“Why Is Water Leaking From My Furnace?”

A common call we get from Westfield homeowners is “Why is water leaking from my furnace? ”The cause? A furnace could leak water for 4 different reasons:

  1. Clog or leak in the condensate line
  2. Broken condensate pump
  3. Undersized flue vent
  4. Issue with the whole-home humidifier

For all these issues, you’ll need to call a furnace repair technician.

But if you’re curious about what these issues are in more detail, read on.

As a bonus, you’ll also see how much it would cost to fix a clogged condensate line—the most common cause of this issue.

Cause #1: Clog or leak in the condensate line

High-efficiency furnaces (90+ percent AFUE) create water when they run (we’ll explain why in a bit). That water needs to be drained out of the home via a PVC pipe called the condensate line.

However, if that line becomes clogged, the water backs up into your home and pools around the furnace. Or if the line breaks the water will leak around the furnace.

One of our Westfield customers, Annie M., had the “clogged” issue. After our technician, Kenny, ran a full system diagnostic, he found the condensate trap (part of the condensate line) was not draining properly.


The solution: Kenny flushed the condensate trap and lines to clear them so the condensate can properly drain.

Why do high-efficiency furnaces create water, anyway?

High-efficiency furnaces create water because they have something that normal furnaces don’t: A secondary heat exchanger.

What’s that?

Well, a primary heat exchanger extracts heat from combustion gases and then imparts that heat to your home’s air. A secondary heat exchanger can extract even more heat from the combustion gases before sending it out the exhaust pipe.

Because the secondary heat exchanger steals so much heat, the combustion gases lose enough energy to condense into a liquid. That liquid is what may be pooling around your furnace.

The condensate line isn’t the only cause of this issue. It could be that the…

Cause #2: Condensate pump is broken

Normally, to let condensation exit out the condensation line, your furnace would let gravity do all the work. But if your furnace is located in a basement, that may not be possible.

Enter the condensate pump. It does what it says on the tin: pumps condensation from the furnace to outside of your home.

If the pump breaks, then the condensation will simply build up and spill over into your home.

Cause #3: Undersized flue vent

Mid-efficiency furnaces can create condensation if their flue vent is undersized. The flue vent’s job is to vent out all flue (combustion) gases from the furnace.

However, the the flue vent is undersized, the flue gases have trouble escaping, causing them to lose energy and condense into a liquid. That liquid can then leak into your home.

Cause #4: Issues with the whole-home humidifier

Do you have a humidifier connected to your furnace? If so, that may be the problem. These humidifiers are connected to your home’s plumbing system and use water to add moisture to your air. So if your humidifier clogs or cracks it can leak water onto your furnace, making it look like that your furnace is leaking water.

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