Renovation & Design

HRV settings key to homeowner's woes

This diagram illustrates the inner workings of an HRHRV unit, a mechanical system that is used to help regulate relative humidity.

QUESTION: I have a new home and have found that my furnace was running almost continuously last winter trying to keep our house temperature at 20 degrees. I also found that the temperature variations within our home fluctuate three to four degrees different than what the thermostat indicates.

My home is a cab-over design and 1,800 square feet. All the literature that I have read indicates that the HRV should run continuously, yet the HVAC company has said that during cold periods you should shut the HRV off. What is the accepted procedure?

Thanks. Allan Rau

ANSWER: There seems to be a lot of confusion, not only among homeowners but also some contractors, about installation and use of Heat Recovery Ventilators (HRVs). I have seen several variations of installation in homes, most relating to the controls for these units. I will provide my insight into the proper use of these devices, which should address your issue.

An HRV is a mechanical system that is used to help regulate relative humidity (RH) in a home. These fairly simple units are often located near the furnace and have ducts which connect to the return air for the heating system, registers in several areas of the house, and insulated ducts that attach to vent hoods on the exterior walls.

The main function of an HRV is to remove excessive moisture from the air in a home and replace it with drier, fresh air from outside. This is done by passing the incoming and outgoing air through a "box," which recovers some of the heat from the outgoing air, making the exchange more energy-efficient. These units often have extra filters to prevent clogging of the components and to prevent excessive debris from being circulated through the house.

One critical component of this system is a control, typically located in the central portion of the living space near the furnace thermostat, which has an integral humidistat. The idea is to set the humidistat to a desired RH, allowing the unit to activate when that level is exceeded in the house air.

Normally, in a new home, the HRV is also connected to on/off switches in the bathrooms. There should also be a switch on the main control which allows the unit to be run continuously, bypassing the humidistat setting. These switches allow quick dehumidification of isolated areas which are not near the main control, preventing excessive moisture from showers, baths, dishwashing and cooking from creating problems in the home.

One of the main problems that I've seen with HRVs is that the main controls aren't functioning properly. The unit operates only with the bathroom switches and on/off switch on the main control, but not when the humidistat dial is rotated. This may due to an installation defect, or simply because the builder has asked that the HRV run continuously near the end of construction to help dry new building materials.

I'm not sure where you found instructions that an HRV should run continuously, but it was definitely not written by someone from this climate. And the advice you received to turn the unit off when it is really cold outside leads me to believe that even some professionals are unsure about the proper functioning of these devices, or they're taking the easy way out.

To understand this better, we only have to look at the simple principle behind this device: We want to use the HRV primarily to replace humid indoor air with drier outside air to reduce the RH in our homes, preventing condensation and mould growth.

In the winter, this works well, as the outside air is much colder and can hold much less moisture that the warm inside air. When the HRV comes on, it will bring in much drier air that we have inside the home, reducing the RH inside the home.

In the summer, especially when we are running the air conditioning, the air outside the house can be much warmer and hold much more moisture than the air indoors. Running the HRV will bring this elevated humidity into the home while exhausting the drier air-conditioned air. This is contrary to the purpose of the HRV and will make the air conditioner work much harder and increase electricity consumption.

Therefore, the HRV should be shut off in warm weather, not cold, contrary to what your HVAC source has told you.

Finally, it may not require very much fresh-air intake at -20C to reduce the indoor air to a reasonable RH level. Running the HRV all the time may bring in far too much cold air, causing your furnace to work harder than necessary. This is likely what is occurring in your home and why the HVAC company told you to shut it off. That's bad advice, but it's true that it will stop your furnace from running constantly.

The proper procedure is to set the humidistat control somewhere between 25 and 40 per cent in the heating season, which will engage the HRV only when the RH in the home exceeds that level. As the weather gets colder outside, you should lower the setting on the control to prevent condensation on colder windows and other surfaces that may lead to mould growth.

It's possible that there is something wrong with the operation of your furnace or thermostat, or it's undersized for your home, but the likely cause of your issue is the HRV control. If the HRV is running constantly, year-round, it's bringing in too much warm, moist air in the summer and too much cold air in the dead of winter, causing the furnace to run constantly. Proper use of the humidistat settings during the heating season and avoiding use of the main control switch in the summer should eliminate your problems.

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


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