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11 Reasons Why Your Furnace May Be Leaking & How to Fix It

Published February 1, 2024
Written by Eric Smith
11 Reasons Why Your Furnace May Be Leaking & How to Fix It

Our Master Tradesman walks you through the 11 most common causes of furnace leaks and how to fix them.

Water leaking or pooling around a furnace can be an alarming find. Our Master Tradesman unpacks the 11 possible reasons why your high efficiency furnace may be leaking on the floor and what you can check before calling a professional furnace repairman to your home to fix the furnace leak. 

Where does the water come from anyway?

This is a great question! In most cases, the water that is leaking from a high efficiency furnace is condensation which is created during the heating process or condensation that is created while pulling ice cold air (or humid as you will learn later) back into the furnace for combustion. When a natural gas furnace combusts, the by-products are carbon monoxide and water. The water vapor is suspended in the exhaust and as the exhaust cools in the secondary heat exchanger, the water condenses and makes it way through the long (and complicated) drain system of the furnace. Read on to learn all of the places to look and the simple solutions that may solve your leaking furnace, along with learning about the other causes of furnace leaks.

Winter furnace leaks vs. summer furnace leaks- this guide covers both

A furnace is being used during all seasons of the year.It can appear to be the source of the water leak near a furnace even if the air conditioner is leaking as well, which is why we identify all major causes of water leaking from a furnace in this guide.  

The furnace may not be what is leaking

In Wisconsin homes, the furnace is usually in the basement near many other appliances that can also leak water such as water heaters, water softeners and iron filters. One of the leading causes of water leaking near the furnace isn’t a leaky furnace at all. It’s some sort of other plumbing leak. Before getting too deep into why your furnace may be leaking on the basement floor, let’s be sure it’s not something else such as the plumbing or a portable dehumidifier.

  • Check the water heater
  • Check the water softener 
  • Check to make sure a portable dehumidifier isn’t leaking or dripping on the floor 

If you are able to rule out those items mentioned above, it’s time to move on and see what is causing the leak.

1) The furnace drain line may plugged

By far the leading reason for furnaces leaking is a plugged drain hose. The drain hose is an external hose connecting to the furnace drain trap (covered in item #2) and runs to the floor drain. Over time, the drain lines that run from the furnace to the floor drain in the basement can clog and get blocked with debris. It’s largely because of this that having an annual tune up on your furnace is so important! If there is water pooling around the furnace or leaking out around it, check the furnace drain line to ensure it’s not clogged.

How to fix a leaky furnace caused by a plugged furnace drain:

Disconnect the drain hose from the furnace and flush it with hot water. Alternatively, you can have Austin Plumbing, Heating & Air completely replace the hose with a new vinyl furnace drain hose, PVC or CPVC. 

2) The condensate trap is clogged

Inside the furnace and a critical part of the furnace’s operation is the condensate drain trap. This trap separates the condensation formed while the furnace is heating from the exhaust that is also flowing through the secondary heat exchanger and aids in the overall operation of the exhaust system. Inside a high efficiency furnace, most of the condensation occurs as the exhaust gasses are cooling in the secondary heat exchanger. This condensation flows out the secondary heat exchanger in many small ports and does so slowly, which leaves time for debris to deposit in the drain trap and allow it to become clogged. When this happens, water backs up inside the furnace and can leak out, making it seem as if the furnace is leaking all over the floor.

The solution if the condensate trap is causing the furnace leak: 

Most homeowners will rely on a professional heating tech such as the ones on Team Austin since the drain trap will need to be removed from the furnace and cleaned thoroughly before re-installation. 

3) Failed condensation pump

Some furnaces are unable to drain to a floor drain by gravity and require a pump to lift the condensation from the furnace to the drain. This furnace pump is called a condensate pump. Without this pump, the condensation from the furnace would leak onto the floor causing a problem. If you have water leaking out of the furnace or the condensation pump, this could be the cause of your furnace leak. 

The solution to condensate pump related furnace leaks:

If you have troubleshot the water leak down to a failed condensate pump, we suggest repair or replacement of the condensate pump and a visual inspection of the drain lines for soundness.

4) Cracked or worn drain hose

Over time, the pipe and/or hose used to drain the condensation from the furnace can become brittle and worn. Furnace drains that are cracked, kinked or otherwise compromised can also happen from being stepped on or having heavy objects placed on them. Carefully examine the drain hose coming from your furnace to make sure the hose is not cracked or leaking. 

How to repair a cracked or leaking furnace drain:

So, what if your furnace is leaky because of the cracked hose? Replacing a cracked or leaking drain hose or pipe outside of the furnace is fairly straightforward, but if the drain hose that is leaking is inside of the furnace, you may want to consider hiring a pro. This is because many of the internal drain hoses require OEM parts and lots of disassembly to remove and reinstall. If you’re not 100% confident in your DIY furnace repair ability, the best course of action if your furnace is leaking is to have a furnace guru from Team Austin out. 

