How To Diagnose, Fix And Prevent Wet Furnace Filters

A furnace filter plays an essential role in removing dust and other pollutants from the air. It protects the furnace and improves the air quality. If the filter gets wet, it won’t efficiently trap airborne pollutants – so it is crucial to fix the problem as soon as possible.

A wet furnace filter usually results from obstruction of airflow due to a dirty filter or blockage within the system. However, build-up in a condensate drainage pipe, external leaks from plumbing and humidifier (sometimes found attached to the furnace), incompatible filters, and undersized furnaces can also cause a wet furnace filter.

A wet furnace filter can lead to many complications within the unit. It also poses a severe health risk. Read this article to diagnose and troubleshoot potential reasons behind the cause of your wet furnace filter.

How Does a Furnace Work?

To understand the underlying cause, it is crucial to know how a furnace works.

A furnace draws cold air from the room into the system, filters, and heats it before dispensing it out of the unit.

  • Once you turn the unit on, cold air starts flowing through it. A circulating fan helps push the air through the unit.
  • The air passes through the filter, located at the furnace’s side or floor.
  • Most furnaces use a generic disposable filter that traps large airborne particles, dust, debris, pet hair, etc., from the air.
  • The filtered air then flows through the heat exchanger, made of a metal chamber and pipes.
  • The combustion process is carried out in the chamber to heat the air.
  • The air is then emitted into the room, while the exhaust gases from combustion are released outdoors through the metal flue and vent.

The general working mechanism is the same for conventional furnaces. However, condensing furnaces also comprise a secondary heat exchanger before the metal flue.

The second heat exchanger absorbs the remaining heat from the combustion exhaust gases. After cooling down, the gases condensate to form water, dripping through the drain pipe.

How to Know If Your Furnace Filter is Wet?

A musty smell near the indoor unit of the furnace or pooling water on the floor around the system are signs of a wet furnace filter. However, you still need to disassemble the furnace to confirm this is your primary issue.

Since each brand has their own furnace design, you may need to refer to your user manual to access the filter. Generally however, furnaces come with an access panel that can easily be removed to detach and replace the filter.

As a safety measure, always switch off the furnace before opening it.

6 Causes of a wet Furnace Filter

Here’s all the reasons why your furnace filter may be wet:

1. Clogged Drain

Although a clogged drain can occur in any furnace, it is most commonly seen in high-efficiency condensing units.

In condensing furnaces, it is crucial to fully drain the condensate (water), as retaining it can cause damage within the system. Inefficient filtration can cause dust and debris to accumulate within the pipes, and if not cleaned adequately, the blockage can prevent water from draining out.

This water can then dampen the filter.

To check for the problem, turn the furnace off and shine a torch through the drain pipe to inspect for clogs. The drain line can be accessed by removing the cap on the PVC pipe.


Build-ups or debris near the pipe end can be removed by poking with a small stick(or similar). However, in case of hardened blockages, you can use a drain brush or vacuum to unclog the drain line.

2. Blocked Filter

A furnace uses generic filters with specific capacity. If the filter is not changed regularly, it becomes saturated and unable to trap more contaminants.

In fact, clogged furnace filters can release carbon monoxide into your home – Plentiful Air has a complete guide on how to prevent this from happening available here.

This allows the airborne pollutants to escape into the system. The buildup can obstruct the inflow of air and impede its movement within the unit. This hampers heating, consequently causing the system to freeze.

The blockage may also prevent drainage of water produced during heat exchange, and this condensate can also make the filter wet.


Look up instructions regarding filter maintenance for your furnace model. Then, remove the clogs/buildup and clean the unit to ensure no dirt or debris is blocking the airflow. Adequate filter maintenance is the key to keeping the furnace running for a long time.

Apart from cleaning and replacing the filter regularly, here’s what else you can do to ensure efficient filtration:

  • Before buying a filter, make sure its MERV rating is compatible with the furnace. MERV rating (filter thickness) should be enough to trap maximum pollutants without obstructing the airflow.

