The #1 Reason Your Furnace Is Leaking Water (And How To Fix It)

Water coming from your furnace is usually a cause for concern.

As a licensed HVAC contractor, I identify and repair all sorts of water leaks from furnaces and air conditioners.

Taking care of a leak quickly is important to prevent mold or additional water damage. Mold growth in the furnace can be blown through the ducts and create a significant problem down the road.

If left unchecked, water drips can cause your furnace to fail and be expensive to repair.

Don’t worry though, most of the time it’s fairly easy to identify what’s causing your furnace to leak water and can be corrected quickly.

Why Is My Furnace Leaking Water? (#1 Reason)

If water is dripping from your furnace, your condensate drain is probably clogged.

If you have a high efficiency gas furnace, the condensation lines or drain lines can become clogged and prevent proper flow to the floor drain or condensate pump.

Newer high efficiency models create a lot of water when they run. They have a secondary heat exchanger that removes additional heat from the flue gasses which lowers the exhaust temperature.

The lower flue gas temperature allows water to condense inside the furnace and flue pipe and inevitably has to be drained away for the furnace to operate properly. This type of furnace uses PVC or other plastic pipe for its flue pipe.

If your furnace uses metal pipe for its exhaust, it’s a mid efficiency model and should not create condensate.

RELATED: 5 Reasons Your Furnace Keeps Turning Off

This happens when you run your AC as well. The cold temperature of the evaporator coil with the hot humid air moving through it removes moisture from the air.

Think of how a glass of ice water sweats on a hot summer day, same idea. There’s a pan under the coil that collects the water and allows it to drain away through a condensate line.

Debris can accumulate in these lines as well and cause the water to overflow on to the furnace and all over the floor.

Additional Reasons For Leaking

A clogged drain line is the most common, but it’s not the only reason you may have water dripping or leaking. 

Condensate Pump 

If your furnace uses a condensate pump, it can clog or the pump can fail causing water to leak.

The discharge line, usually 3/8″ clear vinyl tubing, can get plugged up and cause the reservoir to overflow.

If the pump fails or the float sticks and it doesn’t have a safety switch, water will continue to drain into the reservoir and eventually overflow. 

clean condensate lines to prevent furnace leaking

You also may have slime form inside your pump reservoir or drain lines. It’s a type of bacterial growth that can form and cause lines to get clogged.

A diluted mixture of bleach and water should be used to remove it and help prevent it from coming back.

RELATED: How To Tell If Your Condensate Pump Is Working

Condensate Trap

If your furnace has condensate lines, it should have a trap in it as well.

A condensate trap stops negative pressure from the draft inducer from pulling through the drain lines and allows the water to flow naturally by gravity.

Because there’s water in the trap constantly, it leaves a nice place for algae growth and sediment to form. 

furnace leaking from condensate pump

Some traps are internal and part of the furnace making them more difficult to clean, while others are external and can usually be cleaned with a small amount of bleach or white vinegar.

Whether or not your trap is internal or external depends entirely on what brand and model your furnace is.

READ ALSO: How To Reset Pilot Light on Furnace?

furnace leaking water from condensate trap

Inducer Assembly Leaking

On high efficiency models, the draft inducer assembly is used to pull the exhaust from the furnace and push it through the exhaust pipe.

Water from the exhaust runs into the inducer and if the inducer assembly is cracked, it will cause it to leak inside the furnace.

A/C Leaking Water

Air conditioners create a lot of water during the summer and can be a common reason you see leaks from your furnace.

Frozen Coil

You may see water at the base of the furnace and assume the furnace is the culprit, but that’s not always the case.

If the coil for your AC is iced up on top of your furnace, it eventually thaws and water spills all over the top and into the bottom of the furnace. You can read our guide on how to unfreeze an air conditioning unit of ice right here.

Damaged Drain Pan

Air conditioners use drain pans to collect the condensation that occurs when they run. If there’s a crack or a hole in the pan, the water will leak from there and never make it to the drain lines.

