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Why Is My Furnace Leaking Water?

Finding water dripping from your furnace or pooling around it is an understandable cause for concern. Not only is it a nuisance, but a leaking furnace can also be a sign of a more serious issue with your heating system. Before you can solve this issue, you must first ascertain whether the leak is actually coming from your furnace – there are two main types of furnaces, and only one of them involves water in their normal operations.

If you have a high-efficiency furnace, these units can leak water due to the condensation that they produce. The issue may lie with any of the components involved in draining the condensation, including the inducer assembly, condensate drain line, condensate trap, or condensate pump.

If you have a conventional furnace, these units do not involve water in their normal operation; any water around these units must therefore be leaking from elsewhere. 

How Do Furnaces Work?

There are two main types of furnaces, ‘conventional’ and ‘high-efficiency’, that work in slightly different ways to heat your home. We’ll get into the specifics of these different types of furnaces later, but first, let’s discuss the basic operation of both types.

Furnaces have in-built thermostats that monitor the indoor temperature of your home. When the indoor temperature drops below the thermostat’s setting, it sends a signal to the control board of the furnace. This begins the furnace’s heating process by opening its gas valve, activating its ignition switch, and turning on its fan. The combination of gas, air, and the ignition element causes combustion, in turn warming the furnace’s heat exchanger. 

The waste flue gases from the combustion process pass through an exhaust pipe to the outside of your home. Meanwhile, the blower fan intakes cool air from inside your home and passes it over the heat exchanger. The heat exchanger warms up the cool air, which is then channeled into your home’s ventilation system. The furnace will continue to work in this way until the indoor temperature meets the setting of the thermostat. From this process, your home is warmed up to the desired temperature. 

Now you know how your furnace works, this raises the question of where the water comes from if you spot a leak. The answer to this question lies with the type of furnace you have – as we’ll go on to explain in the next section. 

Does My Furnace Produce Water?

If you have a conventional furnace, or standard efficiency furnace, your unit does not produce water. However, if you have a high-efficiency furnace, these units do produce water from the condensation that forms on their secondary heat exchanger. For this reason, conventional furnaces are known as non-condensing furnaces, whereas high-efficiency furnaces are known as condensing furnaces.

High-efficiency furnaces have a second heat exchanger that grants them their high-efficiency operation. The burning gas in these furnaces passes from the primary to the secondary heat exchanger, where it undergoes a secondary heat exchange. This process creates water vapor, which transitions to a liquid form, releasing more heat into the secondary heat exchanger. The furnace then drains this water into a condensate pump or floor drain.

With that in mind, it’s only possible for a high-efficiency furnace to directly leak water. So, if you’ve noticed water around your furnace but you have a conventional unit, the water must be leaking from elsewhere.

Is My Furnace Conventional or High-Efficiency?

To determine whether your furnace really is the source of the leak, the first step is confirming that you have a high-efficiency unit. If you’re unsure, there’s an easy way to tell which type of furnace you have.

Take a look at the exhaust pipe on your furnace. If the exhaust pipe is a white plastic PVC pipe, this means you have a high-efficiency condensing unit; i.e., this is an indication that your furnace is the source of the leak. If the exhaust pipe is made from metal, you have a conventional furnace, therefore the leak must be coming from elsewhere.

Why is My Furnace Leaking Water?

Assuming you’ve confirmed you’ve got a high-efficiency unit, we can now explore the reasons why your furnace is leaking water. In short, a high-efficiency furnace leaking water is a sign that the condensation is unable to properly drain. 

As we’ve explained, high-efficiency furnaces create a lot of condensation when they release their exhaust gases. The moisture forms on the PVC exhaust pipe and drains via a floor drain or condensation pump under normal operations. Your furnace will therefore leak water when the condensation fails to drain properly. When draining, condensation passes through a few different parts of the furnace that can leak water if clogged or damaged; we’ve gone through each of these parts below to help you find the source of the leak.

