Furnace Humidifier Not Working: Aprilaire and Honeywell

By Ray Bennett · Updated 23 October 2026 · 6 min read · Checked against the sources at the end

Furnace Humidifier Not Working: Aprilaire and Honeywell

Short answer

A whole house humidifier only runs when three things line up: the furnace fan is moving air, the house is drier than the setting, and water reaches the pad. Aprilaire's stop at 45 percent. A scaled pad, a saddle valve not fully open, a damper left on its summer setting or a frost setting on the control are the usual reasons one does nothing.

A whole house humidifier is a box on the furnace ductwork with a pad that water runs through. Air from the furnace passes through the wet pad and carries the moisture round the house. When one stops working, it is usually because one of the few things it needs has gone missing.

Aprilaire says its humidifiers need heat, water and air movement. Honeywell’s manual walks the same chain: the humidity control calls, a water valve opens, water runs through the pad, and the furnace fan carries the moisture away. This page follows that chain, using both makers’ own guides.

For an AprilAire digital control with a flashing service light, use the AprilAire humidifier code guide to separate E1, E2 and E3 from the water-panel reminder.

Before you start

What to carry: a torch, and a new pad if the old one is due. Fifteen minutes.

Gas or electric? Either. The humidifier sits on the ductwork of a gas or an electric furnace, and it runs on electricity of its own.

Which kind you have. A bypass model has no fan and borrows the furnace’s airflow. A fan model, which Aprilaire calls a power humidifier, has its own fan.

Two things before you open it. 120 volts. Honeywell warns that its humidifier uses 120 volts AC. Unplug it, or switch off the power to the furnace, before the cover comes off.

Too much is worse than too little. Honeywell says to lower the setting if condensation appears on the windows. With condensation on the walls, turn the humidity control and the water off until it has completely dried: wet windows and dry air in winter.

What it needs to run

A whole house humidifier needs all three at once. Most that do nothing are missing one.

Check 1Free, ten minutes

Check the fan and the setting

The furnace fan. Honeywell’s first check for low humidity is the furnace blower: reset a tripped breaker or a blown fuse, and check the furnace power is on. Aprilaire says its bypass humidifiers run whenever the heating is running and the house is drier than the setting. If the blower will not run at all: when the blower motor is the problem.

The setting. Aprilaire says its humidifiers will not run at 45 percent humidity or above, because that is the highest setting on its controls.

A frost setting. Honeywell says some controls lower the setting on cold days to keep frost off the windows. The humidifier can then sit idle with the house below the number you chose.

Test mode left on. Aprilaire says a flashing green light on its digital control means test mode, which lasts a minute. Left there, the humidifier will not run. Turn the knob back to where it was.

The outdoor sensor. In automatic mode, Aprilaire’s digital control needs its outdoor temperature sensor to work.

Water to the pad

Check 2

Check the water

The saddle valve. Aprilaire says the valve on the water pipe should be turned fully open. A small orifice in the humidifier sets the flow, and a valve that is not fully open cuts the output.

The solenoid click. Aprilaire says to turn the setting up while the furnace fan runs. The water valve should click. No click means it may need replacing, which is a job for the installer.

Water in the drain is normal. On Aprilaire’s drain-through models, some water runs to the drain to flush minerals off the panel. A model 500 or 600 takes about five minutes to fill a quart jar at the drain.

Water when the furnace is off. Aprilaire says the wiring or the solenoid valve needs checking by the installer.

The pad

Aprilaire's and Honeywell's pad rules. A pad left in too long holds the minerals and stops evaporating.

Check 3

Replace the pad, the right way up

Once a season. Aprilaire says its water panels are made for one humidifier season, and the model 400’s every 3 months during the season. Honeywell says at least once a year, at the beginning or end of the dry season.

Why it matters. Aprilaire says minerals left behind by evaporation stay trapped in the panel. A clogged panel evaporates less water, and it can make the humidifier leak.

The right way up. Aprilaire says most of its replacement panels have a paint dot or stripe on one short end, which goes up, toward the water distribution tray. On Honeywell’s, the arrows inside the tray point into the duct.

Clean while it is open. Honeywell says to wipe the sediment out of the tray and the pad frame.

The damper

Check 4

Open the damper for winter

Aprilaire bypass models. Aprilaire says the summer and winter knob on the ductwork is the bypass damper, and it is closed for the summer on models 400, 500 and 600. In winter it must be open.

Honeywell’s basic models. At the end of the season, the damper knob on the HE100 and HE200 is turned across the bypass outlet. Honeywell says to turn it back to open at the start of the next dry season. Its HE150, HE250 and HE300 have an automatic damper.

The water and the control too. Honeywell’s end of season steps turn the humidity control off and shut the water valve, so check both are back on.

What the lights mean

The light	What the maker says
Aprilaire, flashing green	Test mode. Turn the knob back, or the humidifier will not run
Aprilaire, flashing yellow	Change the water panel, then turn the knob to Test/Reset until the green light flashes, about 5 seconds
Aprilaire, flashing red, E1 to E4	A control fault: E1 humidity sensor, E2 control knob, E3 outdoor sensor, E4 no link to an 8570 thermostat
Honeywell, Change Pad	Replace the pad, then press RESET under the cover. Never reset it without changing the pad
Honeywell HE150, HE250, Service	The bypass damper did not open fully, or no airflow was sensed. Press RESET
Honeywell HE300, Service	The fan motor failed, or no airflow was sensed. Press RESET

Honeywell says that if the Service light comes back after RESET, call an HVAC professional. Aprilaire says a flashing red light means the installer needs to check the control.

Still dry with everything working? Honeywell’s other cause of low humidity is air changing too fast. Keep doors and windows shut, close the fireplace damper when it is not in use, keep exhaust fans to a minimum and seal round doors and windows: finding the drafts.

And give it time. Honeywell says a dry house can take up to a week of continuous running to reach the setting.

Questions people ask

Why is my whole house humidifier not working?

Check the chain: the furnace fan running, the house drier than the setting, the water valve fully open, a pad that is not scaled, and the damper open for winter. Aprilaire’s stop at 45 percent.

How do I know my furnace humidifier is working?

On Honeywell’s advanced models, the Humidifying light is on while it runs. On Aprilaire’s digital control, the green light is solid during a call. You should also hear the water valve click open.

How often should I change the humidifier pad?

Once a season. Aprilaire says every season for most models and every 3 months for the model 400. Honeywell says at least once a year.

What should I set my humidifier to?

Honeywell’s table goes by the weather outside: 35 percent at 20°F and warmer, 30 at 10°F, 25 at 0°F, 20 at -10°F and 15 at -20°F. Lower it if the windows run with condensation.

Why is my furnace humidifier leaking?

Aprilaire’s list: a clogged water panel, a humidifier not level and plumb, a blocked drain line, a cracked scale control insert, or an orifice that is enlarged or missing.

Should I turn it off in summer?

Yes. Both makers say to turn the control off, and Honeywell says to shut the water valve. On bypass models, close the damper, then open it again when the heating season starts.

Sources: AprilAire owner center "Frequently Asked Questions" (humidifiers), and the Honeywell (Resideo) humidifier homeowner's operating manual 69-2414EF for the TrueEASE HE100 to HE300