Furnace Leaking Water? Why and What to Do

gas furnace

A furnace leaking water can be a cause for concern, but understanding the reasons behind it and taking appropriate action can help resolve the issue. Here’s a summary of why a furnace might be leaking water and what to do about it:

Possible Causes of Furnace Leaking Water

  • Condensate Drain Line Blockage: High-efficiency furnaces produce condensation as a byproduct of combustion. If the condensate drain line becomes clogged or blocked, it can cause water to back up and leak from the furnace.
  • Cracked Heat Exchanger: A cracked heat exchanger can allow combustion gases and moisture to escape, leading to water leakage. This is a serious issue that requires immediate attention due to potential safety concerns.
  • Humidifier Leak: If your furnace is equipped with a humidifier, a leak from the humidifier can be mistaken for a furnace leak.
  • Clogged Drain Pan: Furnaces often have a drain pan underneath them to collect condensate. If this pan becomes clogged or damaged, it can lead to water overflow.
  • Improper Installation: Poor installation or maintenance practices can result in water leakage, especially if components like the condensate drain line are not correctly connected or sloped.

What to Do If Your Furnace Is Leaking Water

  • Turn Off the Furnace: If you notice water leaking from your furnace, turn off the furnace immediately to prevent further damage and safety hazards.
  • Inspect and Identify: Try to identify the source of the leak. Check the condensate drain line, drain pan, and humidifier (if present) for visible issues.
  • Clear Blockages: If you suspect a blocked condensate drain line or drain pan, carefully clear the blockage. Use a wet-dry vacuum or a pipe cleaner, and ensure the drain line is properly sloped for drainage.
  • Professional Inspection: For persistent or severe leaks, or if you suspect a cracked heat exchanger, contact a qualified HVAC technician for a thorough inspection and repair. A cracked heat exchanger is a serious safety concern that requires immediate attention.
  • Regular Maintenance: To prevent future leaks, schedule regular professional maintenance for your furnace. Technicians can clean and inspect components, ensuring they are functioning correctly.
  • Monitor: After addressing the issue, monitor your furnace for any further signs of leakage to ensure the problem has been resolved.

How a Furnace Works

To understand why your furnace is leaking water from the top or bottom, I believe it is important for you to first understand how a furnace works in the first place. I will mainly focus on high-efficiency furnaces since those are the ones prone to leaking water.

The difference between standard and high-efficiency furnaces as their names imply is that standard furnaces are less efficient while high efficient furnaces are highly efficient. Efficiency is basically how well the furnace utilizes the heat from the fuel (natural gas).

Standard furnaces have an Annual Fuel Utilization Efficiency (AFUE) rating of 80% while high-efficiency furnace have an AFUE rating of 90% or higher.

Although both furnaces work the same in the beginning of the operation, high-efficiency furnaces a have a secondary heat exchanger which draws heat from the hot exhaust gases from the primary heat exchanger.

When the thermostat calls for heating, a signal is sent to the furnace control board to start the heating process. The ignition switch turns on, gas valve opens and the gas burners starts to heat the primary heat exchanger inside the combustion chamber.

When the heat exchanger is hot enough, the blower kicks in and starts pulling cold air from the house. The air first passes through a filter and is then pushed across the heat exchanger where it absorbs the heat (gets heated in the process).

As you already know, when a natural gas is burnt, it produces exhaust gases. The exhaust gases are removed from the combustion chamber through the flue/exhaust pipe, with the help of the draft fan.

In standard furnaces, the exhaust gases flow straight to the outside. And that is where these furnaces differ from high-efficiency furnaces.

In high-efficiency furnaces, the exhaust gases are diverted to a secondary heat exchanger. In this heat exchanger, the hot gases lose so much heat until condensation occurs, where water and carbon dioxide are formed.

As a matter of fact the condensate is a weak carbonic acid that is drained out through a plastic drain line. This is the reason why high-efficiency furnaces are also known as a condensing furnaces.

