How to replace a furnace humidifier pad
If your house feels dry, static zaps you at every doorknob, or your nose feels raw, the whole-house humidifier on your furnace may need a new pad. The pad, often called a water panel, crusts over with minerals until water and air can't pass through it. A new one usually costs about $15 to $40 and slides in by hand. First, check that the humidistat is turned up and the bypass damper is open for winter. Turn off the furnace and close the water valve before you open the cover.
How a furnace humidifier adds moisture
A whole-house humidifier mounts on the furnace ductwork. When the furnace runs and the humidistat asks for moisture, a small electric valve, called a solenoid, opens. Water comes from a thin tube tied into a nearby pipe, often with a saddle valve. It drips onto a tray at the top of the water panel and soaks down through the pad. Air passes through the wet pad and picks up moisture. Water the pad doesn't use runs out a drain tube to a floor drain or condensate pump.
A bypass model uses the furnace blower. A round duct carries warm air from the supply side, through the pad and back to the return side. The damper in that duct opens for winter and closes for summer. A fan-powered model has its own small fan and no bypass duct. Either way, minerals stay behind as water evaporates. Over a season they build up and turn the pad hard. Aprilaire suggests replacing the water panel once per heating season, and sooner with hard water.
Why the house is still dry
It never asks the humidifier to run.
No warm air moves through the pad.
Scale blocks water and air from getting through.
The saddle valve or shutoff was turned off.
Water can't reach the top of the pad.
Water backs up and may leak instead of draining.
| You notice | Likely cause | Start at |
|---|---|---|
| Dry air, static, nosebleeds | Humidistat or damper setting | Step 1 |
| Ready to open the unit | Power and water off first | Step 2 |
| Need to reach the pad | Cover latch or thumbscrews | Step 3 |
| Pad stiff, heavy, crusted white | Worn water panel | Step 4 |
| No water on the pad or wet floor | Clogged tray, orifice or drain | Step 5 |
| Windows fog after the swap | Humidistat set too high | Step 6 |
Let's fix it
- Check the humidistat and bypass damperStart with the free checks before you buy a pad. Find the humidistat, a dial on the wall or on the return duct near the furnace. Turn it up a notch and set the thermostat 2 or 3 degrees higher so the furnace runs. On a bypass model, find the damper in the round duct between the humidifier and the furnace. It has a small lever or knob. Many models mark it summer and winter, or open and closed. For heating season it should point to the winter or open position. If someone closed it last spring, the humidifier can't move air through the pad. Open it and wait for the next heating cycle before you change anything else.
- Shut off power and the waterTurn the humidifier off at its own switch if it has one. Then turn off the furnace power switch. It usually looks like a light switch on or near the furnace. If you can't find it, switch off the furnace breaker. Next, find the small saddle valve or shutoff on the copper or plastic water line feeding the humidifier. Turn its handle clockwise until it stops, but don't force it. Water flows only when the furnace calls for heat, so the pad may still be wet. Keep a towel and a small bucket under the unit. If the valve leaks at the stem or won't close, stop. A plumber or HVAC tech can replace it with a proper shutoff.
- Open the cover and slide out the panelMost furnace humidifiers open from the front. Some covers have a latch or clip at the top. Others use 1 or 2 thumbscrews or knobs. Release them and lift the cover off. Inside, the water panel sits in a plastic or metal frame, sometimes called the scale control insert. Grab the frame and slide it straight out. It may be heavy with crusted minerals, so support it from below. Note which side faces up and where the small water tray sits on top. Many panels have an arrow or the word top printed on them. Take a phone photo before you pull anything apart, and you'll have a guide for putting it back.
- Swap in the matching water panelLook at the old pad. A working pad is soft and open so water and air pass through. A worn one is stiff, heavy and crusted white or tan. Don't try to reuse it; scrubbing can tear the media, and scale comes back fast. Find the part number on the old panel, the cover label or the owner's manual. Aprilaire, Honeywell and GeneralAire each use their own sizes, so match the number exactly. A pad that's too small lets water and air bypass it. Unclip the frame halves or slide the old pad out. Set the new pad in the same way, with the top side up. Close the frame so the clips snap.
- Clean the tray, orifice and drainWith the pad out, look at the distribution tray on top of the frame. Water drips from it through small slots or holes. Soak the tray and frame in a bucket of white vinegar for 20 to 30 minutes to soften scale. Scrub with an old toothbrush and rinse. Find the feed tube where it enters the humidifier. Many units have a small brass orifice there, a fitting with a pinhole. Clear crust from its opening gently with a soft brush, never a drill bit. Then check the clear drain tube at the bottom. Pour a cup of water into the housing; it should flow out to the drain. Flush a clogged tube with warm water.
- Reassemble, test and set the humiditySlide the frame back in, tray on top, and put the cover on. Open the water valve counterclockwise. Turn the furnace switch and humidifier back on. Raise the thermostat so the furnace runs, and turn the humidistat up. Within a few minutes, water should wet the top of the pad and trickle to the drain. Then set the humidistat by the outdoor temperature. Aprilaire's charts suggest about 35 percent at 20 °F outside and about 25 percent at 0 °F. Keep indoor air within the EPA's 30 to 50 percent range when it's mild. If windows fog or drip, turn it down a step.
Watch it done
- Water trickles down the pad within a few minutes of a heat call
- Water runs out the drain tube, not onto the floor
- A hygrometer shows humidity rising over a day or two
- Less static and fewer dry-air complaints
Furnace runs but the pad stays dry? The solenoid valve may have failed. Call an HVAC tech.
If it comes back
If the house dries out again partway through winter, the new pad may already be scaling up. Hard water does this faster. Pull the pad and look at it in the middle of the season. If it's crusted, replace it again. If your whole house has hard water, see the guide for a water softener that isn't working.
If the humidifier runs but windows fog or drip on cold mornings, the setting is too high for the outdoor temperature. Turn it down and see the guide for condensation on windows. A clogged furnace filter can also cut airflow, so see the guide for changing your HVAC air filter.
If the furnace itself won't start, the humidifier won't run either. See the guide for a furnace that won't turn on. For a portable unit in one room, see the guide for a humidifier that isn't misting.
What a pro typically charges (a rough estimate; prices vary by area): A water panel usually costs about $15–$40. An HVAC service call to fix a solenoid or valve often runs about $100–$300, plus parts.
Shop for a humidifier water panel