top of page

How to Fix Your Furnace from Leaking Water?


A leaking furnace can be a headache.

It can lead to water damage and can be a reason your ac is not working.

If you are wondering why your furnace is leaking water and how to fix it, this guide is for you.

Jump to Section

  1. What causes your furnace to leak water?

  2. Signs of Leaking Furnace

  3. Safety first

  4. Water leaking when AC is on

  5. Water leaking when the heater is on

  6. Faulty condensate pump

  7. Dangers of leaking furnace

  8. Conclusion

What Causes Your Furnace to Leak Water?

There are a few reasons why your furnace could be leaking water.

It can happen while your AC is working or even if your heater is on.

The more common reason your furnace is leaking is when your AC is on.

This might be confusing to grasp because the leak is coming from the furnace. However, this is not entirely true. The evaporator coil is part of the AC system but it is located right above the furnace.

Let’s dive into what causes your furnace to leak water and what you should do in each case.

Signs of a Leaking Furnace

There are a few signs your AC is leaking and if you are here you probably noticed them.

They are:

  1. Water build-up or rust on and around your furnace

  2. Water spots on your ceiling near the furnace location

  3. Your AC or furnace stopped working (safety features)

Safety First

You should always be sure no electricity is reaching your furnace as you may get an electric shock if you are not careful and aware of what you are doing.

Furthermore, many manufacturers’ warranties are void if you tamper with your system.

Therefore you should always be aware and contact an HVAC professional. This will ensure things are done safely and can help prevent any void in warranties.

Frozen Evaporator Coil When AC is Running

The most common reason your furnace is leaking is that you have a frozen evaporator coil and a clogged ac drain line.

This combination can cause your furnace to leak. Leading to water damage in your home or your HVAC system.

An evaporator coil generates condensation. This condensation can freeze up into a block of ice.

As the ice defrosts the water needs to drain through your AC drain line. If the AC drain line is clogged, it will lead to your furnace leaking water.

You can check out this guide on how to unclog an AC drain line.

A frozen evaporator coil is a sign your HVAC system has other problems as well. You can read more about why your AC system is freezing up here.

Leaking Furnace When Furnace is Working

If your furnace is leaking water when the heater is running, you have a high-efficiency furnace.

High-efficiency furnaces produce condensation as they operate.

The condensation generated is designed to make its way to the drain. A leak can occur anywhere on the way.

A clogged condensate drain hose, for example, can back up the water causing it to leak.

You can disconnect the drain hose and ensure it is clear, and allowing the water to flow through it.

You may also have a leak in your inducer assembly. Which can happen because of a blocked drain hose. A damaged inducer can be expensive to replace.

The best way to avoid this is by having regular maintenance for your HVAC system.

Regular maintenance will ensure your drain hose is clear, and your high-efficiency furnace is draining correctly.

In rare cases, a low-efficiency furnace can leak as well.

As the exhaust air goes through the FLUE vent pipe it may condensate if the temperature difference is large enough.

Condensation occurs when there are extreme temperature differences.

One way to avoid this is to have a double-walled or insulated FLUE vent pipe.

If the FLUE vent pipe condensates, it can make its way to the inducer motor causing damage to it as well.

If your furnace is leaking water internally, it can be very harmful to your system.

In some cases, you will need to replace an inducer motor and in others, you will need to replace the whole control board. These repairs can get costly.

The best way to avoid this is by having regular maintenance on your furnace which will ensure it is operating correctly.

Faulty Condensate Pump

Another reason your furnace may leak is if you have a condensate pump that isn’t working.

A faulty condensate pump can overflow and leak. Or it can back up the line and cause leakage inside the furnace itself.

Not all HVAC systems have a condensate pump.

Usually, if your system is located in a basement or where gravity can not be used to drain out the water from the system, then you will have a condensate drain pump in your HVAC system.

If you have a faulty condensate pump, you will need to replace it.

You can either buy one online or contact an HVAC professional to get one installed.

Dangers of leaking furnace

There are a few dangers when dealing with a leaking furnace.

The first is electrical danger.

Any time water and electricity mix, it can be dangerous.

The HVAC system has many electrical components such as control boards and AC capacitors.

So be careful when dealing with a leaking furnace.

The other danger involves damage to your heat exchanger.

Although very rare, if you live in a humid area, your heat exchanger may begin to form rust.

Making it more susceptible to cracking due to the extreme temperatures they reach.

If your heat exchanger is cracked it is a serious health and safety risk.

It will introduce carbon monoxide into your home from combustible gases.


Most often than not, your furnace is leaking water when your AC is running.

If you are lucky, you will have minimal damage to your home.

Always look for signs of leakage and periodically check your furnace to ensure you don’t see any signs of water or rust.

The best way to ensure your HVAC system is operating correctly is by having a yearly HVAC maintenance check on your systems.

You can contact your local HVAC company and see what offers they have available.

And please, contact us if you have any questions.

bottom of page