Heat recovery ventilator helps control relative humidity

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This article was published 23/09/2023 (714 days ago), so information in it may no longer be current.

Question: My home has hydronic heating, with a hot water boiler. With no furnace creating any flow of warm air, and no real issues with humidity, when and for how long do you think I should be running my HRV in the cold weather? I have a separate gauge that indicates RH in our home. Should I just choose a target humidity and run it until it gets to that percentage? And, if yes, what do you think is the ideal humidity in a home? Cheers and thanks in advance for your advice, Douglas S.

Answer: The ideal goal for indoor relative humidity (RH) in the heating season may be somewhat dependent on your geographic location, but more so on the outdoor temperature. A normal range of 30 to 35 per cent RH should be good in many areas of Canada when the ambient temperature is near or below freezing.

Because your home does not have a forced air heating system, or furnace, it will have some additional challenges for using your HRV. Keeping the RH a little lower than usual may be a good idea to prevent condensation and moisture issues in areas with stagnant air movement. Typically, the ventilation system is connected to the furnace ducting to help improve the airflow through the small unit. Without the furnace blower and additional ductwork available for this function, the HRV will likely have to work longer to achieve its goals of ventilation and moisture reduction in your home.

Most HRV main control systems have an integral dehumidistat function that allows you to set your desired RH, and easily adjust it at will. This may be an actual numerical equivalent or a scale from a low humidity end to a high point. This is the best way to use the device, other than the timed switches that should be installed in each of your bathrooms. Those should also be used every time you bathe or shower, to remove the excessive moisture produced from those normal hygiene activities.

Now, to address your specific question, an ideal RH level should be dependent on the environmental conditions and ambient temperature near your home. If you live in an area with high year-round humidity issues, like near the ocean or lake country, you may find it more difficult to maintain a proper RH level. That is because the ventilation system draws outdoor air into itself to replace the stale air exhausted.

Once the weather cools enough that you have to turn on your boiler for heating, the HRV should be used. When this is in the shoulder seasons, when the outside temperatures are still above freezing, you should set your dehumidstat between 35 and 40 per cent.

Following the RH chart on the dehumidistat setting on your HRV control, or using the guide and a stand-alone hygrometer, you should be able to maintain a good level to prevent moisture-related issues even in your hot-water heated home. The actual goal will depend on your location and outside temperatures, but shooting for a range of 30 to 35 per cent RH when it is below freezing outside should be ideal.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com