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Furnace Leaking Water? | Common Reasons And Troubleshooting Tips

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

For a lot of homeowners, water damage is a nightmare that they do not want to face.

Once you notice a water leak, you should want to resolve it as soon as possible as it can only get worse.

If the water leak has originated from your furnace then that could quickly lead to huge repair costs, or the furnace could eventually fail and have to be replaced. 

Keep checking your furnace as any small issues can be rectified in a short space of time before they escalate into bigger problems. 

In this guide, we will look at your first steps to take when you see a water leak from your furnace, how a furnace works, and the common reasons why a leak can occur. 

What To Do When You Notice That Your Furnace Is Leaking

Once you do see a puddle of water underneath your furnace (see also ‘ How Does A Furnace Work? ‘), you will want to take immediate action.

Any delays could cause the eventual costs to ramp up so you should undertake the following steps to avoid some huge plumbing costs. 

First, turn off the furnace (see also ‘Furnace Pressure Switch Stuck Up? Common Causes And Troubleshooting‘) to prevent the leak from escalating. Without any further water coming out, you are free to clean up the leak and you use a mop and bucket or a wet/dry vacuum to clean up the mess.

An excessive amount of water could quickly lead to water damage so make sure the leak is contained first.

With the furnace off and the water cleaned up, you should clean up any excess water you can find inside the furnace after unscrewing the front panel and wiping it away. 

Take the time to check if the air filter is dirty or clogged as this could be the actual cause of the leak which is easily solved by replacing it.

After exhausting all possibilities for how the leak has been caused and how you could fix it, you can call an HVAC specialist to take a look and remedy the problem.

How Your Furnace Works And Creates Water

The furnace is one of the most intrinsic parts of your HVAC system (see also ‘What Is HVAC System?‘) as it generates the heat once the internal temperature of your home has dipped below the temperature set by the thermostat.

A signal is sent to the furnace’s control board and the heating cycle starts when an ignition switch is turned on. This opens the gas valve, turns on the draft fan, and creates combustion.

This combustion cycle warms up the heat exchanger and the flue gasses that lie inside the heat exchanger are led outside through the exhaust pipe. 

The blower fan then blows cold air from your home’s internal space past the heat exchanger which then creates warm air when it absorbs the heat. 

Finally, the warm air is pushed through your HVAC system to effectively heat your home.

The water leak is unlikely to occur, though it still might if you have a conventional furnace (which could be called a standard-efficiency furnace or a non-condensing furnace) as it does not produce water. 

Should you have a high-efficiency furnace of around 90% efficiency (also known as an Annual Fuel Utilization Efficiency (AFUE) rating) then the condensation can occur due to a second heat exchanger.

Once the burning gas leaves the primary heat exchanger it naturally goes to the second one where the flue or exhaust gas gets another heat exchange which creates water vapor. 

This vapor becomes a liquid as more heat makes its way into the secondary heat exchanger which is then drained into a floor drain or condensate pump.

There are some quick ways to identify which kind of furnace you have in your home.

The most obvious one is the exhaust pipe and if it is made from white plastic PVC then you should have a high-efficiency condensing furnace.

An exhaust pipe made from metal will come from a standard-efficiency furnace, which is also known as a non-condensing furnace.

The Common Reasons For A Leaking Furnace

The Common Reasons For A Leaking Furnace

While it can be quite simple to notice a leaking furnace, it is much harder to diagnose what has caused the problem.

This could be attributed to a clogged filter, a heat exchanger issue, or a direct leak with the AC unit (see also ‘Here Is What To Do When Your AC Blows A Fuse‘), plumbing, condensation, or humidifier. 

Thankfully, each of those issues can be straightforward to identify and maybe even easy to resolve in some cases.

If you do struggle to find out the reason for your leaking furnace then an HVAC specialist will quickly discern the issue and look to remedy it promptly. 

Condensation Leak

Of course, any process in a furnace that creates moisture is likely to cause a water leak. If your furnace has a high-efficiency rating it should have two heat exchanges (see also ‘How Do I Know If My Furnace Heat Exchanger Is Bad?‘) that eventually cause condensation. 

Typically, this condensation escapes through your home via the white plastic PVC exhaust pipes or a floor drain.

Should those pipes crack or clog with dirt or debris, and dust then you could easily have a water leak (see also ‘Slab Leaks: How To Find A Water Leak Underground‘).

As this is one of the most common causes of a leaking furnace, an HVAC specialist should be able to resolve it.

Malfunctioning Heat Exchanger

If the leak cannot be attributed to the condensation from the pipes then the heat exchanger may be the issue.

