Short answerReplace a whole-home humidifier's water panel every fall, clean the distribution tray and drain, check the solenoid valve and orifice for mineral buildup, open the bypass damper for winter and close it in spring. Set the humidistat for 30 to 45 percent and lower it when outdoor temperatures drop below 10°F so windows do not frost.
The forgotten box on the side of the furnace
A good share of the furnaces we see in Washtenaw and Wayne County have a whole-home humidifier mounted on the plenum or return duct: a bypass model with a 6-inch duct looping from supply to return, or a fan-powered model with its own small blower. And a good share of those have not been opened since the furnace was installed. The water panel inside is a concrete-hard block of mineral, the solenoid is stuck, and the humidistat is set to whatever the installer left it at. The homeowner has static shocks and cracked lips and assumes the humidifier is running. It is not.
The maintenance is an annual 20-minute job. Here is what it involves, in the order we do it during a fall furnace tune-up.
Replacing the water panel
The water panel, also called the pad or evaporator pad, is a rectangular block of expanded aluminum or coated paper that sits in a plastic frame. Water trickles down it from a distribution tray on top; air moving through it picks up moisture. Every drop of water that evaporates leaves its minerals behind on the pad, which is why in our moderately hard municipal water and very hard well water it goes from pliable to brittle in one heating season.
- Turn the humidistat to off and shut the water supply valve, usually a small saddle valve or quarter-turn valve on a copper or plastic line near the furnace.
- Open the cover. On most Aprilaire and Honeywell units the cover pulls straight off; a few have a clip or a screw.
- Lift out the pad frame. Note which side faced up and which way the arrow or the colored edge points, because the frame is directional and the tray feeds from the top.
- Snap the frame apart, discard the old pad, and seat the new one with the same orientation.
- Scrub the distribution tray and the scale trough at the bottom with vinegar and an old toothbrush, and make sure the small feed holes along the tray are clear.
- Reinstall, restore water, and confirm water is flowing when the furnace runs with a humidity call.
Buy the pad number stamped on the frame or the cover; a #35 and a #45 look similar and are not interchangeable. Replace once a year on city water and twice a year on a well in Augusta, Salem or the rural townships where iron stains the pad orange within months.
Checking the solenoid and orifice
The solenoid valve opens when the humidistat calls and the furnace blower is running. It has a tiny orifice, often a yellow or green plastic insert, that meters flow to about a gallon or two per hour. Mineral scale clogs the orifice, and a solenoid that has sat dry all summer sometimes sticks.
With the furnace running and the humidistat turned up, you should hear a click at the solenoid and see water trickling into the drain line within a minute. If not, pull the orifice and soak it in vinegar, and check the inline strainer screen just upstream of the solenoid. A solenoid that hums but does not open is a replacement part, inexpensive and a same-visit fix on a humidifier repair call.
Follow the drain line, a 1/2-inch flexible tube, to wherever it ends. It should drop steadily to a floor drain, a condensate pump or a laundry sink with no sags that hold water. Algae grows in it over summer; a flush with vinegar clears it.
The damper and the seasonal switch
Bypass humidifiers have a damper in the 6-inch duct, a round plate with a handle marked "Summer" and "Winter" or "Open" and "Closed." Open it in October, close it in April. Leaving it open in summer lets warm, humid supply air recirculate into the return and robs the air conditioner of capacity, and it keeps the pad wet, which is where the musty smell comes from. Fan-powered units have no damper but should have their water supply shut off for summer.
While you are at it, the humidistat. Most are dial-type and sit on the return duct or on a wall near the thermostat; newer furnaces fold the control into the thermostat itself. The target through most of winter is 30 to 45 percent relative humidity, which is the range in ideal indoor humidity for Michigan. Drop toward 30 or just below when outdoor temperatures fall under 10°F, and lower still during a polar vortex stretch, because the cold glass of a 1960s single-pane window in a Westland ranch or an older aluminum slider in a Ypsilanti Township condo will frost at a humidity level that a 2010 Canton colonial with low-e glass handles fine. Frost on the inside of windows is the signal to turn it down; static shocks and nosebleeds are the signal to turn it up.
Common failures and what they look like
- No water flow at all. Supply valve closed, strainer clogged, solenoid dead, or the humidistat wiring to the furnace board is loose. Saddle valves in particular seal themselves shut over years.
- Water running constantly, even with the furnace off. Solenoid stuck open or debris holding it off its seat. Shut the supply valve until it is fixed; a stuck solenoid can run hundreds of gallons down the drain in a week.
- Water on the floor under the unit. Drain line sagged or clogged, pad installed upside down so water runs off the frame, or the drain fitting cracked. A bypass unit mounted on the return duct can also pull water into the ductwork if the drain backs up, which rusts the furnace from the inside.
- White dust on furniture. Not a problem with evaporative furnace humidifiers; that is a portable ultrasonic unit. If you have one of those, the whole-home unit is the better answer.
- Humidity never reaches the setpoint. Pad spent, orifice restricted, damper closed, or the furnace is simply not running enough hours in a mild stretch for the humidifier to catch up. In a drafty prewar house in Ann Arbor's Old West Side, a bypass unit may also be undersized for the air leakage; a fan-powered unit or a steam humidifier does more.
- Musty or sour smell when the furnace runs. Standing water in the trough, an old pad, or a damper left open all summer. New pad, vinegar scrub, and the smell goes with it.
When the humidifier itself is done
Plastic housings crack, distribution trays warp, and bypass units from the 1990s with the old-style float-and-reservoir design become more trouble than they are worth. If a unit is over 15 years old or the frame no longer seals, a new evaporative unit is a reasonable upgrade, and a steam humidifier is the answer for a large or leaky house where the furnace does not run enough to carry an evaporative pad. Our humidifier installation page covers the options, and whole-home humidifier benefits in a Michigan winter explains why the right humidity level matters for the house, not just for comfort.