The reason your furnace is leaking water is because the condensate from the secondary heat exchanger is not being drained out properly. That is usually caused by a clogged condensate drain pipe.

Do You Have a Standard or High-Efficiency Furnace?

It is quite easy to tell if you have a standard or high-efficiency/condensing furnace. Just look at the exhaust gas pipe material of your unit.

If your furnace has a white PVC/ABS pipe as its flue/exhaust pipe, you have a high-efficiency furnace. On the other hand, if the furnace has a metallic exhaust pipe then you have a standard furnace.

With a standard furnace, the exhaust gases are so hot that you need a metallic pipe to successful vent them out of the house. Since the exhaust gases in high-efficiency are quite cold (after losing their heat to the secondary heat exchanger), they can safely be vented using a plastic pipe.

Why Your High-Efficiency Furnace is Leaking Water

Although I said that the source of a high-efficiency furnace leaking water is a clogged condensate drain line, there other causes as well. Let us look at them in more details.

1. Clogged Condensate Drain Line

Due to the humid nature of the inside of the furnace condensate drain pipe, it is a perfect breeding ground for algae, mold and mildew. Dirt particles which penetrate through the air filter will also find their way inside the drain pipe.

The gunk builds up and after sometime and completely blocks the flow of the condensate from the secondary heat exchanger. If the condensate cannot flow out, the drip pan overflows and water leaks from the bottom of the furnace.

Luckily, you can unclog a furnace drain line without needing to call in an HVAC technician. And there are several ways to do it but first make sure that the unit is turned off.

  1. Disconnect drain hose and literally blow into it. It is gross but you have a very high chance of clearing the clog by just doing that.
  2. Pour a cup of distilled white vinegar down the drain line and after 30 minutes flush it with hot water.
  3. Clear the drain line using a plumber’s snake or a brush mounted on a flexible cable.
  4. Force the clog out using a drain gun (remember you will need to separately purchase the CO2 cartridges).
  5. Connect a shop vac to the drain line and suck out the clog. You may need to use a rag or duct tape to create a good seal between the vacuum’s hose and the drain line.

2. Leaking Inducer Assembly

The inducer assembly consists of a small fan that pulls exhaust gases from the heat exchanger and vents them to the outside. In standard furnaces, the inducer assembly is made of metal but that of high-efficiency furnaces is made up of plastic.

The condensate drain hose goes through the inducer assembly and if there is a crack or the seals are worn out, the condensate/water will leak out of the furnace.

Unfortunately, a leaking furnace inducer assembly is hardly something the average homeowner will fix. It needs to be done by a professional HVAC technician.

The technician will recommend sealing the leak or replacing it altogether. While sealing is cheaper than a replacement (it costs $200 or more), there is usually no guarantee that the leak will stop after applying the sealant.

3. Faulty Condensate Pump

A condensate pump pumps the water/condensate out of the furnace and into the drainpipe. As is the case with all pumps, this pump is likely to malfunction and fail to remove water from the unit.

If the furnace condensate drain pump is faulty, the condensate will overflow and leak from the bottom of the furnace.

Again, the average homeowner isn’t likely to troubleshoot and fix/repair a faulty furnace condensate pump. You will need to hire an HVAC technician for that.

4. Leaking Humidifier

Does your furnace have an in-built humidifier? It could be the one leaking and the water dripping from the bottom of the furnace.

To create humidity, the humidifier needs water to flow in and drain out but if the water line develops a leak due to a crack or clog, the humidifier will surely leak. If you suspect that your humidifier is leaking, contact an HVAC professional immediately to prevent water damage-related problems.

5. Improper Vent Pipe Installation

Is water leaking from a new furnace? If that is so, there is a high probability that the flue pipe was not properly installed.

The flue/vent pipe removes exhaust gases produces during the combustion of natural gas to the outside. However, the pipe needs to be installed properly for it to work well.

If for instance the vent pipe is too big or lacks a good slope, it will allow too much air to circulate, and the gases get trapped inside the pipe. When that happens, the gases cool and condense causing water to leak from the furnace.

