Renovation & Design

ASK THE INSPECTOR: Don't use HRV unit during summer months

QUESTION: My question is concerning the family cottage in Whiteshell Provincial Park. My parents built a 1,900-sq. ft., two-storey cottage about five years ago. They have an electric forced-air furnace with a heat recovery ventilation (HRV) system and central air conditioner. They go to the lake a limited amount during the winter months and go every weekend during the spring, fall and summer months. During the winter they keep the cottage at 10 C and do not run the humidistat.

During the summer months the humidistat is set so that the fan is continuously running while family is there. The humidistat was set to run constantly when the system is on and the two bathroom fans were disabled. The air conditioner and HRV is turned off when no one is using the cottage during the week.

When the weather is hot we run the air conditioner periodically. Often, the air conditioner is set around 21-23 C. When the weather is moderate we do not run the air conditioner and often open windows.

With our central exhaust system at home, when we engage the bathroom fan all the heat vents and returns work to bring fresh air to the rooms and take out stale air. The cottage bathroom fans exhaust stale air but we can't feel fresh air coming through the vents into the cottage. Is this normal for the electric forced air furnace system?

Recently we have noticed that the relative humidity in the cottage is high. Often, there is condensation on the inside windows when we wake up. We have noticed a small amount of mould around the windows. When sleeping at night the rooms get very stagnant and stuffy. The furnace/HRV system is located in the four-foot crawl space under the cottage, which sits directly on the Canadian Shield. We have noted that there is lots of moisture in the crawlspace.

Our question is how should we be running the HRV system at the lake? Should the HRV fan be running all the time? Does the system sound like it is working the way it should be?

Thanks for your help. Leanne and Mark Newman

ANSWER: To answer your excellently detailed question, and as a good way to remember proper procedure for using your HRV in the future, we will have to explore moisture in the air inside and outside the cottage. This will provide you with a good guideline to help prevent repeated moisture issues in the living space.

The main thing to remember when determining whether to run the HRV is that this system is used to exhaust stale air from the home, but it replaces it with fresh air from outside the home. The important part of this axiom to remember is the part about bringing fresh air into the building. For this device to work effectively it should only be used with the humidistatic control to reduce high humidity and moisture in the living space. So, it follows that we only want to run it when the air outside the home contains less moisture than the air inside. Otherwise, you will defeat the purpose of the HRV if you're bringing in wetter air than you are exhausting.

It's obvious to anyone who has lived in a northern climate that the moisture content of ambient air is much drier in the winter than the summer. This is because warm air can absorb much more water vapour without precipitation than cold air.

In sub-zero temperatures, during the heating season, the air outside your cottage will only be able to hold a very limited amount of moisture, even at maximum relative humidity. Once this cold air is drawn inside through the fresh-air intake ducting of the HRV and warmed up, it is able to absorb considerably more moisture and the relative humidity (RH) of the fresh air will drop significantly. This is important since this dry air can replace the relatively wet air that is being exhausted by the HRV and absorb excess moisture within the building, preventing condensation and moisture problems.

However, if the HRV is used in the warm summer months, it will draw in warm air that already may contain a considerable amount of moisture, particularly in lake country, and replace air that me be cooler inside. This may have exactly the opposite effect than desired and raise the RH of the indoor air rather than reduce it.

In a nutshell, you should shut off the HRV humidistatic control as soon as the furnace is not being used on the heat setting. You've noted that you open the windows in the shoulder seasons, which will passively add significant fresh air and naturally dry the warm air inside the cottage. So you shouldn't need to run the HRV when nature is doing the same job for you.

When the outdoor temperature rises and the air conditioner is used, the windows should be shut to prevent warm, moist air intrusion. It doesn't make any sense to close the windows to prevent warm air intrusion and then run the HRV continuously, which also brings in outside air. The unit may still come on periodically if it's connected to bathroom registers, but that will only be to remove steam when showering or noxious odours.

The damp-feeling air and moisture on the windows in the summer months is proof that running the HRV unit at this time of year is detrimental. Air conditioning will significantly remove large amounts of moisture through its condensate drain system and help dry the inside air, so it should be used more often. Shutting off the HRV and running the A/C on hot summer days will combine to alleviate your moisture issues, and setting the humidistat at levels below 50 per cent RH over the heating season is the proper methodology for our climate.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and 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


Browse Homes

Browse by Building Type