Basic Setup: Where to Place It and What to Fill

An Aprilaire humidifier connects directly to your home's water line and furnace ductwork, so placement matters. The unit mounts on the return air duct of your furnace — the large pipe that pulls air back into the heating system. Your furnace manual or a heating technician can show you which duct that is. If you're installing it yourself, you'll need a cold water line nearby; if one doesn't exist close to the furnace, a plumber can run one.

Fill the unit's water reservoir if it has one (some models draw water directly from the line). Check the water level by looking at the sight glass on the side of the unit — a clear plastic window that shows how full the tank is. Top it off when it drops to the lower mark. Use distilled water if your model calls for it; tap water can leave mineral buildup inside the tank and reduce how well the humidifier works.

Key Takeaways

  • Mount the humidifier on your furnace's return air duct, not the supply duct, so it can add moisture to air before it heats.
  • Set the humidistat (the control dial) between 30 and 50 percent relative humidity to prevent mold and condensation on windows.
  • Check and refill the water reservoir every few days during heating season, or switch to automatic fill if your model has that option.
  • Replace the evaporator pad once a year, usually in fall before you turn the heat on, to keep water flow steady and prevent mineral clogging.
  • Lower the humidistat setting in winter if you see condensation on windows or frost on the outside of your home.

Finding and Using the Humidistat Control

The humidistat is a dial or digital display mounted on the humidifier itself or on your furnace's control panel. It measures the moisture level in your home and tells the humidifier when to run. Turn the dial clockwise to raise the target humidity level, counterclockwise to lower it.

Start at 35 percent relative humidity and adjust from there. If you see condensation forming on windows or mirrors, or frost on the outside of your home, lower the setting by 5 percent. If the air feels dry and you're getting static shocks, raise it by 5 percent. The goal is comfort without moisture damage — most homes do well between 30 and 50 percent.

Turning the Unit On and Off

Most Aprilaire models have a straightforward on/off switch. Flip it to "On" when heating season starts, usually in fall. The humidifier will then run automatically whenever the furnace runs and the humidity level is below your target setting.

Turn it off when heating season ends in spring, or if you're leaving home for more than a week. Leaving it running when you don't need it wastes water and can cause mold growth in ducts. If your model has a "manual" setting, use that only for testing — it runs the humidifier continuously regardless of humidity level and can damage your home if left on too long.

Checking Water Flow and the Evaporator Pad

The evaporator pad is a spongy filter inside the humidifier that water flows through before entering the air. Over time, minerals from water build up on the pad and block water flow. You'll notice the humidifier running but not actually adding moisture to the air.

Check the pad once a month during heating season by opening the access panel on the humidifier. If it looks brown or crusty instead of white or tan, it needs replacing. Replacement pads are inexpensive and take five minutes to swap out — just slide the old one out and slide the new one in. Most people replace the pad once a year in the fall, before turning the heat on for the season.

If water isn't flowing to the pad at all, check the water inlet valve — a small lever or knob near where the water line connects to the humidifier. Make sure it's turned fully open. If water still doesn't flow, the valve may be clogged with minerals and needs cleaning or replacing.

Cleaning and Maintenance During the Season

Wipe down the outside of the humidifier with a dry cloth every few weeks to remove dust. If you see mineral deposits building up inside the tank, drain it and rinse it with distilled water. Don't use vinegar or harsh cleaners — they can damage plastic parts.

If the humidifier stops adding moisture but water is flowing to the pad, the distribution tray (the part that spreads water across the pad) may be clogged. Remove it and rinse it under running water, using a soft brush to clear any mineral buildup. Reinstall it and the humidifier should work normally again.

Seasonal Shutdown and Storage

When heating season ends, turn off the humidifier and drain the water tank completely. Leave the access panel open so the inside can dry out. This prevents mold and mildew from growing during the months you're not using it.

Before you turn the humidifier back on the following fall, replace the evaporator pad and rinse the tank with distilled water. Check that the water inlet valve opens smoothly and the distribution tray is clear. If you notice any mold smell or visible growth inside, clean the tank with a solution of one part white vinegar to three parts water, then rinse thoroughly with distilled water.

Troubleshooting Common Problems

The humidifier runs but humidity doesn't rise: The evaporator pad is likely clogged. Replace it. If that doesn't work, check that water is actually flowing to the pad by looking for moisture on the pad itself. If the pad is dry, the inlet valve is closed or the water line is disconnected.

Water leaks from the unit: Check that the tank is seated properly in its frame and that the access panel is closed tightly. If water drips from the ductwork, the distribution tray may be overflowing — this usually means the pad is clogged and water can't flow through it. Replace the pad.

The humidifier won't turn on: Make sure the power switch is set to "On" and the furnace is running. The humidifier only operates when the furnace is heating. If the furnace is running but the humidifier isn't, check that the humidistat isn't set to 0 percent — if it is, the unit won't run because it thinks the home is already humid enough.

Frequently Asked Questions

Can I use tap water instead of distilled water?

Tap water works but leaves mineral deposits inside the tank and on the evaporator pad, which clogs the pad faster and reduces performance. If you use tap water, plan to replace the pad more often — every six months instead of yearly. Distilled water costs a few dollars per gallon and extends the life of your pad significantly.

What humidity level should I set it to in winter?

Start at 35 percent and watch your windows. If condensation forms on the glass or frost appears on the outside of your home, lower the setting by 5 percent. Most homes stay comfortable between 30 and 50 percent. Very cold climates may need to stay below 30 percent to prevent condensation damage.

How often should I refill the water tank?

That depends on how much you run the furnace and your home's size. Check the sight glass every few days during heating season. Most people refill every three to seven days. If you refill more than twice a week, consider installing an automatic fill kit so water flows in from your main water line without manual refilling.

Is it normal for the humidifier to make noise?

A quiet hum or occasional clicking is normal. A loud grinding sound usually means the evaporator pad is clogged or the distribution tray is blocked. A high-pitched squeal may indicate the water inlet valve is partially closed or mineral-clogged. Clean or replace the pad and check the valve.

Can I run the humidifier in summer?

No. The humidifier only works when the furnace is running, and the furnace doesn't run in summer. In humid climates, running it would add moisture your air conditioning is trying to remove, which wastes energy and can cause mold. Turn it off when heating season ends.