HRVs, Humidifiers and Indoor Humidity in Winter

For winter indoor humidity in Ontario, aim for roughly 30 to 40 percent relative humidity, and use your HRV and humidifier as opposing controls to stay there. The HRV lowers humidity by swapping damp indoor air for dry outdoor air. The humidifier adds moisture. Newer, tighter homes usually need the HRV running and little or no humidifier. Older, drafty homes tend to be dry and may need the humidifier on a low setting.

What indoor humidity should you aim for in winter?

Health Canada suggests keeping indoor humidity between about 30 and 50 percent, and toward the low end in cold weather. The reason is simple. Cold outdoor temperatures make windows, exterior wall corners and attic surfaces cold. The colder those surfaces get, the less moisture the indoor air can carry before it condenses on them. A level that is fine at zero degrees can leave water on the glass at minus fifteen.

Buy a digital hygrometer for about $15 to $20 and keep it on a shelf in the main living area, away from the kitchen, bathrooms and supply registers. A second one in the basement or a bedroom gives a fuller picture. Guessing by feel is unreliable.

What an HRV does in an Ontario winter

A heat recovery ventilator has two fans. One pulls stale air out of the house, usually from bathrooms and the kitchen area. The other brings in the same amount of fresh air from outside. The two airstreams pass through a core where the outgoing air warms the incoming air without the two mixing, so you get fresh air without losing most of the heat.

Cold winter air holds very little moisture. When the HRV brings it in and it warms up, it is dry, and it replaces air loaded with moisture from showers, cooking and breathing. That is why an HRV is the main humidity control in a tight house. Many Ontario homes built in recent decades have one. It is usually a metal box hanging near the furnace with insulated ducts running to the outside wall.

How to set and maintain an HRV for winter

• Leave it running. Most units are meant to operate continuously on low speed, with a high-speed boost when needed. Switching it off for the winter is a common cause of wet windows.
• Use the dehumidistat. This wall control turns the HRV to high speed when humidity rises above the set point. In winter, set it around 30 to 40 percent and lower it during a cold snap if the windows fog.
• Clean or replace the filters every two to three months. They sit inside the unit's front door.
• Wash the core as the manual describes, usually once or twice a year.
• Check the two hoods outside. Clear snow, leaves and lint from the screens. A blocked intake stops the system.
• Make sure the condensate drain line at the bottom of the unit is connected and runs freely to a drain.
• Have the airflows balanced by an HVAC technician if this has never been done or the ductwork has been changed.

When to use a humidifier, and when to turn it down

Signs the air is too dry include static shocks, dry skin and nasal passages, and gaps opening in hardwood floors. A humidifier mounted on the furnace can help, but it is easy to overdo. Treat its dial as something to adjust through the season. Set it lower as the outdoor temperature drops, and turn it down the day you see water on the windows.

Maintain it as well. Replace the pad or filter each heating season and check that the water feed and drain line are not leaking onto the furnace or the floor. A humidifier left on a high setting can wet the inside of the ductwork and raise humidity throughout the house.

Running an HRV on high and a humidifier at the same time means the two are working against each other. If the hygrometer reads in range, leave both alone. If it reads high, turn the humidifier off first. If it reads low, raise the dehumidistat setting on the HRV before you turn the humidifier up.

Signs your winter humidity is too high

• Water or frost on windows most mornings.
• Damp spots or dark spotting in upper corners of exterior walls and inside closets on outside walls.
• A musty smell in rooms that are kept closed.
• Frost on the underside of the roof boards when you look in the attic during cold weather.
• A hygrometer that sits above 45 or 50 percent in cold weather.

Homes without an HRV

Many older homes in London have no HRV and leak enough air that winter humidity stays low without help. If such a house has been tightened with new windows and insulation, moisture may start to build. Use the bathroom fans and the range hood more, run a bath fan on a timer for a few hours a day, and ask an HVAC contractor about adding an HRV if the windows still drip. An ERV is a related unit that also transfers some moisture between the airstreams. A contractor can advise which type suits your house.

Start with the hygrometer. Record the reading and the outdoor temperature for a week, then adjust one control at a time and watch what changes.

Common questions