Furnace Leaking Water When Heat or AC Is On (Cause and Troubleshooting)

Furnace Leaking Water When Heat or AC Is On

Leaking water soaking through the walls and floors of your house is never a happy sight. This not only is detrimental to the house in a way that it damages the walls and floors, but it can also indicate a faulty household appliance that may cause damage of its own if it’s not immediately looked after.

Leakages are especially common with heating and cooling units, and they usually lead to more problems as consistent moisture can breed things like mold, etc, that could spread and pose health risks to the people in the house. Moisture in itself paired with any electrical equipment is not a good sign either as it can damage the parts. 

So all in all, what we’re trying to say is- your furnace leaking water is something to be wary of. A quick clean-up with a mop isn’t enough to prevent it from happening again.

With enough furnace leaks, you might even start noticing a pattern. That is, your furnace might start leaking when you turn on your Air Conditioner or your furnace. In that case, there are specific ways to find out what’s triggering the leaks. 

At the end of the day, the best way to deal with leakage is to consult a professional so you don’t risk putting yourself in harm’s way. But if you just want to do some surface-level research for yourself, these pointers below should help. 

Furnace Leaking Water When AC Is ON (Causes & Fixes)

Furnaces leaking when ACs are turned on is a common scenario across households, so these problems are generally easier to diagnose than leakages caused by heat. 

A Frozen Evaporator Coil

One of the most common reasons that lead to a furnace leak is if the ACs Evaporator Coils are frozen because of reduced airflow. In such a situation, you’re likely to witness water leaking all over the place once the AC finishes working, as this causes the ice to melt and leak in the form of water all over your floor. 

Usually, reduced airflow happens because of a clogged-up filter or a damaged drain pan that’s allowing the moisture to freeze on the evaporator coils. Proper maintenance of your unit by regularly cleaning out the dust and debris blocking your air filter would help prevent this problem and dodge a bullet entirely. 

A Blocked Condensate Pump

If your AC is one of those models with a condensate pump to help the condensate from your AC coil to be exhausted outside, there’s a possibility that you might run into a blockage or a clogging up. This, in turn, might lead to the condensate dripping out of your unit in a leak. In such cases, it’s recommended that you check the drain pan or the drain hose to ensure it is not damaged. 

If the drain pan doesn’t show any signs of being the cause of this problem, then you can be sure that your pump has a part to play in this leakage. The easiest solution to this mess would be to repair or replace your condensate pump.

With so many tutorials online, any person with little mechanical experience should be able to repair the pump themselves, especially if they’re dealing with a simple blockage. But, in general, most owners prepare to replace the pump entirely. 

Either way, you must deal with your clogged-up condensate pump quickly because it can burn out the motor and decrease its lifespan considerably. 

The Dehumidifier Problem

If your AC has a dehumidifier or humidifier function- and most do- you might want to check their drain pans. Much like evaporator coils, the dehumidifier unit also extracts moisture from the air and condenses it with low temperatures.

This moisture, then, is to successfully trickle down into the drain pan and be shed outside. However, again, a damaged drain pan could prevent this process from going smoothly, and cause the moisture to leak out instead. With a humidifier, the water pipe could start dripping water. 

For a leaking dehumidifier, the reduced airflow could be the main instigator, again. Check the filters and the vents for blockages, and clean out any clogged-up spaces.

You might want to consider changing the drain pan if your air filter is not the problem. For a humidifier, check the water pipe’s connection to the water supply, and make sure it’s not cut or damaged anywhere. 

Condensate Drain Pan

As we’ve identified in the problems above, the drain itself could prove to be the problem if it is damaged or dirty, as it refuses to collect the moisture from the other components and dispel it all out. Mold, dust, or debris left too long u cleaned on a drain pan can be the cause of this. 

You could also face a clogging up of the drain line that carries the moisture outside. In that case, you can clean it out by releasing water into it through a hose or compressed air in a can. If you don’t want to deal with this problem personally, you can always call in a technician to fix this problem efficiently. 


If you type out a Google search for help with a leaking furnace, you are likely to come across many people reporting that their furnace leaks when their AC turns on. Fewer people deal with Furnace leakages when the heat is on. And for the few unlucky ones running into this problem, we’ve listed out some of the reasons why:

The Condensate Pump

Much like the AC unit, condensate pumps in furnaces that tend to carry the condensate from the AC upwards rather than draining it downwards can pose a problem, as you’ll have to deal with possible blockages. Again, you can call in a technician to repair or replace this part, or you can try to do some of your own repairings. 

The humidifier

Most ACs don’t come with a humidifier, but heating components like furnaces do. The humidifiers are most likely to leak because they carry water to the unit. You might find a problem in the water pipe that fills the humidifier with water.

This might be because of an improper fitting of the pipe to the water supply, or due to any cuts or damages on the pipe that’s allowing the water to escape the pipeline. 

Again, you might be dealing with a blocked drain that’s preventing the water from flowing out properly. Some humidifiers even have pads that are supposed to absorb excessive water.

However, minerals from the wage might make it hard and reduce its ability to soak up the water and prevent leakage. 

The Condensate Drain

If you have a condensate furnace as opposed to a non-condensing one, then you’re going to be dealing with moisture and leakages a lot more than a non-condensing furnace owner. 

A condensate furnace exhausts a mixture of gas and propane as it works, and gas is made up of moisture. If your furnace’s vent isn’t working properly, and the moisture isn’t being drained out as it should, there are increased chances of leakage.

The Heat Exchanger

Cracked heat exchange is a common cause of the problem of a leaking furnace. The Heat Exchanger works by transferring heat from the combustion chamber and sending it out to the rooms through the vents. 

The combustion exhaust contains moisture, which should typically be drained out through a vent or condensate drain outside the house. However, this might be disrupted if your heat exchanger is damaged, as it stops the proper disposal of moisture from the unit. Instead, the moisture might start to leak out right back into your home. 

If you have a Carbon Monoxide detector to alert you, then you know where the problem lies. 

While most components behind a water leak are small and repairable, a heat exchanger is more serious. Especially so, because a damaged heat exchanger is indicative of the end of a furnace’s life, so you’re better off simply replacing your whole furnace instead of spending unnecessary money trying to repair the part.

A cracked heat exchanger isn’t something that happens occasionally, because heat exchangers only show signs of wear and tear after they’ve exhausted their usability. 

You shouldn’t be putting up with a water leakage for different reasons. A water leak is neither safe nor hygienic for you, nor is it a good sign about the condition of your AC or furnace. This is why you must take the appropriate measures to deal with it effectively.

Related Articles You May Like:

  • Our Complete Furnace Repair Guide With DIY Solution Click Here
  • Best Gas Furnace Review and Price Analysis Click Here
  • Complete Guide On Furnace Ignitor Checking and Replacing Click Here
  • Complete Guide On Furnace Blower Motor Checking and Replacing Click Here
  • Why Your Furnace Is Short Cycling? Cause, Troubleshooting And Fixes Click Here
Scroll to Top