Do whole-home humidifiers cause mold build up in ductwork?

Consumer question:

I have been told that Whole house humidifiers can cause a build up of mold in ducts. I have been trying to get an answer before I turn my humidifier back on. I turn my unit off in the summer. Please let me know if these rumors are true.

Aprilaires’ response:

Thank you for contacting us regarding our whole home humidifiers. Our humidifiers are evaporative units, meaning we use moving air to absorb evaporating water. By doing this, our humidifiers add humidity to the air as a fully absorbed gas. Because we don’t spray any mist or use any standing water in our units, there is no concern of mold growth when installed properly by a licensed HVAC contractor.

If you have any further questions regarding this or any of our products, please feel free to contact our customer service department at your earliest convenience.

6 thoughts on “Do whole-home humidifiers cause mold build up in ductwork?

  1. brad

    Thank you for your question regarding a red tint on your Aprilaire humidifier water panel. In order for microbial growth to occur three conditions must exist: There must be an organic food source for the microbes, the temperature must be in a certain range, and water must be present for an extended period of time. These conditions can exist in some types of humidifiers, but Aprilaire evaporative humidifiers are designed to prevent microbial growth. None of the materials in Aprilaire humidifiers provide a food source. Most significantly however, the drain-through technology in Aprilaire evaporative humidifiers leaves no standing water once the humidifier turns off. All the water delivered to an Aprilaire evaporative humidifier is either evaporated or goes down the drain, leaving all surfaces dry between humidification cycles. If your water panel has a red tint this would be a sign the water panel is wet 24/7. This is usually due to attempting to achieve a humidity higher than our design humidity level of 35% at an indoor temperature of 70F and an outdoor temperature of 20F or above. We recommend replacing the water panel and reducing the humidification level to the levels we suggest on page 6 of our owner’s manual(see attached).

    Thank you for giving us the opportunity to be of assistance.

  2. Jim

    Sorry. I just had a rep from our gas company visit my home. She has installed Aprilaire units a long time, has been in the business 25 years. She said there is no way NOT to get mold in the ductwork. Ductboard, which mine are made of, is especially prone, as it is porous. I asked how long can one use the system before mold needs to be addressed? She said within three years. Replacing contaminated ductboard could cost near $2,000. And, the ductwork within the walls could not be reached. I was really disappointed, as I was certain I had found a solution to the dry Winter heat. I cannot afford to have that ductwork replaced every three years and don’t want to wait for symptoms. How does Aprilaire respond to this news?? These systems should work so they stop the humidity a minute or so before the warm air… so that for the final minute or so, the dry air could dry the lining of the ductwork. I would appreciate your solution to this problem. Also, the system works only when the furnace kicks in. My home has many south windows – and on a sunny Winter day, passive solar can get my home up past 72 degrees. That means the furnace isn’t on for many hours during the afternoon. Meaning, I’d be getting no humidity over that time either.

    1. aprilaireadmin

      Hello Jim,

      Thank you for addressing your concerns, and I would like to alleviate any misunderstandings.
      With duct board, it is porous and there are misconceptions that the porosity will inhibit mold growth. This is not true.
      In order to have mold growth, we would need to 65% RH (Relative humidity). Given the fact that this is a winter issue, RH in the duct work settles around 20%-30%, and the most we can displace is 45% . We have had contractors calling us, and asking if duct board will be an issue. We have always told them that there will be no deterioration, and there never has been. You have more relative humidity in the summer going through your duct work than you have in the winter. Remember that as it gets colder, heat travels to the coldest source, and with that being said, you lose the humidity in your house that was there from spring and summer.

      As for the furnace fan. It would be in your best interest to contact the tech that installed the furnace. Some furnaces feature a “purge” setting that will allow the fan to run for “X” amount of minutes to evacuate any warm air out of the duct work. A timing relay might help with the furnace fan issue, and that is something that you HVAC tech can determine for you.

  3. Mike Hogan

    I have an aprilaire unit and when I clean it out after the winter, there is a build up of a white, slimy substance. What is it? And is it ok that particles of this could be blowing through the house?

    1. aprilaireadmin


      I am not sure what model you have, but you’re doing the correct thing by preforming maintenance on it. The best thing to do if you are unsure is to have a service technician look at it and advise you.

      If you have any other questions, do not hesitate to contact us.


      Technical Support
      Aprilaire – Division of Research Products Corporation
      Call Center hours are M-F, 7:00 a.m. to 5:00 p.m. CDT


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