This is a particularly rare problem yet it does occur as the heat exchangers move heat across your HVAC system and condensation is an inevitable part of that process. 

You may want to exhaust all other possibilities before honing in on the heat exchangers being the problem as it can be costly to repair them and you may even need to replace the heat exchangers.

If you believe that the heat exchangers are the issue as you cannot find another issue then you should contact an HVAC specialist to perform a diagnosis. 

Humidifier Leak

Another component in your furnace that can create moisture, and therefore a leak, is the humidifier.

The humidifier is typically attached next to the furnace and adds some moisture to the room’s air to affect the internal humidity.

As with many other components, the humidifier can suffer from a broken part or a clog which can cause a leak.

This can be one of the simpler problems that may only take a few minutes to solve. It is still a serious concern so you should call out an HVAC specialist as soon as you can. 

Inducer Assembly Leak

As the condensation runs from the exhaust or flue pipe, it reaches the condensate drain hose and then the inducer assembly.

While there may not be a crack in the condensate drain hose, there could well be one in the inducer assembly. If there is then water can leak out of the furnace. 

Plumbing Leak

The leak that looks like it is coming from your furnace may just be coming from your plumbing system. That may be a relief as the issue could be easily solved but not easily ignored.

Your furnace will be connected to the plumbing system so try to track where the leak has originated from and it may well be a clogged or damaged pipe instead of your furnace.

In this case, a plumbing expert is the best person to contact once you can safely discern that the issue has been caused by your plumbing system and not the furnace. 

AC Unit Leak From The Condensation Pan

A further problem that may look like the leak is coming from your furnace but isn’t can be attributed to a leaking air conditioning unit.

Particularly if you reside in a mild climate, you may end up using both your furnace and air conditioning system at the same time.

The air conditioning unit will create moisture and the condensation pan can soon fill up with water which can then overflow.

This may look like a furnace leak but emptying a full condensation pan can soon make you aware of what the problem is. 

Clogged/Dirty Furnace Filter

A working furnace requires a fully functioning furnace filter and once one becomes excessively dirty and clogged, that can cause a water leak.

The primary function of the furnace filter is to clean the air to improve the air quality. However, debris can be caught in the filters which then leads to a restricted airflow via the furnace coil. 

Without a reliable airflow, the coil may freeze or it could leak water. Feel free to check the coils and if you notice any ice or excessive moisture that is leaking then an HVAC specialist can fix it.

Checking the various filters around your house, including the furnace or air filters in the HVAC system, should be scheduled as maintenance so this problem does not occur. 

Poorly Designed Vent Pipe

The flue pipe can be a really important component in the conventional furnace as it removes gasses that are produced during the combustion process.

A poor design can mean that the exhaust pipe fails to have a slope or is just too big, allows far too much air to circulate, and even results in gasses getting trapped.

If those gasses are trapped in the flue pipe, they can then cool to form condensation and a leak.

Final Thoughts

A furnace that leaks water can become very expensive to repair if not treated promptly and properly.

Even the smallest of leaks can cause rust and eventually destroy parts of the furnace causing an inefficient HVAC system and necessitating a repair.

Should a hole or a crack develop on a heat exchanger then you may expose your home to harmful gasses.

If you do find a leak and can attribute it to the furnace then you should contact an HVAC specialist as soon as possible.

Frequently Asked Questions

What Happens When The Condensate Line Is Clogged?

Simply put, a clogged condensate line can freeze your air conditioning system (see also ‘Frozen AC Line: Why It Happens & What To Do About It‘). Once the condensate line becomes clogged, water will be trapped inside your air conditioning system (see also ‘ The Pros And Cons Of High Velocity Air Conditioning Systems ‘).

One of the key components is the evaporator coil which will eventually freeze along with the moisture from the drain line.

With frozen components, your air conditioning unit will fail to function and effectively turn itself off.

How Do You Unclog A Blocked Air Conditioning Drain Line?

Start by turning off the air conditioning unit at the thermostat and the circuit breaker to avoid any electrical damage.

Find the drain line which could be near the condenser unit if you have a family home, or in the closet with the furnace if you have an apartment. 

Remove the cap from the drain line and take a quick look for any obstructions or debris, you may need a flashlight to locate it and you can manually remove some. 

With the debris removed, pour a single cup of distilled white vinegar directly into the drain line and put the cap back on.

Give it around half an hour before turning your air conditioning unit back on. The distilled white vinegar should clean and unclog the air conditioning drain line and you can perform this as regular maintenance. 

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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