Why Is My Furnace Leaking Water?

Your furnace is leaking water, and you need to know what to do next. You can’t just let it spread because it could ruin your flooring or ceiling. And maybe there are no service companies that can get to your home until the next day. You need something done quickly, so we have put this article together to give you some solid information about why your furnace might be leaking and what you can do about it.

What is causing the leak?

Clogged condensation line

There are many things that can cause the condensation line to become clogged, but the most common reason will be algae buildup. Your indoor coil (the component inside the unit that looks similar to a car radiator) is meant to be cold during your home’s cooling cycle. Because it’s cold, it collects condensation from the humidity in the air. This creates a cool, damp, dark, enclosed space that is perfect for algae growth.

Depending on the orientation of your furnace, your coil may have a large shallow pan under it if the unit sets horizontally. If the unit is vertical, it will be a trough-like structure that wraps around the entire base of the coil. All forms of pans will need to have a drainpipe connected to them for the condensation to leave your home. As the algae forms in the pan under the coil, it will begin finding its way into your drain piping. Once it is in your drain piping, it continues to grow until it blocks the pipe, which will lead to the water backing up. If the water backs up enough, it will leak over the edges of the pan. 

Condensation on piping

Another cause of excess water around your unit is the larger copper pipe connected to your coil. This copper line is the larger of the two and is called the suction line. It flows refrigerant through it from the indoor coil to your outdoor unit. If this copper line is not insulated, it can create a surprising amount of condensation.


Condensation on ductwork

If supply ducting is uninsulated, it will commonly cause condensation. Your ductwork has a large amount of surface space and can create a lot of condensation. If the supply ducting is exposed to a conditioned space, such as running across your living room ceiling, it is not required to be insulated.

Broken or cracked drain pan

If there has been any damage to a drain pan, it won’t be able to do its job and hold any collected condensation until it evaporates.

Damage or leaks in the drain line

If the leak is occurring further away from the unit, it’s possible that the drain line itself is leaking.

Condensate pump not working

When gravity isn’t enough to move water from the condensate pan to a drain or to the exterior of your home, a condensate pump is required. If this fails, water cannot be adequately removed from the drain pan and will cause it to overflow.

How to determine what the cause is?

With water leaks, it is usually pretty easy to determine the cause. If you follow the leak back, you should be able to tell where it is coming from. For example, if you see water running down the side of your furnace, you follow it up to the coil case. So, you take the coil cover off and see that the pan under the coil is full of water. You know that the drain is clogged. 

The same concept will go for the duct or copper piping. If you follow the leak back and determine that the highest point of water residue is the copper piping or ductwork covered in water during your inspection, you will know that a specific item is the culprit.

What steps to take?

Many of the steps you can take are relatively easy and can be done by someone without a lot of experience.

If the issue is a clogged pipe, the most common fix is taking some PVC cutters and cutting the line before the trap. Make sure there is room to connect coupling on both sides of the pipe you plan to cut, which is generally made out of ¾” PVC pipe.

Once you have the pipe cut, you can blow through the drain. Your lungs should be powerful enough to blow it out if it is just algae causing the blockage. Install a coupling and glue the drain back together.

You can prevent algae growth by using an HVAC-specific product that will help clean your drain. Or you can use a small chlorine tablet meant for a pool. These options will eat all of those algae and keep your drain from clogging again. 

If the leak comes from condensating copper lines or ductwork, somewell-placed insulation will solve the problem.

You could also consider a drain pan switch to prevent condensation pan overflows. When the pan fills to a critical level, the switch will turn your AC unit off until the pan can drain or the water evaporates.

Summer vs. Winter


The moisture level within your home is a key component of your comfort level on a hot day. The higher the humidity, the less comfortable the heat can be. An air conditioner removes the water from the air by collecting it as condensation on the coil. It then slides down to the drain pan and out the drainpipe.

Suppose the unit develops air restrictions, like a dirty filter or coil, for example. If the unit does not have adequate airflow, the coil will start to freeze and develop a thick layer of ice around it. Once the ice begins to melt, it is likely that it will not all go into the drain but will drip wherever it can, which will make your unit appear as though it is leaking.


If you have a high-efficiency furnace, it can also leak in the winter due to condensation. This only applies to high-efficiency furnaces with PVC exhaust vents. If you have an older, less efficient model that still utilizes a metal vent, it will not condensate.

If your high-efficiency furnace begins to show signs of leaking water, the most common reason is that the drain has frozen. When your high-efficiency furnace drains outside, it has a much higher chance of developing a frozen drain. As the water tries to escape outside, it freezes at the end of the pipe. As the ice builds up in the drain, less water can escape from the furnace, resulting in a leak.

Why could my furnace be leaking in the winter?

Your high-efficiency furnace will condensate year-round through the coil condensate drain line in the summer and the secondary condensate line during the winter. Older and less efficient furnaces should not have any condensation in the winter.

If your system is not draining correctly or begins to freeze, you can quickly develop a leak during the winter months. An excellent way to troubleshoot if your high-efficiency furnace has a backed-up drain line is to try and turn the unit on. If you can hear your inducer fan turning water inside of your unit, then more than likely, it has a backed-up drain. If the water has backed up enough to hear it, it will probably not run either.

Word of caution about high-efficiency furnaces

These furnaces are identifiable by the PVC exhausts. They condensate year-round and cannot be installed in locations affected by the cold, such as an attic or an unheated room. However, crawl spaces and basements are acceptable because the area retains the heat from the ground and home’s floor and rarely gets below freezing. If the condensation freezes inside the unit, it can cause severe damage.

Why is my furnace leaking when the AC is off?

The drain might be leaking when the AC turns off because the coil had inadequate airflow from a dirty filter or coil. It could also be due to a low refrigerant level. Either of which could cause ice to build upon your coil. Once you turn the furnace off, the ice that has formed will melt, and excessive moisture will flow into the pan. If your drain is clogged or the ice has blocked the drain, this can cause the water to overflow from the pan.

Wrap up

Whenever we have cold components in a warm and humid environment, condensation occurs. This condensation is the cause of the majority of water or leaking issues that occur with your furnace.

Leave a Comment

Your email address will not be published. Required fields are marked *