Furnaces | 6 Min.

Why Is Your Furnace Leaking Water?

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A furnace leaking water can be a cause for concern, but it is not necessarily a reason to panic. Your furnace is a crucial part of your home’s heating system, and water leaks can be indicative of minor issues that can escalate into more significant problems if left untreated. In this comprehensive guide, we will discuss the potential causes of furnace leaks, their solutions, and how to prevent them in the future.

Types of Furnaces

Before diving into the reasons behind furnaces leaking water, it is essential to understand the two primary types of furnaces: high-efficiency condensing furnaces and conventional furnaces. Each type has its unique characteristics, which may contribute to water leaks differently.

High-Efficiency Condensing Furnaces

High-efficiency condensing furnaces, also known as condensing furnaces, have an Annual Fuel Utilization Efficiency (AFUE) rating of 90% or above. These furnaces use a secondary heat exchanger to extract additional heat from the exhaust gases, which increases their energy efficiency. The process of extracting heat from the exhaust gases creates condensation, which is typically drained away through a condensate drain line.

Conventional Furnaces

Conventional furnaces, or standard-efficiency furnaces, have an AFUE rating of around 80%. They use a single heat exchanger and do not produce condensation like their high-efficiency counterparts. Conventional furnaces vent exhaust gases through a metal pipe and do not typically experience water leaks due to condensation.

Types of Furnace Leaks

There are three main causes of water leaks in furnaces:

Condensation leaks – Common in high-efficiency furnaces, these leaks occur due to damaged or clogged tubing, or a clogged floor drain.

Malfunctioning secondary heat exchanger – If your secondary heat exchanger is malfunctioning, it can cause a costly repair or may require a furnace replacement.

Broken humidifier – If a leak is coming from inside your furnace, the humidifier may be the culprit. Regular maintenance can help detect a leaking humidifier before it becomes a major issue.

Common Causes of Furnace Leaking Water

Now that you understand the two primary types of furnaces let’s explore the possible reasons for water leaks in both high-efficiency and conventional furnaces.

1. Water Condensation

Water condensation is the most frequent cause of furnace water leaks, especially in high-efficiency condensing furnaces. As the furnace extracts heat from the exhaust gases, condensation forms and needs to be drained away. If the condensate drain line becomes clogged or damaged, water may pool around the furnace, leading to a leak.

2. Blocked Air Return

A blocked air return can also cause furnace water leaks. If the air duct leading from the furnace to the outside of your home becomes clogged with dust, debris, or dirt, the air coming through it may no longer be adequately filtered by your furnace. This can lead to overheating and potential leaks.

3. Blockage in the Vent Pipe

A blockage in the vent pipe of your furnace can cause water leaks. If the vent pipe becomes clogged, the exhaust gases cannot be expelled from your home, leading to condensation buildup and potential leaks. Ensure that the vent pipe is clear of any debris and that there are no signs of condensation around the vent or in the return air duct.

4. Blocked or Damaged Pipe

A blocked or damaged pipe running from your furnace to the upper floor of your home can cause water leaks. If there is a crack in this pipe or if condensation has built up around it, water may leak from the furnace onto the floor below.

5. PVC Drain Line Plugged

A plugged PVC drain line can cause your furnace to leak water. The drain line carries excess water away from the furnace. If it becomes clogged, water may start pooling around the furnace and cause a leak. Clearing the line may involve pouring a mixture of hot water and white vinegar through the line, flushing it several times if necessary.

6. Filter Plugged Up

A plugged-up filter can cause furnace leaks. A dirty air filter forces the furnace to work harder, pushing air through the house and leading to condensation buildup around the filter. This can cause water to leak out onto the floor below. Replacing a clogged filter can often resolve the issue.

7. Drain Pan Has a Leak

A leak in the drain pan of your furnace can cause water leaks. The drain pan is responsible for collecting condensation and draining it away from the furnace. If the pan becomes clogged or damaged, water may overflow and leak from the furnace.

8. Problems with the Condensate Pump

A malfunctioning condensate pump can cause furnace leaks. The pump is responsible for pushing the condensation out of the unit and into the drain pipe. If the pump fails, it can cause water to back up and leak from the furnace. A professional may need to replace the pump, which typically lasts between 3 to 5 years.

9. Broken Furnace Humidifier

A broken humidifier can cause furnace leaks. Whole-house humidifiers attached to furnaces use water to add moisture to the air. Leaks can occur when water lines crack, break, or become clogged due to a lack of maintenance. Calling an HVAC professional to address the issue is the best course of action.

10. Malfunctioning Secondary Heat Exchanger

A malfunctioning secondary heat exchanger in a high-efficiency furnace can cause water leaks. The secondary heat exchanger extracts additional heat from the exhaust gases, which creates condensation. If the exchanger is malfunctioning, it may cause a condensation leak that will need to be addressed by an HVAC professional.

12. Bad or Broken Blower Motor

A bad or broken blower motor can cause furnace leaks. If the blower motor is not working correctly, it can cause condensation to build up inside the furnace, leading to water leaks. An HVAC professional will need to address the issue and repair or replace the blower motor if necessary.

Identifying the Cause of a Condensation Leak

To determine the cause of a condensation leak in your high-efficiency furnace, follow these steps:

If you have a standard-efficiency furnace and notice condensation, contact a certified technician immediately.

How To Fix Condensation Leaks In Furnace?

Once you’ve identified the cause of the condensation leak in your furnace, it’s time to fix the issue. Here are some steps you can take to resolve the leak:

Damaged or Clogged Tubing

Regular Furnace Maintenance Can Help Prevent Leaks

One of the most effective ways to prevent furnace leaks and other issues is by scheduling regular maintenance with an HVAC technician. It is recommended to have your furnace and air conditioner tuned up once a year – ideally, before the heavy usage seasons of winter and summer.

Maintenance Agreements

Investing in a maintenance agreement with an HVAC company can help keep your system in top shape and prevent issues before they become serious. Maintenance agreements often include services such as equipment tune-ups, filter replacements, and evaporator or blower cleanings. These services can extend the life of your system and improve its overall efficiency.


If you notice water pooling around your furnace, do not hesitate to take action. Address the issue promptly by following the steps outlined in this guide and contacting an HVAC professional if necessary. By taking care of furnace leaks, you can prevent costly repairs and ensure a comfortable and healthy living environment for you and your family.