Humidifier Drain

Some furnaces have a humidifier tied in to the duct work with a water line and a drain for the excess water. If the drain outlet is clogged or if there’s an issue with the water line feeding it, it will leak on or around the furnace.

Water Heater Leak

Most water heaters are located pretty close to the furnace.

If the tank or valve is defective, water can migrate under the furnace and you’ll see water around the furnace making it look like it has a leak.

READ ALSO: Will a Heat Pump Work During Very Cold Weather?

Is Furnace Leaking Water Dangerous?

While condensation leaks are concerning and quite a nuisance, they pose no immediate dangers.

Water is corrosive and can be detrimental to your home and equipment. If not corrected, your furnace will most likely be damaged and will need more significant repairs.

Rust forming on your heat exchanger is a significant concern. If a crack or hole forms from corrosion, you risk CO entering your home. Typically a cracked heat exchanger is the final straw for a furnace and it has to be replaced, which is quite costly.

Furnaces today have multiple electrical components. The most common electrical failure from water damage is a circuit board short or failure. Whether it’s directly related to the water or from corrosion later on, if a circuit board gets wet it will eventually cause the furnace to fail.

How To Fix Condensation Leaks in Furnaces (5-Steps)

Step 1 – Turn Power Off

Shutting down your system will stop additional water from leaking and getting worse. There should be a service switch within arms reach of your furnace that you can use to turn power off. If not, you can turn off the circuit breaker for it.

Step 2 – Locate The Leak

Try to identify where the water is coming from so you can try to correct the issue.

Check the condensate lines inside and outside of the furnace. If your system uses a condensate pump, see if water’s dripping there as well.

Step 3 – Clear Accessible Lines

Removable vinyl tubing can be cleared easily. Remove the condensate lines and try blowing through them.

If you blow through them and there’s resistance, it’s probably clogged. You may even have to use compressed air or a shop vac to clear significantly clogged lines.

PVC lines are usually glued together and will have to be cut to access some of them. I usually use a shop vac at the end of the drain line to remove debris from these.

Keep in mind that once the clog is cleared, a large amount of water will flow and create quite the mess. So be ready with towels to help keep this from happening.

Step 4 – Clean Up Water

Use some towels to remove any excess water that has leaked. This helps prevent mold growth and additional corrosion.

Step 5 – Restore Power And Call A Professional

Once the water is cleaned up and dry, put everything back together and turn the power back on.

If you’re confident you’ve corrected the problem, monitor the furnace or AC and see if it continues to leak. If not call a professional to check everything out and make sure you don’t have any additional issues.

Read Also >> What Is The Correct Heat Pump Register Temperature?

Frequently Asked Questions

Should I Be Concerned About Furnace Water Leaks?

A furnace dripping water isn’t usually an emergency, but you shouldn’t allow it to go on for very long. The problem should be identified as soon as possible and corrected to prevent additional damage or mold growth.

What Is The Best Way To Unclog Condensate Line?

Clogged condensate drain lines can usually be removed fairly easily. Once removed, blow through the line to clear any clogs.

If it’s a significant blockage, you can use a shop vac or compressed air to add additional pressure to remove the debris.

How Do I Stop My Furnace From Leaking Water?

Removing and clearing condensate lines is the first place to start to stop your furnace from spilling water.

Regular maintenance will usually prevent drain blockages and other causes of water damage.

How Can I Prevent Water Leaking?

A dirty filter doesn’t usually cause the furnace to leak, but it can cause the coil to freeze when the AC is running and spill water onto the furnace and the floor.

This typically looks like water is leaking from the furnace, but it’s actually coming from the coil that sits on top of the furnace.

Final Thoughts

I see condensate problems on a regular basis during all seasons, so if you have a leak you aren’t alone. While the consequences of water damage can be expensive, steps can be taken to prevent this from happening and keep your system running properly.

The key is to have everything maintained regularly to stop these water problems before they happen.

Regardless, if you have water coming from your furnace, don’t panic! It’s usually an easy fix you can take care of yourself. If you can’t take care of it yourself, call a pro. They’ll be happy to help you! 

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