Furnace Making Gurgling Water Sounds? Here Are the Causes

What’s that gurgly water noise coming from your furnace?

From the time I’ve spent out in the field, I’ve heard furnaces make all sorts of weird noises.

And while some noises are completely normal, gurgling water noises are not.

The worst part about gurgling water noises is that they can be caused by all sorts of things – from clogged pipes to flooded components.

In this article, I’ll go over the most probable causes of gurgling and water noises that come from your furnace. I’ll also explain how to fix these issues once you’ve figured out exactly what’s causing the noise!

The Condensate Drain Is Clogged

The most common reason a furnace makes a gurgling water sound is a clogged condensate drain.

High efficiency furnaces produce condensation as a byproduct of their combustion. This means that the furnace needs to drain this condensation out of a pipe, just like an AC.

If the condensate drain pipe gets clogged, then your furnace will get backed up with water and make a gurgling sound when it runs.

How to Fix a Clogged Furnace Condensate Drain

If your furnace’s condnesate drain is clogged, then removing the clog is a simple process (for the most part).

Most times, you can remove the clog by hand. Another easy fix is to use a wet/dry vacuum to suck the clog out from the drain pipe.

For more drastic clogs, you might need specialized eqipment like a drain snake or a drain gun.

I go over how to unclog your furnace’s condensate drain (and also how to prevent it from getting clogged in the first place) in my article below:

The Condensate Drain Needs to Be Primed

Another thing that I’ve seen (or heard) is gurgling sounds coming from the drain outlet of an AC coil.

The gurgling is caused by the negative pressure of the blower sucking the condensate water back into the HVAC unit.

This only happens if your AC coil is before the blower, where negative pressure ocurrs.

If this is what’s happening to your unit, then there’s a good chance that the drain pipe is completely dry.

How to Prime Your Furnace’s Condensate Drain

To fix this, you’ll need to “prime” the condensate drain pipe with some water.

Open the cleaning port on the top of your drain pipe and pour water into it. The water will sit in the P-trap in the condensate drain and provide a barrier to prevent air from getting sucked into your unit.

This prevents the negative pressure from sucking the condensate back into your HVAC unit and causing the gurgling sound.

Craig from AC Service Tech has an excellent video that demonstrates how this happens, I recommend giving it a watch:

High Efficiency Furnace Exhaust Pipe Is Flooded

If you hear a gurgling sound coming from your high efficiency furnace’s PVC exhaust pipe, then the exhaust pipe is flooded with water.

High efficiency furnaces produce water as a byproduct of combustion. A correctly installed exhaust pipe will be angled in such a way that the water will flow back toward the furnace and into the condensate drain pipe.

But if the exhaust pipe isn’t installed properly, then water will get trapped in the exhaust pipe and cause a gurgling sound when your furnace runs.

High Efficiency Furnace Exhaust Pipe flooded with water making a gurgling sound

This can potentially become a huge issue since water in your furnace’s exhaust pipe will impede the flow of exhaust fumes.

I’ve seen some furnace exhausts get almost completely flooded with water, and had to completely drain them out before the furnace would run again.

How to Fix an Incorrectly Installed Furnace Exhaust Pipe

The most important thing to do is to ensure that your furnace exhaust pipe has the correct amount of slope throughout the entire length of its run.

I’ve written an article about what you need to know about furnace exhaust pipes here:

The Inducer Is Flooded With Water

Another issue with high efficiency furnaces is that their inducers can get flooded with water.

If your furnace is making a gushing sound inside of it when it begins its startup sequence, there’s a good chance that its inducer is flooded with water.

So how does an inducer get flooded with water, anyway?

Well, if the furnace’s condensate drain pipe gets clogged, that clog can sometimes back up the drainage system all the way up to the inducer.

Water starts pooling up in the bottom of the inducer, and before long, the water level gets so high that the inducer’s fan blade starts sloshing water around when the inducer turns on.

How to Fix a Flooded Inducer

So what should you do when your high efficiency furnace’s inducer gets flooded with water?

The first thing is to remove the inducer from your furnace and drain the water out of it.

