Why Is My Furnace Leaking Water? (7 Issues + Fixes For Condensation)

“My furnace is leaking water. Should I panic?”

Finding water around your furnace is not ideal. Luckily, it’s also not tragic (in most cases). There are several reasons why the furnace is dripping water. The main culprit when you see water coming from a furnace is the damaged condensation drain.

Here’s the deal:

If you see condensation from the furnace, you don’t have to panic immediately. The majority of water coming from furnace issues can be easily solved. Yes, you will need a certified technician, and you shouldn’t delay. A pool of water around your furnace can expose you to mold, furnace-room flooding, or, worst of all, flooding inside the furnace.

We will cover the 7 most common reasons why you see condensation from a furnace (and how a technician would fix them). You can check all of them and rule them out one by one.

Why your furnace is leaking water depends on the type of your furnace:

  • Issues #1, #2, #3, and #7 are most common with high-efficiency gas furnaces.
  • Issues #4, #5, and #6 are most common with standard-efficiency gas furnaces.

Educating yourself what might be causing anything from water leaking from the bottom of the furnace to furnace leaking water from the overflow pipe is the smartest thing you can do before calling a technician to fix a gas furnace leaking water.

First of all, you need to check what kind of furnace you have. This will help us more aptly understand what might be causing the furnace leak. We only see condensation from a gas furnace. You need to check if you have a high-efficiency gas furnace or a standard-efficiency furnace. In more than 80% of cases, we see water coming from a high-efficiency gas condensing furnace.

Note: If you will figure out that you need a technician to repair the furnace, we pointed out how to find the right professional at the end of this instructional article.

Before we look at the 7 things (furnace troubleshooting) that might be causing your furnace to drip water, let’s look at how you can check what kind of a gas furnace you have:

Condensation From High-Efficiency VS Standard-Efficiency Gas Furnace

Furnace leaking water in winter is much more common with high-efficiency gas furnaces. That’s because standard-efficiency furnaces are not supposed to leak water at all. Running a standard-efficiency furnace doesn’t involve condensation; running a high-efficiency furnace does. We’ll explain this later on.

Here’s are 2 foolproof ways how you can tell what type of furnace you have:

  1. AFUE Rating. Furnaces with above 90 AFUE rating are high-efficiency furnaces. 90 AFUE (Annual Fuel Utilization Efficiency) rating means that a furnace will have 90% energy efficiency. Furnaces with below 90 AFUE rating are standard-efficiency furnaces. High-efficiency condensing furnaces usually have a yellow sticker on the unit with a listed AFUE rating.
  2. Check Condensation Pipe. High-efficiency furnaces are drained via a white plastic pipe (PVC exhaust pipe). Standard-efficiency furnaces are drained via a metal exhaust pipe or flue pipe. It’s these just in case; you don’t expect a condensation from a standard-efficiency furnace. Condensation pipe is located at the back of the furnace.

The main difference in how a high-efficiency vs standard-efficiency furnace operates is that the high-efficiency furnaces have 2 heat exchangers (standard ones only have 1).

How High-Efficiency Gas Furnaces Operate? (Production Of Condensate)

A high-efficiency furnace can achieve above 90% of the energy produced by burning gas. Obviously, we love the high efficiency, but it also exposes the furnace to the possibility of water coming from a furnace.

A high-efficiency furnace uses the secondary heat exchanger to extract more heat. This, in turn, causes the produced gases to cool down. When gases cool down, you will see condensation.

That’s why every high-efficiency furnace has a condensation drain (PVC exhaust pipe). When the furnace operates normally, the condensation from the furnace is vented out safely via this drain.

The main reason why your furnace is leaking has something to do with the malfunctioning drainage system. This brings us to the first 3 culprits for furnace dripping water:

#1 Condensation Pipe Is Clogged (Most Common Culprit)

When operating normally, there is a free flow of water (condensate is a side product of extracting extra energy from burning gas) in the furnace PVC overflow pipe. A clog will stop that free flow.

A condensation tubing might be clogged due to the accumulation of dust, dirt, mold, or even ice. When you start the furnace, the condensation is created and needs to be channeled out of the furnace via this pipe. If the pipe is clogged, the water will start accumulating within the furnace.

Eventually, the accumulated water will start to drip out of the furnace. We will see this as water coming from the bottom of the furnace.

Solution: Easy, just unclog the pipe. You can use pan tablets to take care of the clog. That’s easier said than done, however. To get to the drain pipe, you need to open the gas chamber. There are quite a few moving parts there. This is best left for a professional furnace technician.

In some cases, the condensate trap itself can be clogged as well.

After the PVC exhaust pipe is unclogged, you have to reassemble the furnace, and the water will drop leaking from your furnace when running. All the produced water will now be channeled out of your furnace via the unclogged pipe.

Note: This is one of the most common reasons why your furnace is leaking water in the winter. If you haven’t used the furnace in the summer, dirt and mold had time to accumulate and create that clog in the PVC pipe.

#2 Condensation Pipe Leak (Furnace Dripping Water)

If your furnace is dripping water (drop by drop), you might have a drain pipe leakage problem. This is similar to a clogged exhaust pipe. In this case, there is a hole in the pipe.

This is a reason why your furnace is leaking water only in the case of high-efficiency furnaces. These modern furnaces use PVC pipes. PVC pipe can be pierced (due to wear and tear) which will create a hole through which a small amount of water will drip. This can’t happen in standard-efficiency furnaces because they use metal exhaust pipes or metal flue (those don’t break that easily).

