Furnace Leaking Water: Troubleshooting and Repair

No homeowner wants to walk into their basement and find a puddle of water beneath the furnace. Is it a foundation issue, a loose washer, or a several thousand dollar replacement bill? Before you panic, a furnace leak can be something simply like an airflow or condensate system issue. The leak might not come from the furnace at all but from nearby plumbing or a hot water heater. 

Here’s how to identify the cause of that puddle of water and how to prevent and fix a leaky furnace.

Addressing Furnace Leaks

What do you do when you see that pool of water? Use the thermostat to turn off the HVAC system. Clean up any standing water around the furnace and on the basement floor to prevent further damage and safety hazards. Check the furnace and around it for any obvious sources of leaks (see below). If you can’t find what’s causing the leak, consider calling an HVAC professional. 

Above all else, don’t grab a beach towel to mop up that water and ignore the problem. It can, and likely will, grow bigger. A furnace leak can indicate some or all of these problems:

Airflow Issues

A furnace needs proper airflow to function efficiently and avoid leaks. If circulation gets interrupted, it can lead to many problems, including water leakage. There are three common causes of airflow issues.

Blocked Air Return

If dust or debris blocks the air return vent, it restricts airflow to the furnace. Condensation (water vapor) will build up inside the air handler, and the excess water may leak out of the unit onto the floor. To prevent this, regularly check your air return vents to keep them free from obstructions.

Clogged Filter

A dirty or clogged air filter also limits airflow, leading to condensation build-up, just like with a blocked air return. To maintain proper circulation and clean air, homeowners should replace their filters every 1-3 months. Before changing the HVAC filter, use the thermostat to turn off the furnace. Neglecting to replace the air filter leads to reduced furnace efficiency, as the furnace is trying to push hot air through a dirty filter, which can end in water leakage issues.

Blocked Vent Pipe

Gas furnaces have a flue pipe (or vent exhaust pipe) that expels exhaust gases outside the house. If snow, ice, or debris blocks the pipe, it can cause condensation to back up inside the furnace. If you live in a cold climate, it’s especially important to inspect and clear the vent pipe regularly. Keeping the flue pipe unobstructed keeps your gas furnace operating efficiently.

High-Efficiency Furnace Issues

The unique design of high-efficiency condensing furnaces makes them more prone to certain leaks. Two components, the inducer assembly or the secondary heat exchanger, may cause leaks.

The inducer assembly pulls exhaust gases through the heat exchanger and pushes them outside the home. If it develops a leak, water may drip into the house. Unfortunately, this problem typically requires professional attention to diagnose and repair. 

The secondary heat exchanger in a high-efficiency furnace extracts additional heat from the exhaust gases. If this component gets clogged, calcium carbonate (a byproduct of the exhaust gases) can build up and increase condensation and, thus, leaks. Homeowners with a high-efficiency furnace will want to schedule annual maintenance calls to keep them functioning at peak efficiency and avoid leaks.

Condensate System Problems

The condensate system within a furnace consists of a pump, drain pan, drain, and trap. As condensation occurs along the evaporator coils, it runs down into a drip pan and then into a drain. From there, it will either flow outside directly or through a condensate pump, depending on your system. Issues with this water flow can lead to leaks which are usually serious, and require an HVAC professional to perform the repair. 

Being vigilant about maintaining each component of the condensate system prevents leaks.

Blockages

If the evaporator coils get dirty, the condensate system can get blocked. In a filthy basement, the condensation mixes with the dirt and leaks onto the floor or any surface below the runoff. Dirty water can, in turn, clog the system anywhere along the line. 

Annual furnace cleaning and properly installing air filters help prevent clogs and improve overall system efficiency. Also, avoid performing do-it-yourself tasks that involve sanding wood or anything that would put more dirt and particulates into the air. 

Since it all heads to the drain trap, it’s the most common source of blockage. Even a small amount of dirt can clog the trap’s bottom. Use a long, flexible cleaning brush to clear any potential blockage, even if you don’t have any leaks yet. Afterward, flush out the trap by pouring water down the cleanout cap. 

