Plumber tests for leaks. Plumber fixing central heating system.

Noticing water leaking from your furnace is alarming, but don’t panic. There are several possible reasons this may be happening. You can address some issues yourself, but others will require hiring a technician.

This guide breaks down how to diagnose why your furnace is leaking water and recommends next steps.



How Does My Gas Furnace Produce Water?

Before diagnosing the cause of the leak, it’s important to understand how a furnace works and why it produces water. Here’s a step-by-step breakdown.

  1. When the indoor temperature drops below your thermostat’s setting, the thermostat sends a signal to a control board inside your furnace. 
  2. The furnace begins the heating cycle by turning on an ignition switch, which opens a gas valve and turns on a draft fan. This combination of events creates combustion.
  3. The combustion warms two coils, referred to as a heat exchanger. 
  4. The gasses inside the heat exchanger are released through an exhaust pipe. 
  5. The blower fan blows cool air from inside your home past the heat exchanger. The air absorbs the heat and is forced through your ventilation system, warming your home.

High-efficiency furnaces have an additional heat exchanger that produces condensation. After the gas exits the primary heat exchanger, it goes to a secondary heat exchanger for further heat exchange. This process creates water vapor. As water transitions from a vapor to a liquid, more heat is released into the secondary heat exchanger. The condensation is drained into a condensate pump or a floor drain.

Conventional furnaces don’t use this secondary heat exchanger, so they don’t experience water leaks from the furnace itself.



What To Do When Your Furnace Is Leaking Water

You can follow these steps to understand why your furnace is leaking. We’ve also explained some repairs you can complete yourself. 

#1. Determine Which Type of Furnace You Have

The following steps depend on whether you have a conventional furnace or a high-efficiency condensing one. You can skip to step two if you already know which one you have. If not, here are the three main differences between the two furnace types to help you figure it out.

  • AFUE rating: Your unit should have a yellow Energy Guide that displays the Annual Fuel Utilization Efficiency (AFUE) rating. Conventional furnaces have an 80% efficiency rating, which converts 80% of the energy created to heat your home. The other 20% is lost through the chimney, leaks, and other openings. High-efficiency furnaces have an AFUE rating of 90% or above.
  • Exhaust pipe: The furnace exhaust pipe material is the most significant indicator of your unit type. The exhaust pipe is often referred to as the flue pipe. If the pipe is polyvinyl chloride (PVC), meaning it looks like white plastic, you have a high-efficiency furnace. A metal pipe means you have a conventional standard efficiency furnace.
  • Home age: Most newer homes have high-efficiency furnaces. If you’ve recently moved into a newly built home or a home where the heating and cooling systems were replaced, you likely have a high-efficiency model. Older homes that have not been updated have a conventional standard efficiency furnace.

#2. Inspect the Most Likely Leak Sources

Both types of furnaces have multiple common causes for leaks. Check the following areas.

Common High-Efficiency Furnace Leak Sources

Here are some possible causes a high-efficiency furnace leaks water: 

  • Condensate drain hose: The drain hose, which you can see sloping down from the PVC exhaust pipe, might be clogged with debris. If any tubes that aid the draining are damaged, disconnected, or clogged, you’ll notice a condensation leak.
  • Condensate drain line: The line where the water exits the furnace is called the condensate drain line. As with the drain hose, loose connections, cracks, or holes in the line can cause a leak. A clogged line can also cause the condensate to back up and create a leak.
  • Condensate pump: The condensate pump is where the condensate eventually drains into. A blocked drain results in a leak. The furnace is not draining properly if you notice water on the floor around your unit or water stains or rust within the cabinet. A malfunctioning pump can cause water to back up and cause a furnace leak. Most condensate pumps last three to five years. 
  • Inducer assembly: The area where the condensation runs down the exhaust pipe, through the condensate drain hose, and into an assembly is called the inducer assembly. If the inducer assembly is cracked or damaged, water will leak out of the furnace.
  • Condensate trap: If you don’t notice any of the above problems, a clogged condensate trap might be the issue. This can cause condensate to back up and overflow out of your furnace. Not all high-efficiency furnaces have a condensate trap, so a professional will likely need to check to confirm.

Common Conventional Furnace Leak Sources

Conventional furnaces don’t produce water, so if you notice water leaking, it’s coming from another part of your HVAC system. Here are some common reasons water leaks around a conventional furnace: 

  • Condensate pump: Your air conditioning (AC) unit’s condensate pump, which drains the condensation from the HVAC, may be malfunctioning. 
  • Drain line: If debris clogs your AC’s drain line, the drain will disconnect from the drain pan. The water will then drip out and pool around your AC unit. 
  • Drain pan: A crack or hole in the pan located under your AC’s evaporator coil or the secondary drain pan can cause a leak.
  • Frozen coil: If the evaporator coil inside your air conditioner freezes, your AC unit will begin to leak. 
  • Hot water heater: Your hot water heater’s drain valve or T&P (temperature and pressure) valve can loosen, causing water to seep out. In addition, any crack in your water heater will cause a leak. If you live in a region with hard water, the water’s corrosive effect causes cracks.
  • Humidifier: Whole-house humidifiers require plumbing for water. Any cracks or clogs in the humidifier will cause leaks.  

#3. Check What Happens When Your AC is On

Even if you think you know the source of the leak, we recommend running your AC to see if it’s the cause. For example, if the puddle of water around your furnace only seems to grow when the air conditioner is on, the problem is likely with your AC and not the furnace.



Contact a Professional

AC and furnace repairs or replacements are best handled by a professional. There are many components and variables that change what kind of repair you need. An HVAC professional can quickly diagnose and remedy these problems.

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Frequently Asked Questions About Furnaces Leaking Water



When to Consider a Broader Home Inspection

When purchasing or selling a home, it is critical to complete a thorough inspection to understand the condition of the property. A standard home inspection includes an assessment of a home’s systems and physical structure. After the process, the inspector will provide a report detailing their findings and recommendations.

If you are thinking about buying a home or putting your home on the market, we strongly recommend finding an ASHI home inspector in your area.