoor relative humidity ó gets problematic.
To prevent moisture damage to a building, lower humidity levels are always preferable to higher humidity levels. In other words, dry is always better than damp. However, some people begin to complain if the indoor relative humidity is too dry, say, 20% or below. (Of course, people have lived healthy lives for thousands of years in climates where the relative humidity is often below 20%, so it’s not at all clear that low humidity levels are unhealthy.
Ventilation can only reduce the indoor relative humidity if the outdoor air is dryer than the indoor air. Since cold air can’t hold as much moisture as warm air, ventilating a building helps lower the indoor relative humidity only when it’s cold outside (or on dry days during the spring and fall). In most parts of the Canada, ventilation during hot weather actually introduces more moisture into the building ó that is, it tends to raise rather than lower the indoor relative humidity.
What do manufacturers recommend?
Unfortunately, you can’t depend on HRV and ERV manufacturers to tell you whether your building is better off with an HRV or an ERV. The qualified Engineer overseeing your properties is the person to make that recommendation.
Myths about ERV and HRV’s
These myths that “the choice between an HRV and an ERV depends only on climate, and that HRVs can’t be used during the summer” are only two of the many red herrings encountered by installers in search of accurate information on HRVs and ERVs.
Other commonly repeated myths include:
ERVs can’t be used in cold climates because their cores will freeze.
In a humid climate, an ERV can act as a dehumidifier or can help address high indoor humidity.
Both of the above statements are false. (Freeze-up problems were solved years ago by the development of controls with a defrost cycle.)
- The units retain energy used to heat or cool, saving money on utility bills.
- Talk to a trained Core One expert to help you get the information you need to make an informed decision.