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.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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.

2 People Made This Project!


  • Furniture Contest

    Furniture Contest
  • Reuse Contest

    Reuse Contest
  • Hot Glue Speed Challenge

    Hot Glue Speed Challenge

23 Discussions


Question 9 months ago on Introduction

water never stops flowing, through tube when humidstat is operating


Question 1 year ago

I have an Aprilaire 600 furnace humidifier. I installed it myself in 2004. It has worked perfectly all those years until this winter when water started running out of the bottom of the furnace on to the basement floor. I have replaced the solonoid valve and restrictor valve in it. I have a new water pad and I've cleaned the feed tray at the top of the pad. I've watched the solonoid work and send water to the top of the tray for dispursement and I've seen it run down the clear plastic overflow tubing I have attached to the spud under the unit. I can't find anyway that the water would be getting in to the cowling at the A coil but it does and I've opend the inspection doors on the furnace and watched water running down inside. Aprilaire tech support says that pad and overlow is designed to handle twice the flow of water and he could offer the only advice and that was to try replacing the solenoid and restrictor, which I did. Do you have any ideas as to a solution?


1 year ago

This is the best article I came across explaining humidifiers. Thanks. I moved into my house and found that humidifier does not work. I have diagnosed as much as I could.

I know solenoid is fine as it clicks when I connect to 24V. Two wires (brown and blue) coming from controller always has about 27V regardless of settings on control switch (dial). I thought it would only have voltage when controller wants to turn on humidifier and let the water flow. But there is no voltage on the yellow wires going to solenoid. It is kind of hard to check if fan was turning on. So I put mark on one of the fan blades. It shows fan never moved. Maybe the plug on fan side does not snugly fit into plug on humidifier side.

Any help would be greatly appreciated. Thanks.

1 reply

Reply 1 year ago

I have to add something to above.

Ok after I typed above I found out that if I unplug the fan part from 110 outlet, there is no current in those two wires I said above. This to me does not make sense. Or I am really missing something here. In my prev house I had simple rotary humidifier. Thanks!


2 years ago

Hi, great article!
I have a bad control board, too. Where do i find a replacement, please? Not finding good hits with searches so far.


2 years ago

I have a unique problem. My solenoid valve opens up if I knock on the body of humidifier when furnace is on. So I thought there might be some loose wire. I followed your instructions and opened up the humidifier. I did not see any loose wire. I did one test by plugging power supply 120 v and I short circuited the wires to humidistat. I could read 27.5 volts across solenoid yellow wires. Do you think I need to replace control board to get 24 volts.



2 years ago

I have an Aprilaire Model 8570 humidifier and I've been getting the following message: service humidifier. When I checked the humidifier downstairs I noticed there was no water coming out of the black wire that is connected to the humidifier. The water valve us turned on.


2 years ago

I am having the fame problem as dep156 any suggestions?


2 years ago

I have a similar Honeywell HE360 which has started leaking water from the bottom of the housing when it is running. I changed the pad, checked the hoses and tray etc. and everything seems to be in order. Is the fan supposed to pull the hot air through the pad and put it back into the furnace through the vents on the sides or vice versa? Any suggestions?



2 years ago

Sorry, but the answer is beyond my level of expertise.


2 years ago

We have had our AprilAire humidifier checked twice recently, and the tech keeps saying it is working fine. We have had it for over ten years and the humidity in our house in the winter has stayed in the 40% range. It is now around 20%. They suggest connecting it to hot water. It has worked for over 10 years on cold water. If there is not a problem with the humidifier how can switching to hot water at this point help? Do we just need a new humidifier?


2 years ago

I'm not always getting 24v!? most times i get 19-21 volts going to the humidifier.. any ideas how i can regulate this?


3 years ago

Just had an Aprilair 700 installed. The furnace runs through the cycle then almost immediately starts new cycle.


4 years ago

I paid a contractor to install a Aprilaire 500 and he can't explain why the central air comes on when the humidity is high inside my home. Anyone have a clue?


4 years ago on Step 4

Please, HELP!

I have a 700 Model Aprilaire (built before 2008) whole house humidifier that operates properly; however, there is a horrendous noise being generated when the fan is on (possibly a vibration of sorts). It only occurs when the fan is blowing. What is causing this noise?

Thank You,

Dragon Lady

I have a unit that is very similar to that in the lead picture ie the humidifier mounted to the furnace bonnet. It is properly installed and operationally functional,

I have noticed (in my sinus's) a slight increase in humidity level, however the rate of increase in humidity in the house is painfully slow (installation 3 weeks old).

On reviewing the install, i felt a slight movement of air BACK into basement from the two sets of vents in the cover ie the fan is too weak to force the humidified air into the furnace bonnet.

any way of increasing the fan volume or pressure - any way of making the fan "positive displacement"?


4 years ago on Introduction

I have one question.

When the humidistat is turned up and the water valve solenoid clicks on, should the water flow be constant or just a brief one second squirt?

I discovered a few days ago that my humidifier was bone dry inside. I took everything out and discovered the water hose to the filter was blocked up with gunk. Everything was cleaned and reassembled. When I tested the electrical system, I had 24 volts at all connection points. However, when the humidistat was turned up, it solenoid clicked and there was a one second squirt of water. The voltage then dropped to zero.

I installed a brand new solenoid kit today and still get the same one second squirt. Does it just take several on/off cycles of the furnace to finally stabilize?


2 replies

Reply 4 years ago on Introduction

The water should be a continuos stream with the flow controlled by how much you open the saddle valve on the waterline to the humidifier. I'd check and make sure it was open. Other than that I'm not sure what the problem is.


Reply 4 years ago on Introduction

The main valve is open all the way and water flow is rapid if you disconnect the water line to check it.

I just find it odd that I am getting 24 volts from the transformer thru the humidistat (when it is turned on) and to the bare wires that would connect to the solenoid valve wires. As soon as they are hooked up, the solenoid clicks open for about a second then stops. As I was checking all of this with the humidifer cover off and water filter unit removed, I was wondering it that may have been affecting the poper operation of the humidistat. However, it has been reassembled about 5 hours ago and still clicking and shutting off.


4 years ago on Introduction

Well, the HVAC repairman just left ($79 later). I chose to call him after reading through this and deducing that it might be a bad solenoid valve, and didn't want to order a part and find out I was wrong. There was no audible "click" in test mode, drain line was dry, GFI was not tripped, and the fan sounded like it was going so I thought it was getting power (it wasn't -- it was just the furnace itself causing the vibration and air movement sound through the humidifier). So, here is what I would add: even if you don't have an ohm meter to check for power as instructed here, you just can follow the FULL path of the humidifier's wires back to the power outlet. In my case, a wire was hanging loose right next to the outlet way on the other side of the furnace (likely got bumped off when I shut the furnace off to change the filter). So, I'd say assume the simplest and most obvious explanation first! Hope that may help someone else avoid an unnecessary repair bill.