Cracked Drip Pan

The drip pan, a thin, inexpensive plastic container, cracks easily from impacts or pressure from the PVC pipe. While replacement parts are readily available due to their fragility, it’s generally best to have a professional replace the drip pan. If you notice that your drip pan has cracked, replace it promptly to prevent further water damage.

Faulty Condensate Pump

In some cases, the condensate pump may have simply become unplugged, which is easy enough to fix. However, if the pump is faulty or damaged, contact a professional to replace the unit. The condensate pump prevents water overflow and is an important part of the system.

External Issues

If you see water under your furnace, look around. Flooding near a furnace may be caused by an attached nearby unit, which is especially true in more moderate climates where multiple HVAC devices may be running daily. These problems are less likely to require professional repairs, so it’s best to check them before you place an expensive service call.

Air Conditioner

Many modern HVAC units combine air conditioners and conventional furnace units for greater efficiency. The condensate drain system is actually a component of central AC systems, not furnaces. Maintaining your air conditioning unit also impacts your furnace and reduces the likelihood of leaks.

Humidifiers and Dehumidifiers

Dehumidifiers remove moisture from the air, and if you have a wet, moldy basement or live in a humid climate, you might have one installed in your basement. Older models have a drip pan that you have to empty regularly, while newer models may connect directly to the drain. If a furnace and dehumidifier are close to each other or share a common drain, overflows or clogs coming from the dehumidifier may be mistaken for a furnace issue.

Many homeowners choose to install a whole-house humidifier that attaches to their furnace to improve air quality and keep their home more comfortable in winter. A humidifier adds water back into the air. These units connect to the home’s water supply and may develop leaks, clogs, or cracks. As a result, water can drip down onto the furnace or nearby floor, so it will look like the furnace is leaking. 

Most whole-house humidifiers send excess water into a floor drain that connects to a sump pit. If you notice water pooling around the drain, check the sump pump.

Plumbing and Appliances

While not a likely factor if your HVAC is in the attic, most furnaces located in basements have plumbing nearby. Leaks can spring up in ceiling pipes, and then water drips down onto the furnace or nearby floor. This is especially likely to happen if the furnace has a drain connected to it since the floor will slope gently to encourage runoff.

Basements with a laundry room also might spring a “fake” furnace leak. The most common culprit is if the washer develops leaks or has a loose drain hose. Depending on how close it is to the furnace and whether the basement has multiple drains, a leaky washer can easily flood the area around the furnace, and you might not be able to tell where the water is coming from.

Washtubs are another potential source of water in your basement. A partial bath that has a sink and toilet would have separate plumbing, but a washtub may share a common drain with the washer. Some washers drain directly into the washtub – look for a hose that’s propped up and over the sink’s rim. Debris from an unfiltered hose can clog the sink drain and cause the basin to overflow. Leaks in the sink’s drain may cause flooding, which will pool around the nearest drain (again, possibly near the furnace). Monitoring basement plumbing and laundry machines can help quickly identify and resolve this type of leak.

Preventing Future Furnace Leaks

To minimize the risk of future furnace leaks, schedule regular HVAC maintenance with a professional technician. Many local HVAC companies offer prepaid service plans, which might save you money and remind you to get it done. 

Put a reminder in the calendar to replace furnace filters every one to three months and keep the area around the furnace free. Regularly inspect visible parts of your system for any signs of wear and damage, and address any minor issues promptly before they grow into larger problems.

Have a Furnace that’s Leaking Water? It’s Not the End of the World

If you know the causes of furnace leaks and what to look for, a leaking furnace doesn’t need to cause a panic attack. Regular maintenance and prompt attention to minor problems help prevent more serious problems down the line. While many homeowners can fix the simpler causes of furnace leaks, other causes may require professional intervention. 

When in doubt, it’s always best to consult with a qualified HVAC technician to diagnose and resolve any furnace-related issues. If you pay proper attention to furnace leaks, you’ll have an efficient and working furnace for years to come.

Additional Resources

Hannabery HVAC provides an article with a detailed chart of which damages will require professional repair.

Standard Heating and Air Conditioning provides a diagram illustrating the components of a basic modern HVAC unit.