Top Reasons Why Furnace is Leaking Water

The most popular central heating systems in the United States, particularly in colder climates, use a natural gas furnace.

These furnaces have improved greatly since they were first introduced in 1835, but they still face maintenance issues, such as leaking water, that most modern technology rarely encounters.

There are several reasons why your furnace is leaking water, depending on the type of furnace your home’s central heating system uses. These reasons can include condensation leaks, plumbing leaks, humidifier leaks, clogged filters, air conditioner leaks, and even a problem with your furnace’s heat exchanger.

Although it is important to know all of the possible reasons why your furnace could leak water, it is just as important to know which reasons can be attributed to which furnace types.

Keep reading to learn more about the reasons why furnaces leak water, which furnaces are prone to specific reasons, and ultimately how to resolve the issue of a leaky furnace.

Why Do Furnaces Leak Water?

There are many reasons why your furnace may be leaking water, which was iterated in the previous section. However, it is also important to know how to discern each reason from the next.

This section will outline these various reasons and detail the methods for discerning each reason from one another.

Here is a list of all of the reasons why furnaces leak water.

  • Condensation Leaks: This is the most common reason why furnaces leak water. This issue particularly affects high-efficiency furnaces and is generally caused by clogs or breaks in the pipes that carry the heat produced by the furnace around and out of the house.
  • Plumbing Leaks: The water leak could also be attributed to a plumbing issue that may or may not even is related to the furnace. Most likely, plumbing leaks will not be related to the furnace; however, the leak could be caused by a condensate pump malfunction, which is directly tied to the furnace’s operation.
  • Humidifier Leaks: Another common reason why furnaces appear to leak water is, in actuality, a humidifier leak. Humidifiers are often used harmoniously with central heating systems. When a clog or breakage occurs in a humidifier’s pipes, it may cause a leak that appears to be extruding from the furnace.
  • Clogged Filters: A furnace’s clogged filter may also be responsible for a water leak. Although clogged filters are generally attributed to freezing the furnace’s coil, in some circumstances, this causes water to leak and collect around the furnace.
Replacing furnace filter
  • Air Conditioner Leaks: In some cases, particularly in milder climates, furnaces and air conditioners can be interchangeably used during the same season. Air conditioners are more prone to leaking onto the heating system, particularly when used after the heating system, due to condensation. This can make it appear as though your furnace is leaking.
  • Heat Exchanger Problem: Finally, some furnaces, particularly HVAC systems, require a heat exchanger to transfer air between warm and cool states. A break in the heat exchanger could potentially cause a water leak and will lead to a very expensive repair.

These are all of the reasons why your furnace may be leaking water.

Which Furnaces Encounter These Issues?

Now, let’s take a look at which issues correlate to different types of furnaces and heating systems.

To properly assess which issue correlates to different types of furnaces, it is important to know the different types of furnaces you may have in your home’s central heating system.

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First, let’s go over the different categories for furnaces. Here is a list of the different furnace categories that are typically found in a home’s central heating system.

  • Natural Draft: Natural draft furnaces are older, simpler furnaces that channeled warm air through a system of pipes that emanated heat upward into the house. This furnace category is not typically seen in most modern households, but older houses may continue to use natural draft furnaces.
  • Forced Air: This furnace became a staple of many 1950s to 1960s-era households and is certainly more efficient than natural draft furnaces. This furnace features blowers that force the air traveling through the pipes into the house. Additionally, these furnaces have been adapted to work in unison with air conditioning systems.
  • Forced Draft: This furnace is more popular today than either of the furnaces we just went over. It utilizes a multi-speed blower and a steel heat exchanger to efficiently pump warm air into your home. This furnace is also much smaller than a forced-air furnace.
  • Condensing: Finally, there are condensing furnaces, the most efficient furnaces currently available. These furnaces utilize a combustion area, draft inducer, and two heat exchangers. The heat exchanger makes it possible to remove heat released into the exhaust gas while condensing water vapor and other chemicals, which is a process that necessitates strong pipes to avoid leaks.

Now, let’s take a look at the different types of furnaces within these categories. Here is a list of the different types of furnaces typically found in a home’s central heating system.

  • Single-Stage: Single-stage furnaces open their gas valve at 100% capacity when in operation, which means your furnace will burn at full power when in operation. This will continue until your indoor temperature has reached the temperature set on your thermostat.
  • Two-Stage: Two-stage furnaces use different programming than one stage furnaces, which allow its gas valves to allow gas into the furnace at two separate levels. The first level is at about 60-70% capacity. Then, once the indoor temperature has shifted noticeably, the gas valve will open at 100% capacity.
Two-stage funace
  • Modulating/HVAC: Modulating or HVAC furnaces are constantly running at an optimal level by constantly observing marginal changes in indoor temperature and adjusting the amount of heat being released due to these observations. HVAC is an acronym for Heating, Ventilation, and Air Conditioning, as these three systems work collaboratively.

Finally, let’s take a look at the various fuels that these furnace types use. Here is a list of the different fuels that furnaces typically found in a home’s central heating system use.

  • Electric: Electric furnaces are becoming increasingly popular as more climate-conscious people make the switch over from gas to electric. Electric furnaces are also far less likely to experience water leaks than other furnace types.
  • Natural Gas: Natural gas furnaces are still fairly popular due to their inexpensive installation and maintenance costs. Additionally, natural gas is cleaner to burn than other fuels like oil, propane, and coal. 
  • Oil: Oil furnaces are great at supplying a lot of heat and are fairly easy to maintain. However, oil furnaces are much less clean and less efficient than electric and natural gas furnaces. 
  • Propane: Propane furnaces are popular because they do not require gas lines and can produce hotter air than any other fuel type. However, propane is far more expensive than natural gas and oil and is less efficient and clean than electricity.
  • Wood: Wood furnaces are excellent, affordable options for those looking to eliminate their monthly gas utility bill.
  • Coal: Coal furnaces are also affordable options for those looking to eliminate their gas bill, but they are far less clean than other furnace types. 

Many of these issues are commonly attributed to HVAC furnaces, simply because these furnaces cohabitate with the air conditioning, which is more prone to water leaks than furnaces are.

Additionally, high-efficiency furnaces are more likely to produce water leaks than standard-efficiency furnaces.

High-efficiency furnaces extract heat from the combustion gases for a longer time than standard efficiency furnaces, which can cause these gases to cool and, in some cases, condensate.

Service man fixing furnace

However, standard efficiency furnaces are not immune to water leakage.

Although water leakage issues in standard efficiency furnaces are less common, they are still known to occur, particularly if your furnace’s flue pipes were incorrectly fitted.

Ultimately, regardless of whether you have high efficiency or standard efficiency furnace, you should have a furnace technician evaluate the issue before moving forward with any troubleshooting.

How to Fix a Furnace That is Leaking Water

Now that we have covered the types of furnaces that experience water leaks let’s take a look at how to stop a furnace that is leaking water.

There are five steps that you should take if you find water leaks around your furnace.

  • Turn off the System: The first thing you should do is shut off your central heating system. There should be a shutoff switch next to the furnace and coil but, if you cannot locate it, then shut the system off at the breaker. 
  • Clean Up Water: Next, clean up any water collected around the furnace as quickly as possible to avoid any water damage.
  • Check Filter: Then, you may check the furnace’s filter to see if it is functioning correctly.
  • Contact a Technician: Regardless of whether or not you can locate the issue, you should contact a furnace technician to handle it.
  • Pour Water into Top of Condensate Pump: You may attempt to locate the issue by pouring water into the top of the condensate pump. If it does not react, this indicates a mechanical failure, which you will absolutely need a furnace technician to assist you with.