Why Is My Furnace Leaking Water? (In Summer or In Winter)

Water leaking from your furnace can be caused by many different issues, some are easy to correct, and some are more difficult.

  • Does it leak when AC is on – in summer? There’s a set of causes to explore during warm weather.
  • In winter? There are other potential issues to troubleshoot.
  • Are you running a humidifier?
  • Have a high-efficiency furnace?

You get the point. This furnace leaking troubleshooting article will get to the bottom of the question – Why is my furnace leaking?

It covers the most common causes of leaks that occur, both in the winter and summer, and leaks from conventional furnaces, as well as, from high-efficiency, also known as, condensing furnaces.

We will also detail possible fixes you can try yourself or whether an HVAC professional should be called.

High Efficiency Furnace or Standard Furnace?

High efficiency furnaces are 90% efficient or higher. Standard furnaces are usually 80%.

First, you’ll need to know if you have a high-efficiency condensing furnace or a conventional furnace. If you see a white PVC flue pipe coming from the side or top of the unit, you have a high-efficiency condensing furnace. If you see a metal flue pipe, you have a conventional furnace.

Leaks When the Furnace is On – Winter Furnace Leaks

Let’s discuss possible causes of the furnace leaking water for both types of furnace.

High-Efficiency Furnace is Leaking

What is a high efficiency condensing furnace?

When fuel burns, it creates combustion gases including carbon monoxide and water vapor. As these gases cool, the water vapor turns into condensation. In a conventional furnace, the exhaust is still very hot when vented out the flue, so moisture is vented in the form of vapor.

A high-efficiency condensing furnace uses two heat exchangers. The primary heat exchanger transfers the heat from the air from the combustion chamber then moves the air into the secondary heat exchanger removing even more heat before it is vented out the flue. Because so much heat is removed the water vapor condenses and must be drained from the furnace.

Why is My High-Efficiency Furnace Leaking Water?

You might also ask, what is water doing inside my furnace when it is heating?

High-efficiency furnaces produce a significant amount of moisture through condensation. Water leakage occurs when the moisture is unable to drain properly. Most of these leaks are caused by clogged, cracked, or disconnected drain lines and can usually be fixed by a homeowner. Following are specific causes of leakage and what can be done to remedy them.

Blocked Condensate Drain Hose or Drain Line

These drain lines can become blocked with debris, dirt, mold, and dust. The lines may also be damaged, cracked, or disconnected. The lines will need to be cleaned out or replaced. In some cases, the lines can be opened by pouring hot water into them and in other cases they can be vacuumed out.

Clogged Condensate Trap

A clogged condensate trap or p-trap will cause water to back up and overflow out of the furnace, although not all high-efficiency furnaces have a condensate trap. This video shows how to clean out a blocked condensate trap.

Malfunctioning Condensate Pump

High-efficiency furnaces usually drain downwards, but if the water has to go upwards because there is no basement drain, then a pump is necessary to drain the water out of the furnace. If the pump isn’t working, the water will overflow and leak onto the floor. The pump may just be clogged and need cleaning or it may need replacing. Here is a video on troubleshooting a condensate pump.

Cracked Secondary Heat Exchanger

A cracked heat exchanger will allow the water vapor to exit the exchanger and condense, causing a leak. A cracked heat exchanger can also allow carbon monoxide to leak which is extremely serious. If you think your heat exchanger is leaking water, and/or the carbon monoxide detector goes off, shut off the furnace, and call an HVAC professional.

Conventional Furnace is Leaking

Water leaking from a conventional furnace while the heat is on can be caused by several things. Here are the most common reasons and what can be done to fix the problem.

Exhaust or Flue Pipe

A flue that isn’t installed or fitted properly can be the source of a leak. Water vapor is a by-product of fuel combustion but water vapor is not supposed to condense inside a conventional furnace. The water vapor should exit the home with the combustion gases, through the exhaust flue. If the flue is restricted, cracked, dislodged, or not fitted properly, the exhaust can cool and condensation will form inside the flue and create a water leak.

Inspect the flue looking for any signs of water, cracks in the pipe, or whether it’s loose or dislodged. Check for anything blocking the flue including animal nests and remove them. If the flue needs to be repositioned or replaced, you will need an HVAC professional.

