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Why Is My Furnace Leaking Water? 7 Common Causes

Furnaces are installed in a building to provide heat in the interior space. So, a leak is certainly not part of a furnace’s heating function and is indicative of some dysfunction. While the issue with your furnace may not be cause for alarm, it should nonetheless be addressed immediately.

If your furnace is leaking water, you may have a clogged condensate drain, a faulty home humidifier, a broken air exchanger, a slim drain pipe, or a faulty condensate pump. These problems usually happen with high-efficiency (condensing) furnaces and less with standard-efficiency (conventional) ones. 

In this article, we’ll discuss all the possible causes of water leaking from your furnace. We’ll also tell you what you should do to resolve the issues.

You Have a High-Efficiency Furnace

Condensation is the most common reason a furnace will be leaking. But that depends on the type of furnace that you have.

If you have a high-efficiency furnace, you have a condensing furnace, and any faults in the condensate drain can cause your furnace to leak water.

If instead, you have a standard-efficiency Furnace, you have a conventional furnace that shouldn’t have condensation and, therefore, should not leak unless from other non-condensation issues.

You can tell if your furnace is a condensing furnace or a conventional furnace by looking at the exhaust pipe or vent. Condensing furnaces usually have a PVC white plastic exhaust pipe, while conventional furnaces have a metal pipe.

Also, high-efficiency furnaces have an Annual Utilization Efficiency (AFUE) rating of 90% or more. Standard-efficiency furnaces have an AFUE rating below 90%. You can check your furnace’s AFUE rating on the yellow energy guide fixed on the unit or simply look at the pipe.

The bottom line is that a high-efficiency furnace is more likely to leak water from the condensation process, while water leaks from a standard-efficiency furnace are likely caused by a different issue.

Your Furnace’s Condensate Drain Is Clogged

The condensate drain is the exit route for the moisture formed when the evaporator coil removes humidity from air passing over it. You will find it in the utility closet or the attic covered by a removable panel.

The condensate drain sends out the moisture from the furnace through a ground drain. However, if the drain is clogged, this moisture is retained in the furnace, causing a moisture buildup and a possible water leak.

You Have a Wrong-Size Condensate Pipe

A furnace’s exhaust pipe should be wide enough and properly fixed to allow the moisture and other gases to exit the furnace before they condense and turn into water. 

If the exhaust pipe is too thin or has bends that block the exit of the moisture and gases, these will turn into water before they are expelled, and water accumulation will lead to a leak.

The Furnace’s Condensate Pump Is Faulty

Some high-efficiency furnaces are fitted with a condensate pump, which looks like a box connected to the exhaust pipe. The condensate pump is meant to direct moisture into the drain before it can cool down and turn into water.

If the condensate pump is broken, the moisture is retained in the furnace and eventually finds an exit path, which is why you’ll notice a water leak.

Your Home Humidifier Has Malfunctioned

Another cause of the furnace leaking water is a faulty home humidifier. A whole-house humidifier is supposed to add moisture to the air in your home to prevent dryness and discomfort.

If you have a house humidifier connected or attached to your furnace, it might malfunction and leak moisture to the furnace, which in turn leaks from the same moisture. 

Note that this may happen whether you have a high-efficiency or a standard-efficiency furnace.

You Have a Clogged Shared Drain

If your furnace leaks even when it’s not in use and shares the same drain with the air conditioner, the most likely cause is that the shared drain is clogged. So, when the air conditioner is running, the moisture is not correctly drained but instead leaks back to the furnace, causing it to leak in turn.

You Have a Crack or Hole in the Heat Exchanger

A heat exchanger is an essential part of the heating process in a furnace. Its function is to separate the breathing air that comes into the furnace and the heated air from the furnace that is distributed into your home.

Because heat exchangers are made of metal, they will expand and contract continuously while playing their function, which adds up to wear and tear and eventually breakdown. You can easily make out if your heat exchanger is worn out by observing and noticing the holes or cracks.

A dysfunctional air exchange facilitates the retention of moisture in the furnace and can be the culprit causing a water leak.

You Have a Clogged Furnace Filter That Is Causing a Coil Leak

A water leak from a clogged furnace filter may sound impossible. But if your home has a central air conditioner, you have a coil above the furnace to suck the heat from the air and cool it for your home.

If the filter is clogged, the air flowing over this coil is limited and can cause it to freeze, consequently sending water into your furnace below and causing a water leak.

What to Do About Water Leaks From a Furnace

If your furnace is leaking water, you have the option of troubleshooting easy-to-resolve problems or calling an HVAC professional.

Troubleshoot easy furnace issues that cause leaks by:

  • Unclogging blocked exhaust pipes or shared drainages to create a clear exit for gases and moisture from the furnace.
  • Passing soapy water with a bit of bleach through the exhaust pipe to remove dust and other forms of filth from the pipe surface. These reduce the pipe’s diameter and allow water to stick to the sides.

If you are not sure about the cause of the leak or are afraid, you might cause more trouble to your furnace.

Call an expert to:

  • Remove and small-size exhaust pipes that cause poor moisture removal and replace them with the recommended size.
  • Replace old or damaged furnace parts such as a condensate pump or secondary heat exchanger. 
  • Regularly service and clean your furnace filter so that proper airflow is not compromised. 

Above all, stick to the regular maintenance of your furnace to ensure all parts of your heating system work properly. This should necessarily be done before the long winter use of the furnace. 


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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