Confused Why Your Furnace Is Leaking Water? Here’s Why

Furnace has lots of dust

Are you confused as to why your furnace is leaking water all over your flooring?

You’re not alone! Although most people wouldn’t think of furnaces and water together, having these appliances leak affects thousands of people worldwide daily, leaving them cold and scratching their heads.

I understand how annoying it can be to have your furnace leak water and make a mess all over your floors. But don’t worry; you’ve come to the right place for answers. Below, you’ll find a list including 5 of the most common causes behind a furnace that’s leaking water and simple ways to address each one.

When your furnace is leaking water, it can be due to condensation, a humidifier leak, an issue with the heat exchanger, or frozen coils. A clogged Air Conditioner drain can also be to blame for the situation.

Keep reading to stop your furnace’s leak!

Why Your Furnace Is Leaking

Leaking furnaces are more common than you’d think. There are many possible reasons that can explain why your furnace is leaking, and they vary in degree of complexity, difficulty of repair, and commonality.

To help you save time, effort, and money, here are some of the things I typically consider first when dealing with a leaking furnace.

#1 Condensation

The first thing you must check when your furnace is leaking water is for condensation. In my experience, high-efficiency furnaces can produce condensation, which is usually channeled away by pipes or a floor drain. However, if there’s a blockage or break in the pipes or if the floor drain is clogged, water can leak.

Is Window Condensation A Bad Thing?
Condensation can cause your furnace to leak water and promote the formation of rust, corrosion, and mold.

If your furnace isn’t high-efficiency, it shouldn’t have condensation issues; however, even in non-high-efficiency units, the issue is not unheard of. Sometimes, improperly sized vent pipes can cause some condensation to flow back into the furnace.

Solution: In cases of condensation, I find the best course of action is to call a certified HVAC technician to pinpoint the source of the issue, ensure the floor drain isn’t clogged, and check whether the vent pipes are the right size.

#2 A Humidifier Leak

The next possible reason why your furnace is leaking water is a humidifier leak. Furnace humidifiers draw water from your home’s pipes and combine them with the furnace’s air. Leaking often indicates a loose or broken pipe.

When the humidifier is not working normally, it cannot only cause leaking but also draw dirty air and send it to your furnace, where it will be recirculated, contaminating your home with bacteria and increasing the chances of mold formation.

Solution: Checking a furnace’s humidifier isn’t the simplest nor the most DIY-friendly task, so my usual advice is to leave the job to a professional.

The technician will check whether there’s a leak or anything else wrong with your furnace and address it as necessary. Depending on your furnace’s brand and model, the humidifier’s location can vary, so try to have your User Manual handy to make locating the component easier for the technician.

If you no longer have the manual, that’s okay; please refer to our free resource below.

#3 Issues With the Secondary Heat Exchanger

A bad or broken heat exchanger can also explain why your furnace is leaking water like crazy. Depending on your furnace’s model, it might have 1 or 2 heat exchangers. From what I’ve seen, furnaces with 2 heat exchangers are not only more efficient but also more durable than their counterparts.

Heat exchangers are metal shields installed between the combustion chamber and the blower wheel assembly, and they’re responsible for transferring heat from the furnace to the ducts in your home, resulting in the warmth you know and love.

Over time, the heat exchanger can crack due to the extremely high temperatures inside the furnace, leading to many issues, including a flickering furnace flame, air drafts, and, of course, water leaks.

Solution: The only way to fix a broken heat exchanger is to replace it. The process isn’t simple, so again, I wouldn’t recommend you try addressing the situation yourself.

The cost of replacing the heat exchanger will vary greatly depending on your furnace’s brand and model; however, the average replacement price for the part is about $1,500. It’s important to check that the heat exchanger is, in fact, broken before buying the replacement, as the issue could lie somewhere else, so you don’t want to spend money for nothing!

#4 Frozen Coils

Frozen coils can also explain why your furnace is leaking water. The situation can arise from several different factors, but the most common ones are low winter temperatures and refrigerant leaks.

If you haven’t used your furnace in a long time and you’re trying to fire it un for the colder winter months, there’s a good chance that there’s a bit of ice forming in and around the appliance. When you turn up the heat, the ice melts, and it appears as though the furnace is leaking. In such cases, the leaking is completely normal and unrelated to the appliance, so there’s nothing to worry about.

Cleaning fridge condenser coils
Check that your furnace’s coils aren’t frozen and that there’s not a refrigerant leak causing problems.

On the other hand, if the leaking comes from a refrigerant problem, it’s an entirely different story. If refrigerant leaks in your furnace, it can completely freeze the appliance’s coils and other components. As you turn on the furnace, the ice will melt, and leaking will appear.

The main difference between leaking from weather-related ice and refrigerant-related ice is that the latter signifies a larger issue with your furnace and its internal components, so it must be addressed as soon as possible.

Solution: Furnace refrigerant issues are serious and must be inspected by a certified professional with the tools and knowledge necessary to make the proper diagnosis and address the issue as soon as possible. Sometimes, if the refrigerant problems are too severe or have affected the appliance too much, you’ll need to consider the cost of repairs and replacement vs the cost of buying a new furnace.

#5 A Clogged AC Line

Last but not least, let’s consider a clogged AC drain line to explain why your furnace is leaking water. You’re probably wondering what one has to do with the other, as one appliance is used for cooling, whereas your furnace helps with warming up your home.

The answer is simple – shared pipes.

Because ACs and furnaces can be massive appliances, many houses share pipes to exhaust condensation and water. When one of the shared pipes gets clogged, water from the AC can flow back into the furnace and cause leaking.

Clogged pipes and water backing into your furnace can cause many problems, including rust, corrosion, and more, so it’s important that you address the issue as soon as possible.

Solution: Unless you’re well-versed in checking your home’s pipes, you should leave the job to a professional. Certified HVAC technicians know their way around most homes’ piping layouts and will likely be able to identify the source of the problem sooner.

Do Warranties Cover Leaking Furnaces?

To be honest, it depends.

Most furnace manufacturers offer limited warranties for many of their parts, whereas others are covered for up to 10 years! In my experience, whether your warranty will cover the damages and causes of a leaking furnace will depend on one thing – the source of the leaking and the cause of the damage.

If your furnace is leaking and has sustained damage due to a clogged pipe that the appliance shares with the AC or due to improper use, it’s unlikely that the warranty will cover anything. That said, to be 100% sure, please check your warranty document carefully to see the specific terms and conditions of the policy and call your manufacturer if you have further doubts.

Stopping Your Furnace’s Leak

That about covers it!

When your furnace is leaking water, it’s normal to feel worried about the potential cost of repairs you’ll have to pay to get things back to normal.

Luckily, as I hope this piece has helped you better understand, addressing the most common causes behind a furnace leaking water can be easy, quick, and cheap. More often than not, something as simple as checking for ice buildup before firing up the appliance can do the trick.

Thanks for reading. If this article was useful and answered your most burning questions, please check out our other resources and free guides below and consider subscribing to our newsletter.

Have a wonderful day!


I've been helping homeowners with appliance repair since 2016. Starting out as an enthusiastic amateur, I've since worked with many Appliance, HVAC, and DIY experts over the last 7+ years. My mission is to help fix your appliances and prevent future issues - saving you stress, time, and money. Visit my author page to learn more! Read more