Furnace Leaking Water When AC or Heat is on

Avatar photo
Updated April 19, 2023

Causes of furnace leaking water

The most common cause is that the AC condensate drain is blocked or clogged. The blockage can develop over time as the evaporator coils collect particles from the air and mold or mildew may grow in the moist drain pan – especially if it isn’t draining effectively.

The drain pan is located below the system’s evaporator coils and while the AC is running in the warmer months, it catches the condensation that forms when moist, warm indoor air passes over and is cooled by the cold evaporator coils. The water is then drained from the pan and out of the HVAC system down the PVC/rubber drain pipe which is located on the side of the furnace:

To a nearby drain or sump pump pit:

In our case, the technician pointed out that the drain pan valve is ¾”, while the drain pipe connected to it is smaller (½”) when it should have been the same size. This provided a natural bottleneck for particulate and grime to build-up and turn into a clog. 

Signs of a clogged drain line or pan

Water pooling around furnace

One of the signs of a clogged drain line or pan is water pooling or running on the floor around the base of your furnace. The water isn’t coming from your furnace, but rather from your air conditioner’s condensation process that takes place inside the air handler above or near the furnace.

In many cases, the drips from the leak are so minor that the water evaporates faster than it pools and the air in your home is usually dry enough to speed this process along. However, if water is collecting around the base of the furnace, that means the drain pan or line is significantly clogged and if not looked after, the leak can cause damage to the surrounding flooring, framing and walls.

Air conditioner stops working

Once the drain becomes mostly or completely clogged, the drain pan will fill up with water as it has nowhere else to go. Many drain pans have a float that will tell the air conditioner to stop running if the drain pan is full (similar to a toilet or dehumidifier) to prevent overflows and leaks.

Other signs to look for

  • No water coming from the condensate drain line
  • Higher humidity in the home
  • Higher energy bills
  • Odours coming from the HVAC system
  • Mold growth

How to fix a drain line clog

Save yourself a service call to a $100 per hour HVAC technician for what could amount to 2 minutes of work by checking the condensate drain for clogs yourself and cleaning out any build-up:

Step 1. Turn off your air conditioner

Look for an off switch next to the furnace and coil or turn your HVAC systems off at the breaker.

Step 2. Disconnect the condensate pipe

Have towels ready as there will likely be a backup of water This was enough in my case since the clog was at the end of the pipe.

Step 3. Clear out the condensate drain and pipe

Use a thin wire brush or plastic drain snake to clean the drain hole and pipe and as far as the brush reaches. Try cleaning from the other end of the pipe as well – whether that is into a sump pump/drain or a drain line on the outside of you house.

A set like this one includes a basic auger, pipe cleaner brush and plastic drain snakes. They can also be useful for unclogging a sink or toilet.

Alternatively, you can place a shop-vac over the condensate pipe, seal the connection with duct tape and suck the water and clog out of the line. This approach may be better if the drain pan is located in an area that might be damaged by spilled water such as a finished basement or attic. However, it can be difficult as the line is often 3/4″ wide while the vacuum is 2″ to 4″.

Another method is to connect a garden hose to force the clog and sediment out of the pipe.

For stubborn clogs, a clog remover such as Draino may be used, but note that it is not recommended for use on rubber pipes.

Step 4. Clean the condensate drain pan

Mix together 1 cup of bleach with 1/2 cup of water and pour it into the drain pan to kill any mold, germs, and bacteria. You can alternatively use 2 cups of white vinegar or simply boiling water.

Step 5. Dry up any spilled water

Do this as soon as possible as standing water can quickly cause damage and allow mold or mildew to develop.

Step 6. Check your furnace filter

In our case the leak from the drain pan had soaked the bottom of our furnace filter causing it to become damp and misshapen. I replaced it with a fresh filter to prevent mold from developing.

Ongoing. Maintain the condensate pan and line

To help keep your condensate line clear, we recommend pouring a cup of vinegar down the drain line every couple of months. This will help kill mold, algae, and fungi before it has a chance of clogging your drain line. Use vinegar for preventative maintenance throughout the year. It won’t damage your drain line.

You can also use bleach to maintain clear drain lines, but only in the summer when your air conditioner is consistently running. Bleach won’t be able to cause any damage or residue if there is a continuous flow of condensation. Another option is using a stiff brush to clean the inside of the drain line as far as you can reach.

