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Furnace Leaking Water When Heat Is On? 8 Common Causes

A leaking furnace is bad news because water and electricity make a dangerous pair. So, if your furnace is leaking and the heat is on, you might start to panic. But the best move is to find out if the leak is even coming from your furnace and resolve it immediately.

A furnace leaking water when heat is on can be caused by a clogged condensate drain, a faulty condensate pump, a cracked heat exchanger, or a leaking humidifier or AC. Fixes include cleaning and fixing the condensate drain system, replacing the heat exchanges, and fixing your AC/humidifier leak.

Because moisture from a furnace is acidic, leaving a leaky furnace unattended can cause additional damage to both your home and the furnace. Instead, you should resolve the leak by addressing the root causes. This article will tell you about the most common reasons behind a leaky furnace and give you some tips on how to handle them.

1. Overflowing or Clogged Condensate Drain

Condensate is water that forms from the exhaust gases of your furnace’s combustion processes. This water is drained through a condensate drainage system consisting of a condensate pan where the moisture is deposited and an exit route made of a draining vent and an exit pipe. 

Issues with the condensate drain line are the most common cause of furnace leaks. Especially if you have a high-efficiency (90%) condensing furnace, water is a normal byproduct of the combusted natural gas or propane. 

In principle, for every CO2 molecule produced through the combustion of fuels with methane (CH4) (natural gas) in the presence of oxygen, two water molecules are also produced, as shown in the equation.

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) + E (energy)

This means that plenty of water is produced when your furnace’s heat is on, and it has to be efficiently drained. 

Condensing furnaces have a drain vent made of PVC. If this vent gets disconnected by accident or has a crack, water will escape from the outlet and end up on some surface in your home. 

If the drainpipe is clogged, the condensate will flow back to the pan and overflow, causing a leak and compromising your furnace’s overall performance. 

Another issue with the condensate drain line that could cause a leak is if the condensate pan is rusty or has holes and cracks. This often happens with old condensate pans that have done their time.

How To Fix

Call a plumber or HVAC expert to correct the problem if your furnace’s condensate drain pipe is disconnected or cracked and you don’t have the skills to fix it. 

If the condensate pipe is clogged, clean and unclog it by following these steps:

  1. Switch off your furnace and any associated appliances at the circuit breaker. 
  2. Find the condensate pan. It’s usually located in the utility closet or under the air handler. If there’s standing or overflowing water in the pan, there’s probably a clog in the vent. 
  3. Drain the water with a duster.
  4. Clean the pan and the condensate pan hose with soap and water.
  5. While you’re at it, find the condensate drain’s outside outlet and use a Snake Cleaner ( to clear any clogs. The snake cleaner will come in handy for cleaning all drain pipes around your home, not just the condensate drain.
  6. Rinse the drain with hot water and a bit of vinegar or hydrogen peroxide, and a drop of dishwashing soap to clean any remaining dirt.
5 in 1 Sink Snake Cleaner Drain Auger Hair Catcher, Sink Dredge Drain Clog Remover Cleaning Tools for Kitchen Sink Bathroom Tub Toilet Clogged Drains Dredge Pipe Sewers…

If you can’t resolve a condensate drain clog by yourself, call an HVAC expert to help out. If your condensate pan is in rough shape, replace it with a new one. 

In general, you should check your condensate pan and drain it every month during the heating season to ensure proper drainage.

2. Jammed Condensate Pump

High-efficiency condensing furnaces usually have the condensate draining down a pipe. However, if your furnace is installed in the basement, the draining pipes are below surface level, and the water will need to be pumped out. 

In this case, you need a condensate pump to pump the water draining into the pump tank through a draining hose and out through the outdoor drain pipe.

If your condensate pump is jammed or damaged, the pump tank will overflow, and so will the connected drain pipes. This will cause the water to flow back onto the furnace or the floor, creating a massive leak. 

How To Fix

If you’re the kind that’s handy at fixing anything, you can troubleshoot the condensate pump and determine if it’s fixable or needs replacing. Watch this video to learn how it’s done:

Call an HVAC expert to diagnose and resolve your furnace’s condensate pump if you don’t feel like you’re up to the task.

3. The Humidifier Is Leaking

We feel sticky, hot, and uncomfortable when humidity levels are high. But if humidity is low, you’ll quickly feel the effects of dehydration, such as loose skin and dry mouth.

A furnace humidifier creates a healthy balance in humidity levels when it’s on. The humidifier makes for a healthier environment for you and your family as it supports your immune system in fighting air pollutants.

There are several reasons your furnace’s humidifier could be leaking:

  • The water panel is clogged, causing water to run down the cabinet.
  • The humidifier’s drain line is clogged, causing water to flood the cabinet.
  • The humidifier’s drain line is broken or has holes/cracks.
  • The Solenoid valve that sends water to the panel evaporator is faulty, and the water is redirected where it shouldn’t go. 
  • The water pressure is too high (above 125psi (8.61 bar)). 

How To Fix

If your furnace humidifier is leaking, consider one of these remedies depending on the problem:

  • Check the water panel for blockage that could be preventing the water from running through and flowing out of the humidifier’s cabinet. If that’s the issue, clean the water panel to free it from water deposits.
  • Check the humidifier’s drainage to ensure it’s not blocked. Use a fine wire to dislodge and clear the block if there is one. 
  • Call an expert to replace the broken humidifier’s drain line, as this often indicates some other issue with water pressure or clogs. 
  • Call an HVAC expert to adjust the Solenoid valve so that water can run as it’s supposed to.
  • Consult an HVAC expert about adjusting the humidifier’s pressure or have them fix it.