Filters with a MERV rating of 16 or below can be considered. If the people around have a history of dust allergies, you can buy a MERV 11. However, if there are no relevant health concerns, The best MERV rating lies between 8 to 10.

  • Ensure that the filter size is suitable for your furnace.
  • When installing the filter, double-check that it is secured in place. Getting a professional technician to ensure the correct placement of the filter is better.

3. Undersized Furnace

The furnace size must be suitable for the room to produce enough heat. If the furnace is undersized, the system will freeze due to cold. The temperature drop also increases the condensation of combustion gases, leaving you with a wet filter as a result.


Calculating an appropriate furnace size for your room/house is crucial to keep the heating system working and fix the issue. Check for your climate zone and multiply its recommended heating factor with the area of your house (in sq. ft.).

Insulation and ventilation of the area should also be considered when estimating the furnace size.

4. Frozen Coil

The furnace coil acts as a heat exchanger. When the temperature drops, the heat generated by the coils is also used to keep them from freezing. Anything that disrupts the heating system of the furnace can cause the coils to freeze, which can eventually lead to a wet filter.

If you notice cold air coming out of the vent or your furnace makes an unusual noise during operation, you should suspect a frozen system. You can also disassemble the system to confirm the diagnosis.


Frozen coil usually results from an underlying issue impairing the heating system. Turn the unit off for an hour and let the ice melt. Afterward, follow the instructions on the user manual and clean the system thoroughly.

Let it dry for 24 hours, and then switch it on.

If this doesn’t fix the issue, get your technician to replace the coil.

5. Broken Humidifier

In some places, furnaces are attached to the humidifier. Humidifiers use water vapors to moisturize the air, and running them together with a furnace makes the surrounding air warm yet humid.

Any cracks in the humidifying unit can cause water from its reservoir to leak into the furnace, and the filter can also get wet in the process. You may also notice this as a furnace leak.


Navigating around an attached humidifier-furnace unit can be difficult. Therefore, we recommend calling in a technician to fix the issue.

6. External Leaks

Wet furnace filters don’t always arise from internal problems within the system. Instead, external leaks in the roof or plumbing or a leaking AC unit installed near/over the furnace can also cause water into the furnace.

If you have thoroughly inspected your furnace for possible clogging or airflow obstruction, and the heating system is okay, check if any external factors are making your furnace filter wet.


Multiple minor issues like a dirty filter can cause HVAC leaks, and you can fix them at home by cleaning the unit. However, for any complicated problems related to plumbing, we advise getting a professional to take a look.

Frequently Asked Questions

Can a Dirty Furnace Filter Cause Condensation?

Condensation is a normal phenomenon in furnaces and occurs when combustion exhaust gases are cooled down for maximum heat absorption. However, a dirty furnace filter can obstruct the airflow, resulting in a temperature drop and eventual rise in condensation.

Condensation is standard in both condensing and conventional furnaces. When heat is exchanged between air and combustion gasses, the gasses condense to form water and carbon dioxide. This condensate is then removed outdoors through a drain pipe.

However, when dirty furnace filters cause increased condensation, drain pipes will also likely be blocked due to inefficient filtration. Hence, the condensate will not be drained completely, damaging the system.

Can a Dirty Filter Cause a Furnace To Leak Water?

A dirty filter leads to blockage in the drain causing the condensate (water) to leak through the system. Inadequate airflow due to obstruction interferes with the heating process resulting in temperature drop, hence more condensation and eventual leak.

Can You Put a Wet Filter in the Furnace?

A wet filter is not only inefficient for the filtration of airborne particles but also allows moisture to spread through the furnace. This damages the furnace components and also causes mold and mildew growth. Therefore, only a dry air filter of compatible MERV rating and size should be installed in the furnace.

Theresa Orr

Theresa Orr is an Earth Scientist who specializes in determining past climates from rocks using geochemistry. Her passion for clean water, soil and air drives her to provide easy to understand information for everyone to read.

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