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Why Is My Gas Furnace Dripping Water? A Minneapolis Tech Explains

furnace tips and definitions

So you’ve noticed that your furnace is leaking water and you’re wondering what the issue is.
Well, the answer depends on the kind of furnace you have: a conventional furnace or a condensing furnace.

If you have a conventional furnace, the two most likely reasons for dripping water include:

  • A poorly sized flue pipe
  • A leaky humidifier

If you have a condensing furnace, it’s likely leaking due to any of the following issues:

  • A clog or issue in the condensate system
  • Bad/clogged condensate pump 
  • A leaky humidifier
  • A bad heat exchanger

Not sure what kind of furnace you have? Don’t worry, it’s easy to figure out your furnace type. 

We’ll show you how to figure that out and we’ll go over the issues with each one that could cause leaking.

How to tell whether you have a conventional or condensing furnace

You can determine the kind of furnace you have by:

  • Checking what kind of material your exhaust pipe is made of
  • Checking the furnace’s AFUE rating

Let’s look at both of those a little closer...

Exhaust pipe material

Your exhaust pipe is the metal or PVC pipe that connects to your furnace and goes through the roof or side of the house. This pipe directs combustible gases away from your home.

If your furnace has a metal exhaust pipe you have a conventional furnace.

If your furnace has white PVC exhaust pipes you have a condensing furnace.

AFUE rating

The AFUE rating, or Annual Fuel Utilization, measures how efficient your furnace is. You can check your unit’s rating by looking at the yellow Energyguide sheet on the side of your unit.

If your furnace’s AFUE is above 90, you most likely have a condensing furnace.

If your furnace’s AFUE rating is below 90, you most likely have a conventional furnace.

Note: If your furnace doesn’t have an AFUE rating posted on the side, it is most likely a very old unit and is a conventional furnace.

Now that you know what kind of furnace you have, you can skip to the section below that describes the issues that might be causing your furnace to drip water.

Why a conventional furnace leaks water

First off, conventional furnaces were not designed to produce water of any kind. So if it is dripping water, you should have a professional inspect the unit immediately.

There are two main issues that can cause your conventional furnace to drip water:

Poorly designed exhaust pipe

The metal exhaust pipe connected to your furnace is designed to collect and vent away the gases produced during the combustion process.

Those gases are supposed to exit the exhaust pipe and enter the atmosphere before they have time to cool down and condense into water. But if the exhaust pipe is either too large or doesn’t slope upwards enough, it slows down the movement of the gas as it travels outside. 

This allows the gases to cool down and condense into water while still in the exhaust pipe, causing leakage. If this is your issue, you’ll need a professional to inspect your unit and replace or redesign your exhaust pipe.

Leaky humidifier

If you see a smaller device attached to your furnace with hookups for water, electricity and drainage, you have a humidifier on your furnace. These devices help add moisture to the warm air that is blown into your house.

However, issues with the humidifier, such as a clogged filter or drain, can cause the unit to leak. A leaky humidifier is often misdiagnosed by confused homeowners as a leaking furnace.

You’ll need a professional to inspect the humidifier to see what is causing the leak and fix the issue.

Why a condensing furnace leaks water

Unlike conventional furnaces, condensing units are actually designed to produce condensation.

You see, instead of venting away combustion gases quickly (like a conventional unit), a condensing furnace pulls a lot more heat from those gases before allowing them to vent away into the atmosphere.

And of course, absorbing more heat takes more time. Enough time, in fact, for those gases to cool down and condense into water before it leaves the furnace. Normally, high-efficiency furnaces drain away condensation via a draining system. But if that water ends up on your floor, that’s a sign that there may be an issue with your furnace.

Let’s look at some of those issues that can cause your condensing furnace to start leaking water...

A clog or problem in the condensate system

Your high-efficiency furnace has an entire system set up to drain away the condensation it produces. But if there is an issue or a clog anywhere in the condensate line or drain, water will begin overflowing and leaking from the furnace and onto the floor.

Problems with the condensate pump

Some high-efficiency furnaces may use a condensate pump to push condensation toward the proper draining area.

A condensate pump is a small white box connected to the PVC drain pipe and pumps water toward drain. It basically acts like a small sump pump. 

If the condensate pump malfunctions or stops working altogether, you’ll see water dripping or overflowing from the device. Have a professional replace this unit if you see water leaking from the condensate pump.

A leaky humidifier

This issue is a common problem with both condensing and conventional furnaces.

For more information on leaky humidifiers and what to do, check out the “Leaky humidifier” section under “Why a conventional furnace leaks water”.

A bad heat exchanger

A condensing furnace actually has 2 or even 3 heat exchangers as opposed to the single heat exchanger in a conventional furnace.

This allows the furnace to absorb a lot more heat from the combustion gases before they exit the system but it also means that the heat absorption process is much slower.
And by the time those hot gases reach the 2nd or 3rd heat exchanger, they’ve had time to cool down significantly. So the 2nd/3rd heat exchangers usually produce a lot more condensation than the first heat exchanger.

But if the passageway holding that heat exchanger is cracked or damaged, that condensation will find its way onto your floor.

Unfortunately, a bad heat exchanger is a very expensive repair. You should have a professional inspect the heat exchanger and give you professional advice on whether you should replace the part or replace the entire furnace.

Need help from a MN tech?

If your gas furnace is leaking and you live in theMinneapolis-Saint Paul area, give the professionals at MSP.

We offer same-day service to keep you warm through the rough winters.

Just schedule your heating appointment with us today!

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Categories: Heating