This problem needs an experience technician to troubleshoot and fix.

Water Leaking from the Furnace when the AC is on?

Is water leaking from your furnace during summer? As you already know the furnace is usually turned off during the months of summer and instead the AC kicks in to cool the house.

If your furnace is leaking water when the air conditioner is on, you clearly are dealing with a leaking air conditioner.

In vertical HVAC installations, the air conditioner indoor unit (evaporator coil) is usually located above the furnace. Therefore, if the air conditioner is leaking, the water will drip from the furnace (thanks to gravity).

And why would an air conditioner leak water? And where does the water come from?

During the months of summer, the hot air also contains high levels of humidity. An air conditioner apart from cooling the air also takes care of that humidity.

The evaporator coil inside the house contains a very cold liquid known as a refrigerant. When the evaporator coil fan pulls the hot and humid air from the house, the refrigerant cools the air while the water vapor (humidity) condenses on the cold coil and drips on a drip pan located below the evaporator coil.

A condensate drain pipe drains the water from the drip pan to outside of the house or even to the house’s drainage system.  If this system fails to work as designed, the condensation will leak from the air conditioner and drip from the furnace below it.

The following are the causes of a leaking air conditioner:

1. Clogged AC Drain Line

When the AC drain line is clogged by algae, mold and other types of gunk, it fails to remove water from the condensate drip pan.

As a result, the drip pan overflows with water which then leaks from the AC to the furnace below it. Connecting a shop vac to the AC drain line outside the house will suck out the clog and clear the drain line.

You can also clear the drain line using distilled vinegar, plumber’s snake or drain gun as I wrote in this post.

2. Hole in the Drip Pan

After many years of usage, the AC drip Pan will corrode and develop a hole or holes. When that happens, the condensation will leak out of the air conditioner as soon as it lands on the pan and drip from the furnace.

While a hole in an AC drip pan can be plugged using a water-resistant sealant, sometimes you will need to bring an HVAC technician over to replace it.

3. Frozen Evaporator Coil

If the flow of warm air from the house to the evaporator coil is blocked/restricted, the temperature of the refrigerant inside the coil will drop below freezing point. That means that the condensation on the surface of the coil will ice over instead of dripping in the pan.

When eventually the ice starts to thaw, the water will leak from the AC and land on the furnace below it.

A frozen evaporator coil is caused by:

  • Dirty air filters
  • Dirt evaporator coil
  • Clogged air ducts
  • Leaking air ducts

Low levels of refrigerant (caused by a refrigerant leak) will also cause its pressure and temperature to drop. When that happens, the evaporator coil will freeze too.

4. Faulty Condensate Pump or Float Switch

An AC float switch prevents the drip pan from overflowing when there is a problem with the drain line. If the float switch is faulty, it will fail to turn off the unit resulting in production of more condensation even when the pan is already overflowing.

A condensate drain pump helps to pump the water out of the drip pan when the HVAC unit is located farther away in the attic or basement. Again, when this pump fails, the AC drip pan will fill with water and overflow through the furnace.

Is water Leaking from Furnace Dangerous?

As I mentioned, the condensate in the secondary heat exchange combines with carbon dioxide to form a weak carbonic acid. However, the condensation (leaking water) does not pose any harm to you and your family.

However, a leak in the house is never a good thing. Water as we know it can cause water damage to the ceiling, wooden floors and even dry wall resulting in expensive repairs. That is why you need to have the water mopped up immediately and the problem fixed promptly.

Unlike the AC unit, a furnace has lots of electrical components that can easily be damaged by water. Water can even damage the entire unit and you may need to replace the whole of it.

The problem with furnace problems is that you discover them when it is too cold to stay a day without heating. Unless the leak is severe, you can decide to wait it out until spring if it requires a part to be replaced.

In summary, when you notice water leaking from your furnace, turn the unit off immediately and clean up all the water. Next check if it is something you can fix otherwise call in a professional HVAC technician.