Furnace water leak explained

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.

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.

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

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.

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 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.