Leaking from the Humidifier

A built-in humidifier on your furnace is a common source of leakage. The filter in a humidifier is known as a water panel or evaporator and can become clogged with hard water residue or debris. Water is distributed onto the water panel where air flows over it and picks up moisture, distributing humidity throughout your home. When the water panel is clogged, it cannot absorb the water and it will run out, creating a leak.

Changing the water panel and cleaning out the drain should resolve the leak. This is an easy fix, in fact, it’s part of normal maintenance and should be detailed in the owner’s manual.

Humidifiers have drain lines to get rid of excess water not picked up by the pad. It can also become blocked. When you remove the humidifier cover, you can see the drain hole. If the line is blocked, the water can’t drain. You can try pouring hot water down the drain or blowing it out. Here is a video on clearing out the humidifier drain line.

A Cracked Heat Exchanger

If the heat exchanger is cracked, the combustion gases, including carbon monoxide, and water vapor may leak out before they can exit through the exhaust. The water vapor will then condense and cause the leak. A cracked heat exchanger is a very serious situation, the furnace should be shut off and an HVAC professional should be called.

Leaks When Air Conditioning is On – Summer Furnace Leak

Water around your furnace in the summer, when the furnace isn’t running, is probably coming from your air conditioning coil in the furnace. Air conditioning removes humidity from the air, condensing it on the evaporator coil. The water then drains away through drain lines.

Note: Water can leak from the air conditioning in either a high-efficiency furnace or a conventional furnace.

There are a number of issues that can cause water to leak from an air conditioning unit. Below we cover some of the most common reasons and what you can do to fix them.

Damaged Drain Pan

All air conditioning units have a primary drain pan located under the evaporator coil. Some air conditioners have a secondary drain pan located beneath the unit. The purpose of these drain pans is to catch any condensate. If these pans develop a crack or hole, water will leak.

Check the drain pans for damage and repair or replace them.

A Frozen Evaporator Coil

The evaporator coil in an AC unit can freeze and become covered with ice. As the ice melts, a leak will occur.

Tip: Lack of air flow due to a clogged air filter is the most common reason that evaporator coils freeze. Turn off the air conditioner and allow the ice to melt. Check and replace the filter if it’s clogged.

Check other causes of poor airflow: Lack of air flow can also be caused by blocked air intake, blocked vents, damaged ductwork, and a malfunctioning blower fan.

If a new filter doesn’t keep the coil from freezing, the next step is to check the drain lines from the unit because water backing up can also cause the evaporator coil to freeze.

There are a number of other reasons an evaporator coil can freeze including low refrigerant, malfunctioning blower fan or motor, and a broken drive belt. In these cases, you’ll need to call for professional repair.

Tip: Have towels and/or a mop ready for when the frozen coil thaws! There could be a lot of ice, and you’ll want to soak up that water as it flows off the frozen coil.

Clogged Drain Pan or Line

An AC unit produces water as it dehumidifies the air. Drain lines allow the water to exit the unit. If the lines are plugged, the water cannot drain and will leak and pool around your furnace.

Debris, dust, algae, and mold can form blockages in the drain lines. Clearing the blockage can be an easy fix, depending on the unit and the type of drain lines you have. Sometimes there is a cap on the main line which can be removed and hot water poured down the line to release the clog. Some lines can be vacuumed out to remove the blockage and some drain lines must be cut to be cleaned out.

If you are handy, you can probably accomplish these fixes yourself. There may be instructions in your owner’s manual or look online for a video to guide you through the process. Here is a video demonstrating drain line cleaning. If your drain line is different than the one shown, check out this video tutorial on how to clean a furnace drain line.

Malfunctioning Condensate Pump

A condensate pump is used to collect and remove the water produced by an HVAC system that is located in an area where the liquid can’t be drained by gravity. If you have a high-efficiency furnace, the furnace and the air conditioning unit will probably share the same condensate pump. Troubleshooting a condensate pump is detailed above under High-Efficiency Furnace.

Get Advice and Help

If you’re still stumped about why your furnace is leaking water – or you figured it out and want a pro to fix it – help is on the way.

Use the convenient toll-free number on this page or the quick Free Local Quotes form, and you’ll be contacted by prescreened, licensed pros in your area. They’ll provide free estimates and advice on the best solution to the problem of a leaking furnace, whether it is in Heat mode or AC mode.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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