Why Water Is Leaking From Your Furnace and How to Fix It

Leah Lopez Cardenas
Updated August 12, 2021
Woman covered in a blanket sitting by the window
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If you see water leaking from your furnace, it's important to address it ASAP to avoid water damage and mold growth

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Noticing your furnace leaking water is a headache no homeowner wants to worry about. However, you shouldn’t despair, as the cause of your leaky furnace could be something minor. Once you understand the source, you can consult a local HVAC contractor to help you make repairs and have your furnace working normally.

Is a Furnace Leaking Water Dangerous?

In rare cases, water leaking from your furnace can be very dangerous because of the risk of electrocution. The most likely danger, though, is the potential for water damage throughout the room or basement where the furnace is placed. It’s important to turn off the furnace, clean up the water leak as quickly as you can, and call an HVAC service professional near you to help you make the necessary repairs.

Common Causes for a Leaking Furnace

Once you identify the cause, an HVAC pro can work to repair your leaky furnace quickly and keep your heater running smoothly. These are a few of the most common reasons your furnace could be leaking.

Condensation Leak

A condensation leak is the most likely reason why your furnace is leaking water.

High-efficiency furnaces—those with an Annual Fuel Utilization Efficiency (AFUE) rating of 90% 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 leads into 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 floor drain becoming blocked.A standard-efficiency furnace—which has a metal exhaust pipe—should not have condensation. If it does, it could mean that the flue pipe is not the correct size. 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 suspect this is your problem, an HVAC expert can diagnose and address the issue. Fixing a furnace is not something you’ll want to DIY.

Faulty Furnace Secondary Heat Exchanger

Detail of a house furnace
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If you have a high-efficiency furnace, the secondary heat exchanger may be the source of water around your furnace. The secondary heat exchanger captures what would otherwise be wasted heat, uses it to heat the house, and as the gases cool, they condense into a liquid.

If this is what’s leaking from your furnace, it could be an expensive fix and might even require a complete furnace replacement. A new furnace costs $5,500 on average but could cost you as much as $6,400. It’s best to schedule an inspection for your furnace so a pro can check for issues with your secondary heat exchanger before purchasing a new furnace. This extra step helps to ensure buying a new furnace is the best next step for you to take.

Issue With Furnace Humidifier

You could also have an issue with your humidifier, which could be leaking inside your furnace.

It’s a good idea to have an annual HVAC service check and furnace tune-up. If you’ve already had yours this year, a faulty humidifier is likely not the culprit of your leak problem because your service technician would have alerted you to the issue early on. 

If you have yet to get that once-a-year tune-up, there’s no time like the present. The sooner you can have an HVAC pro come to inspect the leak, the more likely you can get your furnace patched up and working again.

Internal Drain System Clog

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

What to Do When You Notice Water Leaking From Your Furnace

Leaks can cause huge issues, including damaging parts, flooring, and walls, leading to mold growth. A local heating and cooling specialist should be able to quickly diagnose the source of the leak and offer solutions to your furnace problem.

How Much Does It Cost to Fix a Leaking Furnace?

The average furnace repair cost is around $300, with that amount increasing if you have a gas furnace instead of an electric furnace. HVAC professionals typically charge between $75 to $150 per hour for labor. Due to the complex nature of furnace repairs, it’s safest to hire a qualified HVAC professional to make the repairs for you versus doing it yourself.

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