Whole House Humidifiers And Mold (2020 Guide)

Do you want to learn how whole house humidifiers can sometimes cause mold problems?

Whole house humidifiers are great home comfort devices, but they can sometimes cause too much moisture in a home — and lead to mold growth.

In this guide, I will go over...

  • Auto mode versus manual mode
  • Attic and window condensation
  • Dirty HVAC systems & mold

Let's get started with this guide!

whole house humidifier and mold

Will A Whole House Humidifier Cause Mold?

As a home inspector, I have seen a lot of mold in houses, and whole house humidifiers can be a common culprit.

The main problem with whole house humidifiers is that they can put out an excessive amount of moisture when the temperature drops (especially below around 10-degrees). In addition, if the indoor air handler or furnace is extremely dirty with dust, this can also lead to mold growth when you add moisture from a whole house humidifier.

Basically, if you don't properly control and maintain a whole house humidifer (and your HVAC system) — there is a likelihood of mold growth inside your home.

Read Also: What Are The Best Steam Whole House Humidifiers?

Manual Mode

In all whole house humidifiers, it states in the owner's manual that if you operate it it manual mode, then you have to anticipate lowering the indoor humidity if the outdoor temperature drops.

For example, if the outdoor temperature drops from a low of 40-F to 10-F... you should lower your whole house humidifier to 25% relative humidity.

Of course, with auto mode, this isn't required.

AprilAire 700 Manual

"With the Manual Humidifier Control, it is important to anticipate a drop in outdoor temperature and reduce the setting accordingly to avoid excessive condensation..." 

Auto Mode

With automatic mode, you don't just have an indoor humidity sensor, but you also have an outdoor temperature sensor as well.

So when the outdoor temperature drops, it sends the signal to the humidifier, and the humidifier will drop its humidity accordingly.

Outdoor Temperature

Ideal Indoor Humidity

40-F

45%

30-F

35%

20-F

30%

10-F

25%

Window Mold

One of the biggest indicators that your whole house humidifier is putting too much moisture into the air is condensation on windows.

With moisture in the air, it always wants to condense on the coldest surfaces first. In a home's interior during the cold season, the coldest surface is usually the windows.

Condensation on the windows itself is a cosmetic issue because you can no longer see through it, but this condensation (or frost if it freezes) can damage your window sill and woodwork as it falls downwards. I have seen numerous windows with mold on the frame or in the window nooks due to excessive moisture.

Read Also: What Are The Best Portable Whole House Humidifiers?

Attic Mold

Another place where condensation may quickly form due to excessive condensation is your roof sheathing.

If the home's interior has a high humidity level (compared to the outdoor temperature), this moisture can waft up into the attic, and condense on the cold roof sheathing. This moisture may or may not freeze, but mold may eventually start growing on the sheathing. The moisture may also condense on the rafters, storage planking, or other attic areas.

Dirty HVAC System

Mold can be found almost anywhere, but as a home inspector, finding mold in the HVAC system such as a furnace or air handler (heat pump) is extremely common. Maybe 1 out of every 10 furnaces I inspect (remove cover), I find mold growth of some degree.

Mold can grow in the furnace so easily because it has the three things that mold loves... darkness, food, and moisture. Mold especially likes dust which is mainly human skin cells, hair, and clothing fibers—which it can break down as food.

So if your furnace, blower fan, evaporator coil, motor, and plenum are extremely dirty and caked with dust — add the excessive moisture from a whole house humidifier — and you may have a big mold problem.

The solution for this is to periodically have your furnace (and air ducts) cleaned by a professional air duct cleaning company. I recommend having your furnace cleaned every 3-5 years.

This will help prevent mold growth in your HVAC system, but it will also help it run much more efficiently. And don't forget to clean the exterior A/C condenser as well.

Read Also: What Are The Best Whole House Humidifiers?

Dirty Evaporator Pad

You would be suprised how often I find an evaporator pad in a whole house humidifier that looks like it hasn't been changed in years.

If the evaporator pad isn't changed every season, it can cause your humidifier to malfunction, and it may even cause excess water in your HVAC system. If your evaporator pad isn't draining the water correctly, this can also lead to a mold problem.

Always change the evaporator pad yearly, preferably prior to using the whole house humidifier at the start of the cold season.

Final Thoughts

Whole house humidifiers are a great way to add moisture into the air during the cold season. If you don't have enough humidity, it can cause dry skin, exacerbate allergies, crack wood floors, and cause numerous problems.

However, if you don't manage your whole house humidifier correctly, such as lowering the relative humidity as the outdoor temperature drops, then you may send too much moisture throughout your home.

This excess moisture can lead to condensation on windows, roof sheathing, and other cold surfaces. Condenstion or water droplets can lead to out of control mold growth.

In addition, any excess moisture sent throught your HVAC system can lead to mold growth if your system is very dirty and caked with dust. I recommend cleaning the furnace such as the A-coil, motor, blower fan, and plenum at least every 3-5 years.

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Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping people understand how homes work. He is a licensed home inspector in two U.S states and owns a residential and commercial inspection company (read his full bio on the About page). To ask Arie a question, please use the comment box at the bottom of the relevant article.

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