How often should you change a furnace humidifier filter?

Short answer

Replace the furnace humidifier pad once a year, at the start of heating season. With hard water, change it twice a season, because the minerals that make water hard are what clog the pad. The pad, also called a water panel or evaporator pad, is the honeycomb insert in the humidifier mounted on your furnace's ductwork. Swapping it takes 10 to 20 minutes and a new pad costs $10 to $30.

Why it matters

A furnace humidifier adds moisture to dry winter air by trickling water over the pad while furnace air blows through it. The water evaporates into the air and the minerals it carried stay behind on the pad. By the end of a season the pad is coated in a white crust, and water runs over the crust instead of soaking in. Less water evaporates, so the house gets dry no matter where the humidistat is set, and the extra water just runs down the drain.

A pad left in over summer does something worse. It sits damp in the dark, and the minerals and dust on it feed mold. When the furnace starts up in fall, the first air through the humidifier carries that smell, and whatever grew on the pad, into every room. A fresh pad at the start of the season means the humidifier adds moisture and nothing else.

When to do it more often

Hard water is the only thing that changes the schedule. If your faucets grow a white crust, your glasses come out of the dishwasher spotted, or you already own a water softener because of it, the pad will clog by midwinter. Change it twice a season, once in fall and again around the turn of the year.

A home that runs the humidifier hard, with the humidistat set high through a long cold winter, also goes through a pad faster. If the pad looks crusted when you check it in January, that's your answer.

What you need

A screwdriver, for the cover or the pad frame on some models
A replacement water panel made for your humidifier's model number

Steps at a glance

Turn the humidistat to off and shut the small water valve on the line feeding the humidifier.
Unclip or unscrew the cover on the front of the humidifier.
Slide out the frame that holds the pad and pull the old pad out of it.
Wipe the mineral crust out of the frame and the tray beneath it with a damp cloth.
Fit the new pad into the frame the same way round as the old one, with any colored edge where the old one's was.
Slide the frame back in, replace the cover, open the water valve, and set the humidistat back.

You're done when the cover is latched, the water valve is open, and no water drips from the humidifier when the furnace runs.

Signs it's overdue

A crust of minerals on the pad, white or tan, that flakes when you touch it.
Dry indoor air and static shocks even with the humidistat turned up.
White dust on dark furniture, which is mineral carried out of a pad that's shedding.
A musty smell from the vents when the furnace starts up in fall.

DIY or pro

This is an easy job. The cover comes off by hand on most models, and the pad frame slides out with no tools. The one thing to get right is the model number, so the new pad fits. It's printed on a label on the humidifier housing, and the pad frame is often marked too.

A technician will swap the pad as an add-on to a furnace tune-up for $75 to $150. If you're having the tune-up anyway, hand them the new pad and ask. If you're not, doing it yourself takes less time than finding the phone number.

Safety

Electricity: the humidifier is wired to the furnace, and its solenoid valve and fan, if it has one, run on house power. Switch the humidistat off before you open the cover, and don't touch the wiring or the valve. If the unit has its own power switch or transformer plug, shut that off too.

FAQ

Can I clean a furnace humidifier pad instead of replacing it?

Soaking it in vinegar loosens some of the crust, but the pad is a cheap part that doesn't come fully clean. Replacing it is faster and the humidifier works better.

What happens if you don't change the humidifier pad?

The humidifier stops adding moisture, water runs to the drain, and the old pad grows mold over summer that the furnace spreads in fall.

Should I turn the furnace humidifier off in summer?

Yes. Shut the water valve and turn the humidistat off at the end of heating season, and take the old pad out so it doesn't sit damp all summer.

Which way does the humidifier pad go in?

Match the old one. Most pads have a colored edge or a marked side that faces a particular direction so water spreads across the whole pad.