1. Blocked or Damaged Condensate Drain Hose

The condensate drain hose is a sloping part that attaches to the PVC exhaust pipe. This is the first part that condensation passes through when draining from the furnace. It’s easy for this part to become clogged with dirt, dust, and other debris. If any of the surrounding tubing aiding the furnace’s drainage becomes clogged, or otherwise damaged or disconnected, it can lead to a water leak.

2. Cracked Inducer Assembly

From the condensate drain hose, the condensation then travels through the exhaust pipe and into the inducer assembly. This is another point from which water can leak out if the inducer assembly is cracked or damaged in any way. 

3. Loose, Cracked, or Clogged Condensate Drain Line

Next, the condensation travels through the condensate drain line. If this part has cracks, holes, or a loose connection, water can leak out of it. The condensate drain line can also become clogged with dust or debris; this causes the water to back up which then leads to a water leak. 

4. Clogged Condensate Trap

Not all high-efficiency furnaces have a condensate trap, but if yours does, this is another part that may leak water. If the condensate trap becomes clogged, water may back up and overflow out of the furnace.

5. Clogged Condensate Pump 

The condensation finally drains into the condensate pump. Again, this part can become clogged, resulting in water leaking from the furnace. Even a drain that is only partially blocked can lead to a water leak. Noticing water on the floor around the furnace is a clear sign that the furnace is failing to drain properly; another indication of this is if there are water or rust stains within the furnace’s cabinet. Furthermore, the condensate pump may simply fail to work properly, causing a backup of water and a leak. As a note, condensate pumps typically last 3 to 5 years before they need to be replaced to avoid leaks. 

Is a Furnace Leaking Water Dangerous?

Safety-wise, there isn’t much of an immediate threat if your furnace is leaking water, as this issue isn’t necessarily dangerous. Even so, any source of leaking water in your home is something that you’ll want to address quickly; if you leave the problem untreated, it can lead to bigger issues that may end up costing you more to remedy.

When water leaks out of your furnace, it can cause rust to form on the furnace’s framework and electrical components. This rust will gradually destroy the furnace’s components, first reducing its efficiency, then causing it to stop working entirely. The water can also lead to the formation of mold in and around the unit. To avoid such damage to any part of your home, you’ll need to take action to stop the leak as soon as possible.

What to Do if Your Furnace is Leaking Water

There are some immediate steps you can take when you suspect your furnace is leaking water. There’s no need to panic, as this isn’t a safety concern; however, you’ll want to stop the leak ASAP before seeking professional help to prevent water damage to your property. 

1. Turn Off Your HVAC System

The first step is to turn off your furnace to stop the flow of water. There are a few ways to do this; you can switch it off using the thermostat, or locate the furnace switch, before turning it off here. The furnace switch is typically found on or next to the furnace. If you can’t find this switch, you can cut power to the furnace completely via your circuit breaker box.

2. Clean Up the Leaked Water Around the Furnace

You’ll then want to clean up the leaked water on the floor surrounding the furnace. Depending on the extent of the leak, you should be able to get it all up using paper towels, rags, and/or larger towels. If there’s a significant amount of water, it may be easier to use a wet vac for this step.

3. Clean Up the Water on the Inside of the Furnace Panel

If you feel confident in doing so, it’s best to clean up the water from the inside of the furnace too. This is beneficial in preventing mold from forming in and around the unit. Unscrew the front panel of the furnace before using a rag or towel to remove all of the moisture.

4. Get in Contact with Your Local HVAC Specialists

Finally, you should get in contact with your local HVAC specialists for the inspection and repair of your furnace. For instance, we at Metro Heating and Cooling are always here to help with advice when your furnace is leaking water or otherwise malfunctioning. We’ll send out a specialist as soon as possible to assess your furnace and carry out the necessary repairs.


Schedule a HVAC repair anywhere in the Minneapolis or St. Paul area today by calling (651) 294-7798 or by requesting an estimate online.


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