Here is another tutorial:

Other causes of water leaking around a furnace

Leaking drain pan

The condensate pans in older HVAC systems were made of metal which could develop rust and holes over time as the water sits in them for long periods of time. Newer pans are made of plastic which will not rust, but since they are located directly above or below the furnace, they can develop cracks by repeated temperature changes and being dried out by the furnace running during the winter.

Small holes and even cracks can be patched up with waterproof sealant or epoxy glue.

Evaporator coil is frozen

A frozen evaporator coil can lead to dripping water that will leave telltale signs of moisture in your drain pan. The evaporator coil is most likely to freeze if your refrigerant levels are low. If you’re dealing with low refrigerant levels, this often indicates a refrigerant leak somewhere in the system. Our technicians can help you get to the bottom of this issue by identifying leaks and repairing your system. Once it is no longer leaking, we’ll recharging your refrigerant.

If it is beyond a simple repair, however, it’s best to contact a professional for replacement.

Condensate pump failure

If the condensate drain line runs upwards to get out of your basement, you probably have a condensate pump to push the water up and out of your basement.

If there are no obvious leaks or damage to the pump, try pouring some water down the drain line into the pump. If the pump doesn’t start up, this is a sign of mechanical failure. If the pump starts up, but water isn’t leaving the unit, there may be a clog in the outflow line. Here is how to troubleshoot your condensate pump:

Furnace is leaking when heat is on

High-efficiency (condensing) furnace

While less common, if the efficiency of your furnace is 90% or more, it is a condensing furnace. To confirm this, if the furnace’s exhaust vent that carries air outdoors is made of PVC/white plastic, it is a condensing furnace. If it is made of metal, it is not.

Condensing refers to the change from water vapour to liquid water. When natural gas or propane are burned to produce heat in a furnace, two molecules of water in the form of water vapour are produced for every molecule of carbon dioxide.

Similar to the AC, this water is collected and funneled through a condensate drain and likely a condensate pump which can become clogged, cracked or fail as described in the previous sections. The first and easiest step before calling a technician is to check the drain trap and drain line to see if they are clogged.

Conventional (non-condensing) furnace

If the efficiency of your furnace is less than 90% and the flue (exhaust) pipe is made of metal, it is likely non-condensing.


If your furnace has a built-in humidifier, the water is likely coming from a clog or leak in it. The humidifier is typically attached to the outside of the furnace:

Check for leaks and clogs at the water tap line (tighten fittings, add teflon tape or replace the line), drain line (remove and unclog using a small wire or screwdriver) and pad (replace if it is hard, crusty and not absorbing water):

Flue pipe

The purpose of the metal flue pipe is to exhaust the harmful gasses produced as a byproduct of the combustion process out of your home. If the pipe is too large or doesn’t have enough slope, the gases may flow slow enough that they start to cool down and form condensation. If there is water leaking from your flue pipe, contact a professional to fix the size and sloping of the pipe. Patching the leak will only lead to the development rust and mold.

Cracked heat exchanger

The heat exchanger transfers heat from the combustion chamber to the ductwork that directs warm air to the rooms in your home. If it is cracked, the moisture can leak out and the fumes that come with it will set off your carbon monoxide detector in great enough quantities.

Since the exchanger is at the core of the furnace’s operation and the furnace must be disassembled to get to the core, this is not a DIY fix. Call an HVAC company.

Exchangers typically die later in the lifespan of a furnace and out of warranty, so it can be more cost effective to simply replace the furnace than paying 1/3 to 1/2 the price to repair a component of an otherwise old unit.

Over to you

We’re interested to know – what was the solution to your furnace leaking problem? Let us know by leaving a comment below!

About the author

Avatar photo
Alex Wideman
Alex Wideman is a consumer rights advocate, serial entrepreneur and the editor-in-chief of Cansumer. He has a bachelor's degree in electrical engineering from Queen's University. He is passionate about helping others save time and money and has been creating consumer-focused online resources for over 10 years. More about Cansumer Read more

Was this article helpful?

If you found this page helpful, you can send thanks and support future work by buying me a coffee.
Buy Me A Coffee

Leave a comment