How to Maintain a Furnace Humidifier

A whole-house humidifier mounted on the furnace adds moisture to the dry winter air, which helps with static shocks, dry skin, irritated sinuses and cracking hardwood. To work well and stay clean, it needs about 20 minutes of maintenance at the start of every heating season. Neglected humidifiers get clogged with mineral scale, leak, or grow mould in standing water.

Most Ontario homes have a bypass humidifier: a box on the supply or return duct with a round duct connecting it to the other side of the furnace, and a damper to open in winter. A fan-powered humidifier looks similar but has its own small fan and no bypass duct. Both usually use a flow-through evaporator pad (also called a water panel) that water trickles over. Older drum-style humidifiers have a rotating foam belt sitting in a water tray; they need more frequent cleaning because the water stands. Steam humidifiers boil water in a canister and have their own maintenance schedule in the manual.

A replacement evaporator pad for your model (CAD $15–$35; the model number is on the humidifier's label), white vinegar, a small brush, a bucket and towels, and a screwdriver or nut driver to remove the cover.

1. Turn off power to the humidifier and the furnace at the furnace service switch.
2. Close the small water supply valve on the line feeding the humidifier.
3. Remove the humidifier cover and slide out the pad frame.
4. Remove the old pad, noting which way it faces, and fit the new one the same way.
5. Soak the plastic water distribution tray and frame in a 50/50 mix of white vinegar and water to dissolve mineral scale, then rinse.
6. Check that the drain hose is clear and runs freely to the floor drain or condensate pump.
7. Reassemble, open the water valve, and turn the power back on.
8. On a bypass humidifier, set the damper to the winter (open) position.
9. Set the humidistat for the outdoor temperature and watch for water flowing through the pad during the next heating cycle.

Indoor humidity needs to come down as it gets colder outside, or water will condense on your windows. As a general guide, aim for about 35% when it is -10°C outside, 30% at -20°C, and 25% or lower below -25°C. Many humidistat dials are marked with outdoor temperatures for this. If you see condensation on the inside of your windows, turn it down (see our guide to window condensation).

In spring, close the water supply valve, set a bypass damper to the summer (closed) position, and turn the humidistat off. Leaving water running through the humidifier in summer wastes water and encourages mould.

Call an HVAC technician if no water flows to the pad when the furnace is heating (often a failed solenoid valve), if the humidifier leaks into the furnace or ductwork, if you see mould inside the unit or ducts, or if you want a humidifier installed or replaced.