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January 01, 2023

Why Is My Furnace Leaking Water and How to Fix It

Fixing Water Leaking Furnace - LG Home Comfort

When a furnace is leaking water, it can be a sign of a serious problem that needs a prompt solution. Water means moisture that can damage your home if not dealt with quickly. Plus, if it is not draining properly, it can result in your furnace running inefficiently, leading to more costly repairs in the future.

What Type of Furnace Do I Have?

Before answering the question, “why is my furnace leaking water?” you should first determine which type of furnace you have, as this is a factor contributing to this issue.

There are two main types, such as:

  • High-efficiency (condensing). These newer furnaces condense water vapor produced during combustion and quickly vent out the exhaust or flue pipe. They are typically gas-powered but can also operate on electricity. However, it’s more expensive and less common.
  • Standard efficiency (conventional). These are older furnaces that don’t condense water vapor and, instead, release it through the exhaust vent. Since they don’t collect any water, they typically aren’t prone to leaking.

If you are unsure which furnace type you have, look at its vent or exhaust pipe. If the pipe is made of white plastic, it is a high-efficiency system. Another way to tell is if your furnace’s annual fuel utilization efficiency (AFUE) rating is at least 90%, it is also high-efficiency.

On the other hand, if the exhaust pipe is metal, the AFUE rating is below 90%, and the furnace is 15 years or older, then it is a standard efficiency furnace.

Is It Normal if My Furnace Is Leaking Water?

Water leaks in furnaces are not normal. While this is a common problem homeowners encounter, especially for high-efficiency furnaces, you should fix it as soon as possible. As mentioned, it can cause further damage if not dealt with immediately, not only to the unit itself but also to the other parts of your home.

For instance, mold growth may occur when water accumulates near parts of your home that are not well-ventilated, such as the ceiling and walls. This can lead to health problems for you and your family, as well as structural damage to your home.

What Would Cause a Furnace to Leak Water? 

There are different types of furnace units, but these are possible causes for water leaking from a furnace:


Condensation leak is a common problem for furnaces wherein warm and humid air mixes with cold metal, resulting in the formation of condensation. Condensation forms a pool of water underneath or near the furnace, typically draining away through a condensate line.

There are two possible causes of this issue, such as:

  • Damaged or clogged tubing

First, check how bad the clog or damage is by looking at the drain pan and outside where the drainpipe runs. Sometimes, it’s just a minor block that you can fix. Remove the blockage and turn off the furnace before doing so. If it is near the surface, poke your finger into the drain or use a wire or hanger to remove it. Use a drain brush or snake if you believe it’s in the middle of the line. A brush can also scrub the inside of the pan to remove any remaining residue.

If this is unsuccessful, get a wet or dry vacuum to suck up any debris. Attach the vacuum hose to the drain line’s end, and you may need to seal it with duct tape to avoid air. Turn on the vacuum and let it work. If the clog is more serious, use a drain gun to fire compressed gas into the line to break up any blockage.

If these all fail, you may need to replace the condensate line or have a professional inspect and repair it.

  • Floor drain clog

The floor drain is the part where the condensate line enters the floor. If it is clogged, water will back up from the line and spill out from the drain. Cleaning or replacing the floor drain can help in this situation.

Remove the drain cover and scoop out any debris you can reach. Then, use a plunger to push the clog down and flush it out with hot water to liquefy all the gunk. You may also pour a mixture of vinegar and baking soda, then flush it with hot water after 15 minutes. Don’t use harsh chemicals, as they can damage the drain.

  • Broken condensate pump

The condensate pump’s function is to push the water away from the furnace, but water will accumulate near or underneath the unit if it fails. If this happens, replace your condensate pump as soon as possible by following the instructions in your manual.

If you’re not confident in doing this yourself, hire a professional. Note that condensation only occurs in a high-efficiency furnace, so this is not an issue with standard units.

Broken Humidifier

Water dripping from a furnace may also come from the inside of your unit, specifically the humidifier. A leaking humidifier can cause water to go back into your furnace and drip from the bottom. This can cause damage to your walls, ceilings, and floors. To avoid this, regularly inspect the unit’s water lines for any signs of cracks or damage. It is also a good idea to install a shut-off valve to easily stop water from entering the humidifier when it’s not in use.

Not all furnaces have humidifiers, so this is not an issue if your system does not have one. In addition, this is less likely to happen if you maintain a regular professional checkup.

Malfunctioning Secondary Heat Exchanger

If you own a high-efficiency condensing furnace, it has primary and secondary heat exchangers. The secondary heat exchanger is a metal tube that partially surrounds the primary one that accepts combustion exhaust and transfers it out of your home through a vent. If there is any damage to this component, water dripping in the furnace may occur. This could be due to corrosion or a crack in the exchanger, both of which will need to be addressed by a professional.

To know if this is causing the problem, check the vents and ensure they are not blocked by dust or debris. If they seem to be clogged, here’s a step-by-step guide on how to unclog them:

  • Turn off the unit’s power and the gas or propane by shutting off the valve.
  • Remove any debris from the heat exchanger block with a wire brush. You can also use it to loosen any black soot or scale from the chamber in the heat exchanger. If you don’t have a wire brush, get a damp cloth with a cleaner specifically designed for gas furnaces or metal surfaces to remove stubborn dirt build-up.
  • Vacuum the entire block assembly with a strong hot vac. After then, connect the narrow vacuum attachment to clean all the loosened debris in the block and remove all remaining residue.
  • Reassemble everything, turn on the power and gas or propane and check if the problem has been fixed. Otherwise, you may need to replace the entire heat exchanger with a new one.

Ill-Fitted Exhaust Pipe

This is usually a problem in a conventional furnace. If the exhaust pipe is ill-fitted, it can cause water to drip from the furnace. When the stove is running, the hot combustion exhaust in the pipe condenses and forms a liquid that leaks out from any gaps or cracks.

To fix this problem, ensure all exhaust pipe connections are tight and secure. If there are any holes or cracks, use metal tape to seal them up. You may also need to replace your old exhaust pipe with a new one if it’s too damaged to be repaired.

After fixing all potential problems causing water to drip from the furnace, inspect the area regularly for any signs of moisture or dampness. This helps you detect any issues early on before they cause more damage. It also saves you time and money in the long run, as repairing damages caused by water leakage can be expensive.

Clogged Furnace Filter

A clogged furnace filter is one of the most common reasons for water dripping from furnaces. This happens when the filter becomes so full of dirt and debris that it can’t trap any more particles, allowing them to escape into your furnace coil.

To prevent this, replace or clean your filter regularly according to the manufacturer’s instructions. Depending on your filter type, you may have to clean it every month or two months. If all else fails and you still find yourself dealing with water dripping in the furnace, contact a professional HVAC technician.

Final Thoughts

Various factors can cause a furnace leak. If you detect any signs of water dripping from the furnace, don’t wait for it to get worse, as this can cause damage to your walls, ceilings, and floors. Consider the steps above in identifying and fixing the problem to help you save time and money.

If you need professional help repairing a leaking furnace, choose a qualified and experienced technician who can accurately diagnose and address the problem quickly and efficiently.

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