On this page
A bypass or fan-powered humidifier on the furnace needs a new water panel every heating season, a clean water tray and drain, a humidistat set low enough to keep the windows dry, and the water shut off in summer. Do those four things and it adds comfort in dry winter air. Skip the humidistat setting and it does damage: condensation on windows, then on the inside of exterior walls, then mold in the attic.
The unit sits on the supply or return duct near the furnace. A solenoid valve lets water trickle over a water panel (the evaporator pad) when the furnace fan runs and the humidistat calls for humidity; air passing through the wet pad picks up moisture; the excess water runs to a drain. The pad collects minerals and grows film; the valve and the drain clog; and the humidistat, if it is a simple dial, gets set once in October and never touched as January arrives.
What do you need?
A replacement water panel for your model (the model number is on the unit's cover), a screwdriver if the cover is screwed on, white vinegar, a soft brush, a bucket and rags, and a digital hygrometer for the living space. The hygrometer is the one item people skip, and it is the one that tells you whether the humidistat setting is right.
- A replacement water panel for your model (the model number is on the unit's cover)
- A screwdriver, if the cover is screwed on
- White vinegar, a soft brush, a bucket, and rags
- A hygrometer for the living space (an inexpensive digital one)
What are the steps at the start of the heating season?
At the start of the heating season, shut off the furnace and the humidifier's water, remove the old panel, soak and scrub the frame and trays in vinegar, clear the drain line, fit the new panel the right way up, restore the water and confirm it trickles to the drain, and set the humidistat low enough to keep the windows clear. About thirty minutes.
- Turn off the furnace at the thermostat and, for a fan-powered unit, its power. Close the small saddle valve or shutoff on the water line to the humidifier.
- Remove the cover and lift out the water panel frame. Note which way it faces; the panel is directional on many units.
- Remove the old panel and discard it. Soak the frame, the distribution tray on top, and the drain tray in vinegar for a few minutes to dissolve scale, scrub, rinse, and dry. Done when: the tray's water holes are clear.
- Check the drain line for slime and blockage. Run water through it or blow it out. A blocked drain overflows onto the furnace.
- Install the new panel in the frame the right way up and put the frame back. Replace the cover.
- Open the water valve and set the humidistat. Turn the furnace on and let it run; confirm water is trickling over the panel and reaching the drain, with no drips outside the unit.
- Put the hygrometer in the living space and set the humidistat so the windows stay clear of condensation on cold nights. Lower the setting as outdoor temperatures drop through the season.
What are the steps mid-season and at the end of the season?
Mid-winter, check the panel for scale and watch the windows for fog, lowering the setting as the outdoor temperature drops. At the end of the season, close the water valve, set the humidistat to off, discard the panel, clean the trays, and leave the unit dry for summer so it cannot feed mold or fight the air conditioner.
- Mid-winter, look at the panel. Hard water can scale it in a couple of months; replace it if it is crusted.
- Watch the windows. Fog or frost at the bottom of the glass means the setting is too high for that night's temperature.
- At the end of the heating season, close the water valve, set the humidistat to off, remove and discard the panel, clean the trays, and leave the unit dry for summer.
Why is too much humidity the real risk?
Moist indoor air moves into cold places: the window glass, the inside of exterior walls, and the attic. On the glass it is a nuisance and rots the sash. In the walls and the attic it is invisible, and it soaks insulation and grows mold. The attic check looks for frost on the roof deck for exactly this reason. A humidifier is often the cause, and the fix is the setting, not the attic.
When should you stop and call a pro?
Call an HVAC technician if the solenoid valve does not open or does not close (water running with the furnace off is a stuck valve and a flood risk), if the unit leaks, if a fan-powered unit's motor is loud or dead, or if you find frost in the attic or staining on exterior walls (that is a moisture investigation, not a humidifier fix). Ask the technician to check the humidifier at the fall service visit; it takes them a few minutes.
What should you record?
The panel replacement dates and the part number, the setting you found works for your house at different outdoor temperatures, and the shutoff and restart dates. Set the fall panel change and the spring shutdown as recurring tasks.
Common questions
What humidity should I set in winter?
Low enough that the windows do not fog or frost. As it gets colder outside, the setting has to come down, because the inside of a window gets colder and condenses at lower humidity. Many humidistats print a guide on the dial; a hygrometer in the living space tells you the truth. Condensation on the windows is the signal to turn it down.
How often should the water panel be replaced?
At least once a heating season, and more often with hard water, which scales the panel quickly. A panel that is white with mineral crust, cracked, or slimy is done. Keep a spare so the swap happens when you notice, not when you get to the store.
Should I turn it off in summer?
Yes. Close the water supply valve and set the humidistat to off (or the summer setting) so the unit does not run when the air conditioner is trying to remove moisture. Leaving it on in summer wastes water and can feed mold in the unit.
What should you do next?
Buy the panel now, before the first cold snap, and put the hygrometer where you sit in the evening. The number on it, not the dial on the furnace, is the one to watch.