Overall, the best way to deal with a leaking furnace humidifier is to call an expert to thoroughly inspect the unit and resolve any issues. This might mean cleaning, repairing, or replacing the furnace humidifier altogether.

4. The Heat Exchanger Is Cracked

A heat exchanger is a part that transfers thermal energy. To be more precise, the heat exchanger transfers heat from the combustion chamber to the ductwork. It also works to keep toxic combustion gases away from the heated air delivered into your home. 

If your heat exchanger is leaking water, it’s probably cracked, allowing water that should be drained through the condensate drain to find an alternative route out of the furnace. 

Note that a leaking heat exchanger lets out carbon monoxide and other toxic gases, which are dangerous to your health.

How To Fix

Suppose you were hoping to read a DIY solution to a cracked furnace heat exchanger. Well, sorry to disappoint! 

The only person authorized to deal with a faulty heat exchanger is an HVAC expert. 

Assessing a heat exchanger requires disassembly. That can’t be a trial-and-error fix. Besides, most HVAC pros will advise you to replace your furnace’s heat exchanger as cracks on this unit are correlated with your furnace’s age.

5. The Filter Is Clogged

We know that a dirty filter blocks airflow and causes ignition and overheating problems with your furnace. But did you know that a clogged filter can trigger water leaks in your HVAC system too? 

Even though a clogged filter won’t directly cause a leaky furnace, it can lead to leaks if the furnace and AC share the same ductwork. 

Considering that most AC systems have the evaporator coil just above the furnace, a dirty filter limits airflow to the coil, leading to freezing temperatures and lots of moisture. 

So, when your AC is off, the frozen moisture will melt and flow onto or below the furnace, especially if the condensate drain can’t get rid of the water fast enough.

How To Fix

There are two standard ways of resolving furnace filter issues. The first is to remove the filter, clean it, and put it back in if it’s still good and less than three months old.

The second solution to a clogged furnace filter is to replace the dirty or damaged filter with a new one. Pleated filters like the FilterBuy Pleated HVAC Air Filter ( are a better choice as they are tightly wound to keep away dust and debris as they’re less porous.

FilterBuy 20x20x1, Pleated HVAC AC Furnace Air Filter, MERV 8, AFB Silver, 4-Pack

Fiberglass options such as the Filtrete 16x25x1, AC Furnace Air Filter ( are also a good choice. Plus, they can be washed to prolong their lifespan. However, they’re more porous, providing less resistance to pollutants.

Filtrete 16x25x1, AC Furnace Air Filter, MPR 300, Clean Living Basic Dust, 6-Pack (exact dimensions 15.69 x 24.69 x 0.81)

As a rule, you should replace furnace air filters every 90 days (3 months). But if issues with the filter are causing your furnace or your AC to act up, you’re better off replacing it immediately.

If the leak isn’t resolved after changing or cleaning the filter, there might be a different cause to the leak, and you should ask an HVAC technician to check your furnace. 

6. There’s a Moisture Leak From the AC

If you live in an area with extreme temperature differences between day and night, you might be using the AC and furnace on the same day. Because air conditioners absorb plenty of moisture from your home, which turns into water during the cooling process, they have to drain it through the condensate pan.

If the pan overflows or the drain is clogged, water may flow down the heating system and make it seem like the leak is coming from there.

How To Fix

First, confirm that the leak is indeed coming from the AC. Also, check for clogs and overflow in the furnace pan and drain. Go back to the first step if that’s the problem.

If not, you should have an HVAC expert fix the AC leak. You can also check if the drainpipe inside the AC unit is connected by tracing the leak back to the source.

7. An External Leak Is Draining Near the Furnace

If a pool of water appears underneath your furnace, an external source might be behind it. That can appear as though it was coming from your heater. Here are a few common causes of leaks around your furnace:

  • Furnaces in basements are surrounded by drainage and clean water pipes. If the plumbing in these pipes has leaks, the water may flow below the furnace and cause it to appear as if it’s leaking.
  • Home appliances (e.g., washing machines) or draining facilities (e.g., sinks) in the basement may also have loose drain hoses and develop leaks, flooding the area around your furnace.
  • If your furnace is installed in an attic or has pipes above the ceiling, any leaks in the plumbing can cause water to drip onto your furnace.

How To Fix

You typically have to call a plumber to fix external leaks. Unless you’re an experienced handyman, of course. So, before calling an HVAC expert and paying for a service you don’t need, assess the plumbing above and around your furnace to ensure there aren’t any external leaks.

8. The Metal Vent Pipe Is Causing Condensation

This last cause of a leaking furnace applies only to conventional non-condensing furnaces (with an AFUE rating ˂90%). These furnaces have a metal exhaust pipe, different from the PVC pipe used in high-efficiency furnaces.

The metal pipe removes gases produced in the combustion chamber and sends them outside while they’re still hot. 

If the exhaust vent isn’t installed correctly or is blocked, the gases can get trapped and form condensation once they cool down, causing them to leak onto the furnace.

How To Fix

Check your furnace’s exhaust vent. Look for clogs, bad joints, or disjoints and repair them to allow a clear route for combustion gases. Again, an expert can take care of this issue for you, which may be your only option if there’s a major crack that requires welding.


Apart from causing panic, a leaking furnace could be accompanied by costly damage to your home flooring. Moreover, it could damage the furnace.

So, what do you do when you notice a puddle of water underneath your furnace?

Turn off the furnace immediately! That includes the furnace’s main switch and the switches in the circuit breaker box linked to the furnace.

If you can’t troubleshoot the issues with your furnace or don’t know what to do, calling an expert HVAC is always the safest option.


  • 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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