Introduction: Troubleshoot and Repair a Furnace Mounted Humidifier

About: It's said that to perfect a skill takes about 10,000 hours of work and study. If that's the case I've got around 9,000 hours to go. But, like they say, it's not the destination but the journey.
A properly functioning humidifier will maintain a comfortable moisture level in a home during the winter heating season. However, when a humidifier fails the air in a home lacks moisture naturally present during the warmer months of the year. This extremely dry air makes the skin feel cold while increasing static electricity in a home. Fortunately, a humidifier is a relatively simple device that many people can troubleshoot and repair without the need of a repairman. This instructable will explain how to diagnose and, hopefully, repair a broken humidifier with a minimum investment in time, effort and expense. While my humidifier was an Aprilaire 700 model humidifier, I would still expect that most of this discussion can easily be applied to other brands of humidifiers as well.

NOTE: An estimate from the technician doing our annual furnace cleaning and checkup was in a range of $200-$250.00. I was able to fix the humidifier for $63.00.

Step 1: How a Humidifier Works

How a humidifier works

A humidifier adds moisture to the air by converting water into vapor and circulating it through the furnace ventilation system. A humidistat, as set by the home owner, maintains the desired moisture level in the home by turning the humidifier on and off as needed. Specifically, when the humidistat senses that humidity is needed, the humidifier solenoid valve opens allowing water to flow through the water panel of the humidifier and the humidifier fan turns on. Some of the water passing through the panel is vaporized by thehumidifier fan and is directed into the furnace ventilation system. Water that is not vaporized exits the humidifier via a drain tube at the bottom of the water panel. When the humidity reaches the level set on the humidistat the solenoid valve closes, stopping the flow of water to the humidifier and the unit shuts off.

Parts of a humidifier

These photos show the principle parts of a furnace mounted, whole house humidifier (in this case an Aprilaire model 700):

Water Panel - photo #1
Solenoid Valve - photo #2
Humidifier Fan - photo #3
Control Board & Fan Motor - photo #4

Step 2: Diagnosing Possible Causes of a Humidifier Problem

Diagnosing the Problem

There are several ways to determine if a humidifier is working properly. First and foremost, when the humidifier turns on an audible click, followed by the sound of the humidifier fan should be heard. Additionally, water should be seen running through the drain tube at the bottom of the humidifier. Also, if a humidifier has been working properly, the water panel should need replacement at the beginning of every heating season. A water panel that doesn't show a significant build up of mineral deposits (or scale) most likely hasn't been receiving any water.

Here is a breakdown of the most common problems and the probable cause:

Fan doesn't turn on (no audible sound)

a) No power - bad control board
b) Motor is burned out (unlikely, but possible)

No water to humidifier - no click when the humidifier turns on, or no water exiting drain hose

a) No click - solenoid valve is bad or not getting any power indicating a bad control board
b) No Water exiting the drain hose - solenoid valves have an in-line strainer on the inlet side and an blue orifice on the outlet side of the solenoid, either of which could be clogged requiring either cleaning or replacement
c) The saddle valve connecting the humidifier water line to a household water line may be clogged and blocking the flow of water to the humidifier

Step 3: Disassembling the Humidifier

The pictures in this section illustrate how to disassemble the humidifier. Without the use of a volt meter to verify that power was reaching the solenoid valve and fan motor, my only option was to visually inspect the various parts for any sign of damage that would cause the humidifier to fail. I opted to first inspect the fan motor and control board since the symptoms indicated I wasn't getting any power to the fan or solenoid valve.

Disassembling the Humidifier

1) First, UNPLUG THE HUMIDIFIER from the power source.
2) The humidifier body is easily removed by pulling down on the plastic locking tab (photo #1) and swinging the cover up and toward you while lifting slightly. This will release the humidifier body from the rest of the assembly.
3) Notice the four screws in photo #2 (and also in a closeup in photo #3). They have been loosened so the plastic frame that holds the guts of the humidifier (fan, motor and control board) in position can be removed.
4) Remove the screws and unclip the wiring assembly from the harness (lower left corner of the frame) and remove the frame (photo #4).
5) Lift  the humidifier fan, motor and control board assembly out of the humidifier cover and turn it over (photo #5).

Photo #6 is the original control board. It will need to be removed and the printed circuit inspected for any sign of damage.

6) Before removing the control board make a drawing of the wiring so each wire is easily identified when being reconnected.
7) Disconnect the wires from the control board and remove the two screws that hold the control board in place.
8) Flip the control board over and inspect the printed circuit.

Photos #7 and #8: The circuit board in full view and in close up. As seen in photo #8, the circuit board had shorted out and was the cause of the problem (see the small black spot in the lower right quadrant of the circuit board)

Photos #9 and #10: A new control board (including new wiring) was ordered online at a cost of $55.00 plus shipping (total $63.00).

Reassembly is simple but caution should still be exercised to avoid potential damage to the circuit board:

a) Don't over tighten the mounting screws when installing the new control board.
b) When reconnecting the wiring to the control board, work the connectors back and forth gently until properly seated.

Note: Aprilaire 700 humidifiers come in two series, those built thru 2008 and those built after 2008. As a result, they require different control boards. Make sure you do the necessary research to properly identify the correct parts for whatever brand of humidifier you are attempting to fix. There are so many resources available on the internet that you shouldn't hit any major roadblocks.

Step 4: Troubleshooting Other Humidifier Problems

Fortunately, my repair was relatively easy to diagnose and repair. However, other power related problems may require a more comprehensive analysis. In many cases a volt meter may be required to test humidifier components before the problem can be identified. Volt meters are relatively inexpensive and readily available at

Additional Troubleshooting Tips:

If the fan is running but water isn't reaching the humidifier there are several things that you can check:

1) Fully close and open the saddle valve several times to loosen any deposits that may be blocking the flow of water to the humidifier.

2) Close the saddle valve, unplug the humidifier and disconnect the solenoid at the input and output connections. Remove the in-line strainer and blue orifice. Clean them thoroughly and inspect for any damage. If no damage is present reassemble the solenoid and test again. If water still doesn't reach the humidifier these parts may have to be replaced.

3) Remember: If the fan works but you don't hear the solenoid click when the unit turns on there is a good chance the solenoid valve is defective and needs to be replaced.

In Summary:

This goal of this instructable was to provide a basic understanding of humidifiers and give an overview of how to diagnose and troubleshoot a few of the more common repair problems. While my level of expertise is quite limited on this subject there is a wealth of information available online with more detailed, brand specific information to help you fix your humidifier. A few Google searches can, undoubtedly, answer any questions not addressed here.