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Furnace Not Draining Properly? Top 4 Causes (+ Fixes)

Condensation in your furnace builds up throughout the day. When water leaks from the furnace, it may signal a problem with the device. This problem is often related to the condensate line, a part of the furnace. 

Your furnace won’t drain properly if the condensate lien is clogged or leaking, the condensate pump is faulty, or the flue vent is too small. Fixes include fixing leaks in the condensate line, ensuring the pipe isn’t clogged, checking the condensate pump, and taking a look at the flue vent. 

This article will help you get to the bottom of a furnace not draining properly. I’ll go over the common causes and the solutions you need to follow. 

The Condensate Line Is Clogged

The condensate line is a PVC pipe that allows the furnace to drain water. Water gradually builds up due to humidity in the house. 

Without the condensate line, the furnace couldn’t effectively drain any water that accumulates inside the system. 

While there are a few reasons why the condensate line may cause problems with the furnace’s drainage, clogging is often one of the more common causes. 

If the condensate line becomes clogged, then water cannot drain through this pipe. Instead, water will build up inside the furnace, which can lead to a water leakage, along with other potential problems — there are electrical components inside the furnace. Water and electricity don’t make a good pair. 

How To Fix

It’s a good idea to start with this fix if your furnace isn’t draining properly. You may need access to some special tools. If there’s a clog in the condensate line, then the furnace’s condensation system needs to be flushed. 

  1. Determine where the condensate line is located. You should be able to see pipes coming out of the furnace. These run toward an area where the water is drained.
  2. Remove the pipes. Do this only after the furnace is turned off.
  3. Use an appropriate solution to flush out the pipes. See if you can identify any clogs in one of the pipes. If flushing doesn’t help, you may need to add more pressure to remove the debris that caused the clogging.

This video shows how the condensate line and trap can be cleaned:

There Is a Leak in the Condensate Line

If you can’t seem to find a clog in the condensate line, then there is one more thing to check with this part of the furnace. 

Apart from clogs, another common reason why the condensation system fails to work properly is leaking. When there are leaks in the condensate line, water cannot efficiently flow toward the pump and drain to the designated area. 

A leak may seem small at first, but it can result in something more serious later on if you don’t fix the problem on time. Thus, before moving on to other potential problems, first spend some time on this one.

How To Fix

Checking the condensate line for leaks shouldn’t be hard at all. You do need to ensure the furnace is turned on during this process. This is because you want water to be pushed through the condensate line. This is the only way to see if there are leaks in the PVC piping. 

  1. Turn the furnace on and leave it running for a while.
  2. Take a look at all areas where the line runs. See if there are any areas where water drips from the pipes. If you see a drop of water running out of the pipe, you found your leak.
  3. You should also be on the lookout for small cracks in the pipes. These small cracks allow tiny drops of water to escape. However, the cracks may grow larger after a while, eventually leading to a larger leak.

The Condensate Pump Is Faulty

Some furnaces use a condensate pump to assist with the draining process. However, this isn’t the case with every furnace. Thus, you’ll first determine if your furnace uses a condensate pump. 

A good way to determine if there’s a condensate pump is to consider the furnace’s installation area. A furnace that sits in the basement will likely have a condensate pump. 

If the furnace is installed in the house, a condensate pump may not necessarily be present. This is because gravity helps with the draining process.

If you do have a condensate pump, it may be faulty, causing the furnace not to drain properly. 

How To Fix

There isn’t a simple fix to help repair the condensate pump. 

A professional can repair the pump, but they first need to understand what caused the problem in the first place. 

In some cases, a complete replacement of the condensate pump is the best way forward.

The Flue Vent Is Too Small

Many people overlook one part when determining why the furnace isn’t draining properly — the flue vent. The flue is a term used to describe the gases the furnace produces during combustion. 

A flue vent allows these gasses to escape from the furnace system. In this particular situation, you should look at the size of the flue vent. 

A small flue vent can be the reason behind the furnace not draining properly. When the vent is too small, the combustion gasses start to accumulate inside the furnace. The gases start to lose their energy. When this happens, they condensate — this means that the gasses start turning into liquid. 

How To Fix

Determining if the furnace not draining properly is caused by a small flue vent can be tricky.

A good starting point is to look at the current flue vent installed in the furnace. 

Now, take the size of your furnace into account. A large furnace will produce much more flue gasses than a smaller system used to heat up a two-bedroom apartment. 

When you take the furnace’s sizing into account, you should get a better idea of how large the flue vent should be. You can use a reference guide to help you determine if the flue vent is too small compared to the amount of gas produced by the furnace unit. 

A flue vent that has an insufficient size needs replacement. If you’re not sure, call an expert and have them inspect the system. A pro can quickly determine if the problem lies with the flue vent.


A clog in the condensate trap is the most common cause of a furnace not draining properly. Sometimes, leaks develop in the line or pump responsible for dealing with condensation. The flue vent may be too small, or the problem may lie with the humidifier that regulates your home. 

Call a professional to diagnose the reason and fix the furnace if you don’t want to deal with the problem yourself.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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