Demystifying Furnace Humidifiers

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One of the systems often misunderstood by home buyers during their home inspection is the furnace humidifier. Unless educated during the inspection, many new buyers use improper settings on the humidistat. This can translate into dry, static air if set too low during our cold Alberta winters. Alternatively, if set too high in the winter, you will notice condensation on the windows with ice forming along the bottom of the windows during the real cold snaps. Winter settings differ from summer settings as well. Set too high in the summer, the home will become “swampy” and everything will feel damp. This can lead to a multitude of problems including mold issues.


I have seen filters totally scaled, plugged water-supply and drain lines, rusted humidifiers, and completely removed systems. This is mainly due to homeowners not understanding how to set and maintain their systems. In this article we will take some of the guesswork out of it.

The two most common types of humidifiers I see in the Edmonton area are:

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A typical Flow-Through Humidifier.

⇐ FLOW THROUGH HUMIDIFIERS run air from your furnace through a filter with a constant trickle of water. With this system there is little risk of mold forming in the humidifier. For maintenance, turn off the water supply and turn the humidistat to OFF in the spring. In the fall, the pad can be removed and descaled, or replaced with a new one. The water supply pipe valve can be turned back on, and the humidistat set to 35%. This should be done every fall.

⇒ DRUM HUMIDIFERS have a round pad that turns through a pan filled with water. The moistened pad then evaporates as the furnace air flows through it. These types of humidifiers must be cleaned regularly as there is a high chance of mold growth from the standing water. Every spring, the water supply pipe valve should be turned off and the tray and sponge should be cleaned. Turn the humidistat setting to OFF. In the fall, turn the water back on and set the humidistat to 35%. A mid-winter cleaning is recommended as well.

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Drum Style Humidifier


Every home is different, and every household is used differently. A home with a family of six will, on average, create more humidity than a family of two. Older homes weren’t built as tight as newer homes. Different air handling systems make a huge difference also. This is a topic that I am only scratching the surface of, but without getting into the building and weather science of it all, the bottom line is that each household will have to adjust and fine-tune to what works best for them.

Start with a setting of 35% humidity. If the house becomes too dry raise it up a little. If the windows form condensation, lower it a little and wait a few days to see what happens. This will fluctuate with extreme temperature changes, so you have to pay attention to those wild swings a little bit. Over time you will become familiar with what works best for your household.

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