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Vacuuming up a leak from a furnace

Furnace Leaking Water? Try These Tips

Sean S.Verified

HVAC Owner/Technician

Residential HVAC

292 positive ratings
Using AC and Heating in Same Season?

If so, it’s likely to affect your HVAC unit. Dust and debris and dust can get caught in the unit, plugging the AC evaporator coil or clogging the filter. Both can cause leaks.

Everyone knows that fire and water don’t mix, so seeing a puddle of water underneath your furnace may have you wondering what’s going on. While you shouldn’t ignore a leaking furnace, don’t panic. Although you may need a service technician to determine what is causing the water leak, the problem may be a minor one that is easily repaired. Whether your HVAC unit is in the attic, basement, or somewhere in between, you should be able to repair the leak and prevent it from happening from the safety of your own home.

What causes a furnace to leak water?

Several problems can cause water to pool below a heating system, usually occurring on both electric and gas heating units.

High-efficiency furnace condensation

The most common issue is a condensation leak produced by high-efficiency heating units. Combustion reactions produce water vapor and carbon dioxide. Furnaces are designed to extract heat from these combustion gasses. Low-efficiency units only extract some of the heat, and then vent the rest through a flue pipe.

Condensation on a flat surface
Condensation poses risks when heat exchangers in the furnace fail to burn it off.

Efficient units have two heat exchangers to extract the maximum amount of heat from the combustion reaction. The secondary heat exchanger causes the water vapor to condense. This condensation then drains through a drain in the floor and attached pipe, exiting your home.

When the condensation produced by the secondary heat exchanger can’t drain, it can cause the furnace to leak. If the top of the condensate drain pipe is clogged and producing pooled water, you may be able to fix the issue yourself. However, if the condensation tubing, internal drain line or pump are broken, you’ll need a service technician to diagnose and fix the issue. Large amounts of leftover water pooled in the condensate pan may be cause for concern, and may cause your unit to automatically turn off.

There are two ways to determine whether your furnace is a high efficiency model. First, look at the yellow energy tag. A 90 percent or higher AFUE (Annual Fuel Utilization Efficiency) rating indicates high-efficiency. The second method is to look at the exhaust pipe. White PVC pipe coming out of the heating unit also indicates a high efficiency model.


Regular vs. high-efficiency furnace comparison

Regular model

High-efficiency model

Energy efficiency of 80% or lower. 20% of the heat is vented out and lost.

Energy efficiency of 90% or higher. Includes a secondary heat exchanger that reuses captured heat.

Features a single-stage burner that operates at only one temperature

Features a multi-stage burner with the ability to sense when a lower, more efficient burner can be used in moderate climates

Draws air for combustion from the house. May cause household air to become dry and increase backdrafts.

Contains a sealed-off combustion chamber which promotes healthy humidity levels and lowers backdrafts

Contain blower motors that are either on or off, and which tend to cause fluctuations in temperature

Contain blower motors that are electronically commutated (ECM units), meaning they run almost continuously at variable rates. This tends to eliminate wide temperature fluctuations.

Humidifier leak

Some heating units have built-in humidifiers that use tap water from your home’s plumbing system to keep the air moist when the unit is in use. If the humidifier has any cracks, leaks or clogs, a water leak may appear beneath your furnace. Although it may appear that your your HVAC unit is leaking, it's actually a problem with the humidifier connected to your plumbing.

One common problem with humidifiers is the pad that it rests above. If the pad is incorrectly installed or positioned, water can pour out of the unit and cause problems with the furnace. Pads are cheap to buy and relatively easy to install yourself.

Checking these issues is part of a yearly HVAC tune-up. If you’ve had this service performed, then a leaky humidifier isn’t likely to be a complication. However, if you’ve skipped annual maintenance this year, call a service technician to have it repaired.

Nearby plumbing leak

Leaky pipes get attention from a plumber

If a plumbing pipe near the furnace springs a leak, it may create the illusion that the water leak is coming from your HVAC unit. Backed up drains in the floor can also cause this problem. If the unit is partially submerged, immediately shut it down. It may be safer to shut it off at your home’s circuit breaker panel rather than trying to flip a switch in a flooded area. Leave it off until a repair professional has inspected it and properly determined whether it can be repaired or replaced. If you have a gas furnace, make sure the gas supply line is also shut off and that the pilot light is out.

Leaky air conditioner

In climates with cool summers or mild winters, you may run your air conditioning and heating system in the same season. When both heating and cooling functions are part of the same HVAC unit, they affect how the other works. The biggest problem is dust and dirt buildup. Debris can plug the AC evaporator coil or clog the filter. Both issues can appear as though your furnace is leaking, even though the air conditioner is the culprit.

What to do if water is leaking from a furnace

Take these steps to minimize damage and danger in the event of a furnace leak.

  1. Shut the system off. Look for a shutoff switch near the furnace. It may look like a light switch. If you’ll have to stand in water to shut it off or if you can’t find the shutoff switch, check the circuit breaker. Most homes have a dedicated breaker for the HVAC system. Make sure you shut off the natural gas source and extinguish the pilot light for gas furnaces.

  2. Clean up standing water. The longer a water leak sits, the more damage it can do to flooring and delicate electronics. Use a water safe vacuum or towels to remove excess water as quickly as possible.

  3. Check the air filter for moisture. Since a wet filter blocks airflow, it can cause even more issues. Replace the air filter with a clean dry one if needed.

  4. Use a water safe vacuum to clear debris near the drain line. This removes superficial clogs or blockages, and may fix the problem. When you are certain the system is dry, you may restore power and turn on the air conditioning system. Monitor it to make sure the overflow water is draining. You should only take this step if you are certain that no part of the furnace has been submerged. Otherwise, seek professional maintenance.

  5. Pour a small amount of water into the top of the condensate pump. A non-reacting pump indicates mechanical failure, while a working pump indicates a clear drain.

DIY solutions

In most cases, a leaky furnace is a good reason for calling in the pros and seeking qualified repairs. However, homeowners can safely diagnose and fix minor issues.


Easy fixes

In the middle of a potential crisis, not everyone thinks clearly. Before calling a repair technician for a faulty condensate pump, check to see that it is plugged in. Next, take a look at the drain line. It should be pitched at a downward angle to facilitate drainage.

Suction line insulation

When the HVAC unit is in air conditioning mode, the suction line and evaporator coil produce exterior condensation. The evaporator coil drains into a pan that shunts water to a drain pipe, but the suction line lacks this mechanism. To keep the copper line from “sweating,” it is wrapped in black insulation called armaflex. When the armaflex is damaged or missing, the condensation buildup can cause a water leak. Make sure seams and joints are tightly taped and butted flush against each other.

Occasionally, the coils on the unit can even freeze up. When ice on the coils changes state after warming up, it can escape out of the pan and onto the floor. You should never see ice on your unit, even in the cooling season.

Clogged floor drain

Condensation produced by air conditioners and furnaces must have a clear drainage path to exit the home. Dirt, dust, and debris can accumulate and clog the floor drain, causing water to pool around the base of the furnace. Pick up any large debris, and use a water safe vacuum such as a Shop-Vac® to remove the rest.

Consult a professional for maintenance help. Even if everything appears to be in working order, ask a verified HVAC Expert to determine that your system is in good repair and safe to operate.

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