Next, you’ll also need to clear out the entire condensate drain system in your furnace – from the hoses inside your furnace to the pipes outside of your furnace.

If that sounds like a bit much, don’t hesistate to hire an HVAC professional to do the work, since it is quite a complicated task.

The Humidifier Is Flooded with Water

A commonly overlooked piece of equipment is right there in plain sight: your furnace’s humidifier.

The humidifer’s job is to add moisture to the air in your home during those cold, dry winter months.

A furnace’s whole-house humidifier works by running water over a pad. The air that flows through the furnace also flows through this pad and picks up humidity as your furnace heats your home.

The problem arises when the humidifier gets clogged up and starts flooding with water.

Humidifier clogs can be really stealthy since they are usually only using a trickle of water.

And in the worst cases, the humidifier water can actually back up into your furnace’s ductwork without you even knowing.

How to Fix a Flooded Humidifier

If you hear gurgling noises coming from your furnace’s humidifier, the first thing that you should do is turn off your furnace and shut off the water valve to your humidifier.

Next, put a bucket under your humidifer and open up the front panel on your humidifier. Hopefully not too much water comes gushing out, but if it does, that’s what the bucket is there for.

After all the excess water is drained out of your humidifier, take a look around the inside of the humidifier and see if you can spot any clogs.

Inspect the top piece of the humidifier pad frame – that’s where there are a few holes that distribute water into the pad. All of those holes need to be cleaned out.

The most important part to clean is the drain hole in the bottom of the humidifier. That hole is responsible for removing excess water from the humidifier. Clean out the drain hole and the hose connected to the drain hole to free them of clogs.

The Condensate Pump Is Having Issues

Sometimes, the water gurgling sound might not be coming from your furnace at all: it’s coming from your furnace’s condensate pump.

Condensate pump for a high-efficiency furnace making a gurgling sound

The condensate pump is the device that’s responsible for removing the water from your furnace. It’s used when your condensate drain pipe isn’t high enough for the water to drain naturally by using gravity.

But just like any other piece of mechanical equipment, a condensate pump can break down, even if nothing else is wrong.

How to Fix the Condensate Pump

If your condensate pump is having issues, remove the cover from the pump and take a look inside of it.

Inspect the pump’s basin for any clogs, debris, and buildup. You might need to take it outside to empty it out and spray it down with water.

Also inspect the drain line that connects to the condensate pump for any clogs and clear it out if you find any.

If your condensate pump is still not working correctly after a deep cleaning, you’ll need to replace it.

Fortunately, condensate pumps don’t cost too much and they are pretty much an off-the-shelf item.

Refrigerant Is Leaking from the AC Coil

If you have an AC coil installed in your furnace, then a water dripping or bubbling noise might be coming from a leaky AC coil.

The AC coil contains a refrigerant gas, and if one of the refrigerant lines that connects to the coil springs a leak, then your furnace might start making a watery noise.

If you hear this noise coming from outside your furnace, there’s a good chance that the leak is at the filter drier. The filter drier has two soldered connections, and if these connections weren’t soldered correctly, then it might start leaking out refrigerant and make a bubbling noise.

If year hear this noise coming from the inside of your furnace, then it could be the AC coil itself that’s leaking refrigerant.

In either case, you’ll need to get an HVAC professional to diagnose the issue and make the repair.

Furnace Sounds Like Running Water

If your furnace is just making a gentle sound, like you just turned on the water faucet, then it could be that nothing is wrong at all.

Some gas furnaces make a subtle “flowing” sound when gas moves through their pipes, similar to water running through the pipes in your home.

If that’s the case, it could be that there is nothing wrong with your furnace and everything is working correctly.

What’s Next?

Furnaces are known for making all sorts of noises. Gurgling water noises, while not extremely rare, can be quite troublesome since all sorts of things cause them.

If you’re still not sure what that weird noise that’s coming from your furnace is, I’ve written another article about all the other types of noises that a furnace makes.

You can read it here:

However, if you’ve gone through my entire list and still can’t figure out where that gurgling water noise is coming from, it might be time to call in a professional to diagnose the issue.

As always, feel free to drop a comment below and I’ll try to help!

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

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