Furnace Leaking Water Explained

A furnace water leak can result from a number of potential reasons.  (Photo by Angie's List member David H. of Anchorage, Alaska)

A furnace water leak can result from a number of potential reasons. (Photo by Angie's List member David H. of Anchorage, Alaska)

Dear Angie: Why would a furnace leak water at the base? The filter is wet also. – Brent T., Indianapolis

Dear Brent: There could be a few reasons why your furnace is leaking water, but let’s start with the most likely one: a condensation leak.

High-efficiency furnaces – those with an Annual Fuel Utilization Efficiency (AFUE) rating of 90 percent or higher – have cool exhaust, and as a result, they produce condensation. An easy way to tell if you have a high-efficiency furnace is to look at its vent pipe. If the pipe is white plastic (PVC), you have a high-efficiency furnace.

Furnace condensation

Typically, the condensation from a high efficiency gas furnace is channeled to a floor drain. Your leak could be a result of the condensation tubing becoming clogged or from breaks in the line. It could also be a result of the drain becoming clogged.

A standard-efficiency furnace – which has a metal exhaust pipe – should not have condensation. If it does, it could mean that the flue pipe was incorrectly sized. That could allow the hot exhaust to cool down and condense in the pipe, then drain back to the furnace and leak out.

Faulty furnace secondary heat exchanger

If you don’t have a furnace condensation issue, it’s possible that the secondary heat exchanger is the source of water around your furnace. Let’s hope not, though. That could be an expensive fix and might even require a complete replacement of your furnace.

Issue with furnace humidifier

You could also have an issue with your humidifier, which could be leaking inside your furnace. If you’ve had an annual HVAC service check, as you should, this is an unlikely scenario because your service technician would have alerted you to the issue early. If you have neglected that service, you should call for it now because if this is your problem, the leak could do a lot of internal damage to your furnace.

Internal drain system clog

There is another possibility. If your air conditioning unit is still operating and it shares an internal drain with your furnace, you could have an internal drain system plug, which is sending water to the furnace.

While we can’t diagnose your issue precisely, don’t let this go too much longer. Leaks can cause huge issues including damaging parts, flooring and walls and lead to mold growth. It’s a clear sign that you need to call in professional help. A reputable heating and cooling specialist should be able to quickly diagnose the leak and offer solutions to your water furnace problem. Do your homework and get more than one estimate before you act, but get started now.

Angie Hicks is the founder of Angie’s List, a provider of reviews you can trust on contractors, doctors, dentists and other service professionals. More than 1 million consumers across the U.S. use Angie’s List to help make tough hiring decisions easier.

This is an updated version of a story originally posted on Dec. 12, 2011.

More Like This

Understanding furnace efficiency ratings


Check the big yellow tag on your furnace for the energy efficiency rating. (Photo courtesy of Angie’s List member Cynthia M. of Springfield, Mass.)
Check the big yellow tag on your furnace for the energy efficiency rating. (Photo courtesy of Angie’s List member Cynthia M. of Springfield, Mass.)

Furnace energy efficiency ratings don't tell you how much your bills will be, but they tell you how well your gas or electric furnace uses energy in your home.


My Window AC unit is leaking water inside my window. Why? I just bought it last summer.

The issue may lie here

How do you change the gaskets on pipes output and input to a furnace? thank you,

Have you found the problem? I have the same issue except my gas furnace is located in the basement. When we turn off the whole house humidifier water line, the water seems to dry up considerably. Was the water line installed wrong? Thanks for any input.

This has not been mentioned as a source of the water. If the furnace and AC use a condensation pump to discharge the water, the pump may have failed or the discharge line may have become clogged. My condensation pump is an older unit and does not have a safety switch to shut down the furnace or AC if water overflows the reservoir so it just overflows onto the floor.

my pips froze and it burst i replace the pipe now when i turn on my furnce the next day my furnce is flooded with water wat could that be.

I have a high efficiency furnace in my unheated attic. I have been told that it is not as high efficient as others because of the unheated location. It has a small white plastic PCV pipe to channel condensation to a drain. The Exhaust pipe on the other hand is metal (aluminum) and travels on a slightly upward angle into a nearby chimney to vent. The heating system works fine and has done so for years. My problem is that I have water leaking from the wall only where the chimney and wall meet, nowhere else in the attic. Up until now my assumption was that there was a roof leak where the roof and Chimney meet like the flashing not attached to the chimney properly. I have had the roof patched previously and thought that it fixed it, but this year with the extensive snow, the leak reappeared. My question is: could the exhaust from the furnace be causing the water problem? The water leak appears 3 to 4 feet below where the exhaust pipe connects to the chimney. I don't know why the installer of the high efficiency furnace didn't use a PVC vent pipe directly out the wall instead of going into the chimney.

Add comment


What is Angie's List?

Angie’s List is the trusted site where more than 3 million households go to get ratings and reviews on everything from home repair to health care. Stop guessing when it comes to hiring! Check Angie’s List to find out who does the best work in town.

Local Discounts

Daily deals up to 70% off popular home improvement projects from top-rated contractors on Angie’s List!