Renovation & Design

HRV a counterproductive exercise in summer

System works against, not with, your air conditioning

alex schudtz / the holmes group

Turn your heat recovery ventilator off in summer, or in newer homes, set it so it only comes on when bathroom ventilation is requested.

Question — I have a question after reading an article you wrote about HRV settings. I have lived in my home a few years and was unaware that I had to adjust my Fantech humidistat between winter and summer. I had always kept it in the "comfort zone", but last year after all that rain I noticed it felt a bit damp in my home, so I went looking for answers. My home has central air conditioning and a Fantech HRV system. What setting should I have it at for summer and what for winter? When I put the Fantech humidistat dial past 80 it seems to click and turn off and only come on when I activate the bathroom fans. When I have it lower than 80 the fan seems to run continuously. I am afraid I may have caused damage to my home by not knowing this for all these years. Any guidance would be greatly appreciated. Thanks, Nicole.

Answer — Using your heat recovery ventilator (HRV) in the summer, other than with the bathroom controls, is counterproductive and a waste of energy. The dehumidistat should simply be set to off, or to the highest possible number, once you start opening your windows in the spring. That way, your HRV will not be working against your air conditioner (AC) or bringing in additional moisture to your living space in the humid summer conditions.

While summer appears to be rapidly coming to a more abrupt end than normal, I chose your question about HRV settings because this may be the most misunderstood system installed in all new homes. In a reverse of my typical style, I will answer your question about specific summer and winter settings for your HRV first, and explain that answer in the following text. To be as succinct as possible, you should shut off your main HRV controls for the summer and turn it on and set it to 30 per cent, plus or minus five, for the heating season. If your control does not have percentage numbers on the dehumidistat portion of the main control, set it to the more humid setting in the "comfort zone" when you first turn on your furnace in the fall and to the least humid end of that zone in the dead of winter.

To explain the rationale behind my recommendations, we first have to understand the functioning of your HRV in relation to moisture control within your home. The V in HRV stands for ventilation and the box in your basement hanging from your rafters has two ducts that connect to vent hoods on the exterior of your home. One of these is the fresh air intake duct and the other is the exhaust duct. Just like the names suggest, when the unit is running, fresh air enters the HRV from outside, while indoor air is drawn in before being expelled to the exterior. The heat recovery portion of the name refers to the core of the unit, which absorbs some of the heat from the outgoing air and transfers it to the colder incoming air. This prevents the waste of some of the heat energy, common with traditional exhaust fans or central exhaust systems.

While the ventilation aspect of this system is very important to maintain good indoor air quality, especially in newly built air-tight homes, the moisture management aspect is equally critical in our climate. When our homes have the windows closed and the furnaces blasting during our long, cold winter, we can build up considerable amounts of pollutants and moisture in the living space from our daily activities. The simple act of breathing by the home occupants releases significant amounts of water vapour, as well as carbon dioxide, into the air. Cooking, smoking, and burning wood or fossil fuels can also release a large amount of pollutants into the indoor environment, while using up oxygen from the air. If our homes become too well sealed to allow this polluted air to escape and be replaced with fresh air on a regular basis, the occupants can become very ill. So, having the HRV provide this air exchange is fundamental to the overall health of the occupants of the home.

At the same time the HRV unit is providing clean air for the residents inside the living space, it should also be reducing the amount of moisture that can be trapped in the air, preventing damage to the building materials. The principle of this is straightforward, but is critical to understanding proper use of this device. In the winter, the outdoor air can only hold a very limited amount of dissolved moisture before it condenses. As the air warms up, it can hold substantially more water vapour before condensation occurs. This is why the moisture content of the air is expressed as relative humidity (RH), it is dependent on the temperature of the air. In other words, the air outside your home will be much, much dryer than the indoor air any time the ambient temperature is below freezing.

In winter, drawing in outdoor air with the HRV to replace the warmer, more humid indoor air will lower the moisture content of the indoor air, and the RH. In the summer, the outdoor air is often warmer and more humid than inside, especially when the cooling system is on, so the reverse of the dehumidification process will occur. The wet air being sucked inside the home will raise the RH, causing the air conditioner to work harder to cool and dry the indoor environment. That will be a waste of electricity for both the AC unit and the HRV and furnace blowers. Which is why the HRV should not be used in the cooling season. Because we are more likely to go in and out of our homes much more in the warmer months, more natural ventilation will occur just by opening and closing the exterior doors. Additionally, using the manual HRV controls in the bathrooms to periodically draw out saturated air from showers and baths should provide enough ventilation for decent indoor air quality.

Turning your main HRV control off during the summer is the right choice, which will prevent unnecessarily increasing the RH inside your home. This will prevent a waste of energy and overuse of the cooling system, as well.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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