5) Furnace vent leak

Water doesn’t leak up, but if you’re noticing a leak or pooling around the furnace, it’s a great idea to look up as well! A sneaky culprit of furnace related leaks can be the vent and intake pipe that leads from the furnace to the outside. In this piping (both intake and exhaust), condensation occurs and if there is a faulty connection or failed PVC joint, this condensation can run out and leak on the floor near the furnace. 

What to do if your furnace vents are leaking: 

Repairing an exhaust vent pipe or intake pipe of a furnace should be done by a qualified technician. Call Austin Plumbing, Heating & Air to schedule an appointment to have your leaking furnace vent repaired. 

6) Rain or snow in the vent or intake pipes

If you’re noticing the leaking furnace when it’s raining or snowing outside, don’t rule out water from a driving rain or snow making its way from outside back into the furnace and leaking out. 

What to do if rain or snow is getting drawn into the furnace vent or intake:

If this is happening during an ongoing weather event, there isn’t much you can do other than to clear the snow away from around the intake pipe. The best course of action long term would be to have a professional HVAC contractor such as Austin Plumbing, Heating & Air out to see if the vent pipes can be reconfigured to prevent this from happening.  

7) Condensation forming in the intake pipe

The intake pipe from a furnace is used to draw outside air directly into the furnace during the heating season. This improves efficiency and overall cleanliness of the combustion process. In the summer, this pipe is still connected to the furnace and in some cases the humidity and heat from summer air causes condensation in the intake pipe. When this happens, a small amount of water will run back into the furnace and cause a furnace leak in the middle of summer.

How to resolve intake condensation from leaking:

Since excess water the furnace is not designed to handle is the root cause of this issue, we suggest the installation of a special fitting designed to catch and divert the excess water to the drain. After we install this special furnace part, no more water leaks will happen inside the furnace.

8) Secondary heat exchanger leak

Throughout this guide, we have talked about condensation a lot! The lionshare of the condensation and plugging that can occur in a furnace drain system happens in the secondary heat exchanger. The water that is created during the furnace operation is acidic and very corrosive and if left to lie in the secondary heat exchanger for too many years, it will eat holes in the heat exchanger causing leakage. 

What’s the fix if your furnace is leaking from the heat exchanger:

The diagnosis of a leaking secondary heat exchanger is not an easy (or inexpensive) one to make which is why it should be done by a properly trained heating technician. Have a licensed heating company out to perform a complete diagnostic analysis to be sure that this is in fact the problem. Looking for a licensed heating repair company in Wisconsin? You’re in luck with Austin Plumbing, Heating & Air.

9) Inducer motor is leaking

Connected directly to the secondary heat exchanger collection box is the inducer motor or also commonly called the furnace exhaust motor. If water backs up in the secondary heat exchanger due to any of the previously mentioned drainage problems that cause leaky furnaces, it is possible for water to back up into and leak out of the inducer motor of the furnace. 

Typical solution to a leaking inducer or exhaust motor in a furnace: 

The first step is to eliminate the drainage issue downstream of the inducer motor to see if that will stop water from leaking out of the motor or the housing itself. If that doesn’t fix the problem, then professional furnace repairs will be required and potentially an inducer motor repair or replacement on the heating system will be required. 

10) Humidifier is leaking

Running in the background at the same time as your heating system is your humidification system (if you have one). Humidifiers pass water over an evaporator pad where warm air draws the water vapor into the air stream so it can raise the humidity in the home. This is an absolute necessity in many Wisconsin homes, but also is a common cause for furnace leaks. 

If your furnace leak is actually a leaking humidifier follow these steps:

  • Replace the humidifier pad
  • Pull, flush and re-install the humidifier drain tube 
  • Check to make sure the filter and filter rack is installed properly 
  • Put the cover securely back in place 
  • Put the humidifier in test mode and observe its operation

11) The air conditioner is leaking

In Wisconsin, the furnace sits right below the AC system.So if your furnace leak seems to pop up in the summer months, chances are that the furnace itself is not the one leaking but rather the air conditioner is leaking on top of the furnace. 

Here are some of the things related to the air conditioner that can make it look like the furnace is leaking:

  • Plugged air conditioner drain lines
  • Rusted or leaking evaporator coil pan
  • Frozen AC evaporator coil 
  • Plugged furnace filter 

You can learn more about air conditioner leaks in this guide. 

What is the best step to take when you have water leaking from your furnace? 

Call the professionals at Austin Plumbing, Heating & Air who have been serving Southeastern Wisconsin for 80+ years strong. When it comes to furnace and heating system repairs, Austin is the most trusted because of our commitment to craftsmanship, integrity and serving our clients. To book expert HVAC leak repairs, call 262-367-3808 today.

About The Author: Eric Smith
Eric Smith is a 3rd generation State of Wisconsin Master Plumber, Water Well Pump Installer, Plumbing Contractor, Water Well Contractor, HVAC Contractor and Water Treatment Expert and the owner of Austin Plumbing, Heating & Air.