If the pipe has a hole, you will see the furnace dripping water. Usually, this involves small quantities of water; a clogged drain pipe will generate much more water around your furnace. If you look at the pipe itself, you will see water dripping from the furnace vent pipe.

Solution: Fix the leak or replace the pipe. In theory, you might want to fix the leak. In practice, however, it’s a common practice to just replace the drain pipe. This is a standard PVC pipe that’s cheap and you can buy it pretty much anywhere.

Again, this involves disassembling and reassembling the furnace; that’s the hard part. It’s better to call a professional to do this.

#3 Condensation Pump Issues (Water Accumulation)

The condensation pump creates pressure that pushes the condensate via the PVC vent pipe out of the furnace. That’s a very efficient way to get the condensation out of the furnace.

In many cases, the gas furnace will not function adequately if the condensation pump is not doing its job. If you see water coming from the furnace, chances are you have a faulty condensation pump.

How to fix water coming out of your furnace due to a malfunctioning condensation pump?

Solution: Fix the pump or replace the pump. First of all, you have to see what the issue with the pump is and if it can be repaired. A professional will notice the issue quickly enough. In many cases, it’s easier (and quicker) to just replace the pump. These pumps cost less than $50.

When you have a functioning condensation pump again, you will see that the furnace leaking water from the overflow pipe has stopped.

#4 Incorrectly Sized Exhaust Pipe (Furnace Leaking Water From Overflow Pipe )

As we have talked about earlier, you don’t expect a leak from a standard-efficiency gas furnace. There should be no condensation happening in this type of furnace. But just in case if the condensation starts, most of these older furnaces have metal exhaust pipes as safety procedures.

Now, there are several reasons why an older gas furnace that shouldn’t produce condensate, starts producing condensate. The main thing we see is the furnace leaking water from the overflow pipe.

Normally, this is a case of an incorrectly sized exhaust pipe. That means that the pipe (without a condensation pump; older units don’t have them) is too small to allow for adequate drainage. Obviously, however, the heart of the problem can be anything from a faulty heat exchanger to gas leakage.

Solution: Call a professional. Yes, the first instinct is to install a bigger exhaust pipe, but that’s just treating the symptom, not the problem. It’s difficult to pinpoint exactly why you see condensation from a furnace that shouldn’t be generating condensation in the first place.

A professional who knows his or her way around these older furnaces will be able to detect the problem and repair it. In some cases, you might even have to get a furnace replacement.

#5 Faulty Whole-House Humidifier

Some gas furnaces have a whole-house humidifier connected to the furnace. Needless to say, a humidifier holds quite a lot of water. Since a whole-house humidifier and a furnace are connected, any issue with the humidifier can potentially flood your furnace.

First, you might see water coming from the furnace. However, if you suspect the whole-house humidifier might be the root cause of the water underneath the furnace, check the humidifier first.

If you see that humidifier has to be refilled more often than usual, that’s a good indication that it’s not using all the water you pour into it for humidification. A small or large part of that water might be channeled to the connected furnace, flooding it in the process.

Solution: You need an HVAC professional here. General HVAC professionals can take a look at both the furnace and the whole-house humidifier connected to it, and see if a faulty humidifier is causing the furnace to leak water.

#6 Internal Drain System Clog (AC Furnace Leaking Water In The Summer)

If your air conditioner is connected to the furnace (AC furnace), the leaking AC drain might flood your furnace as well. This is one of the most common reasons for furnaces leaking water in the summer.

In summer, our air conditioner is running and extracting quite a lot of water moisture from indoor air. All that accumulated water is drained via a drain pipe.

In the event of AC furnace leaking water, it’s mostly because the AC and furnace share the same internal drain. When this drain is clogged, the water from the air conditioner can be rerouted toward the furnace. Basically, you will have water coming from the furnace and no idea why this is happening in the summer when the furnace is not even running.

Solution: Unclog the internal drain that is shared by both the air conditioner and the furnace. If you can locate the internal drain without having to disassemble an AC or a furnace, you can DIY furnace leaking water repair yourself. If you can’t, it’s best to call an HVAC professional.

Once the clog is removed, the air conditioner can be adequately drained again, and it will stop sending water to your furnace.

#7 Issue With Secondary Heat Exchanger (Most Expensive Furnace Water Leak Repair)

As we have talked about earlier, only high-efficiency 90+ AFUE rating furnaces have a secondary heat exchanger. That’s the very place where additional heat is extracted from the gas. This causes the gas to cool which in turn produces condensation in the furnace.

If something is wrong with the secondary heat exchanger, more condensate might be produced. When the amount of condensate is bigger than what the PVC overflow vent pipe can handle, the water will start coming from the furnace.

What exactly is wrong with the secondary heat exchanger is hard to say. The main issue is when this heat exchanger starts to malfunction, it’s very hard to repair it. That leads to an unwelcoming solution.

Solution: Replace the furnace. In very limited cases, the secondary heat exchanger can be repaired (costly repair). But in most cases, you will need a new furnace. That’s why this is the most expensive furnace leaking water issue.

Do You Need Professional Help With Furnace Leaking Water?

In most cases, you will need a professional to take a look at your furnace. Disassembling and reassembling the furnace is a complex process and usually, the primary reason why you would need a professional.

It’s best to call your local HVAC company and see if they can help you with a furnace leaking water.

We hope that all of this was at least a bit helpful.

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