Furnace Leaking Water When Heat or AC is On

The cause of a furnace leak can be diagnosed using the tips in this guide.

In some cases, where the cause isn’t obvious, a furnace technician might be needed for the diagnosis.

Some causes are easy to DIY fixes. Others require furnace repair.

Choose the section that fits your furnace issue:

Furnace Leaking When AC Is On

This is more common, so we’ll begin here and list the potential leaking furnace cause from most to least likely.

  1. Plugged Condensate Pan or Drain

When in air conditioning mode, as you likely have known, the system removes humidity from the air by condensing it on the evaporator coil. The moisture is collected in a pan and flows out the condensate drain.

The problem is that mold, dust, and debris form a sludge in the pan that easily blocks the drain.

DIY Solution? Yes, if you’re a handy homeowner, you can find and solve this problem.  In this video, a seasoned AC repairman shows you how to diagnose and solve the problem.

If the blockage is in the line leading out of your home, then you can use compressed air or a garden hose and water to force it out. This handy homeowner shows you how to do it. This is a moderately difficult DIY job, so leave it to an HVAC tech if you don’t want to tackle it.

[shows you how to do it]

  1. Blocked Condensate Pump, if You Have One

If the condensate from your AC coil has to go up and out of the basement, then a pump is needed.

Here’s an image of a condensate pump on a Carrier furnace.

Do you have one of those? Then there are a few issues that could be causing a leak.

DIY Solution? Maybe. First, check the drain as described in problem number 1. If it is clear, and you don’t have a humidifier, then the pump is likely the cause.

Experienced homeowners or those with mechanical skills that enjoy tackling new projects can probably repair or replace the condensate pump. They’re easier to replace than repair, unless a simple clog is the problem.

This video gives excellent advice on troubleshooting a condensate pump. The only suggestion we have is that if you suspect it is blocked, as shown in the video, and you enjoy taking things apart to repair them, then there’s no reason not to.

It’s possible the blockage has caused the motor to burn out, but if not, and you can clear the blockage, you might get more life from the pump. The presenter recommends getting a new pump if you suspect a blockage. Of course, he works for a furnace parts site.

  1. Frozen/Thawing Coil

Sometimes the coil doesn’t get enough airflow, and it freezes. When the air conditioning cycle ends, the frozen coil will warm up. The ice will melt, and there might be so much water it will overflow the drain and leak onto your floor.

This is caused by a very dirty filter keeping the furnace or air handler from getting sufficient air.

DIY Solution? Yes. Check the filter. If it is dirty, replace it. If you don’t have a fresh one, it’s OK to let the system run for a day or so without a filter.

  1. Leaking Dehumidifier or Humidifier

Is your furnace equipped with one or both of these?

A dehumidifier does the same thing as the evaporator coil. It gets really cold and condenses moisture out of the air, so it has a drain too. Check for a clogged pan/drain.

A humidifier shouldn’t run in the summer, of course, but the water pipe that feeds it could leak any time.

DIY Solution? Yes. If it’s the dehumidifier, the solution is the same as above. If it is the humidifier, turn off water to the unit. If the leak stops, you’ve diagnosed the issue. Check the fitting on the water supply. It might need to be tightened or Teflon tape on the threads might need to be added or replaced.

Furnace Leaking When Heat Is On

This is less common, but it can happen. Here are the most common reasons.

  1. The Condensate Drain is Leaking

If your furnace’s efficiency is 90% or higher, it is called a condensing furnace. That term refers to moisture, and that moisture has to drain.

Water is a byproduct of the combustion of natural gas and propane. In fact, for every molecule of carbon dioxide produced, two molecules of water are produced. That’s a lot of water produced by a condensing furnace.

If you’re not sure whether your furnace is condensing, you can tell by locating the vent that carries exhaust air out of your home.

Is it PVC – white plastic? That’s a condensing furnace vent.

If it is metal, you have a non-condensing furnace, and it doesn’t have a condensate pump.

DIY Solution? Yes. This video from a DIY homeowner gives you options and guidance for clearing a condensate drain. If your furnace setup is a little different, for example if it is in the attic, there are many videos on this common subject. So you should be able to find one that fits.

  1. The Condensate Pump Isn’t Working

High-efficiency condensing furnaces typically drain down.

But if the water has to go up and out of a basement or crawlspace, a pump is required.

We discussed this issue above since the pump runs during air conditioning cycles too. See number 2 under Furnace Leaking When AC is On.

  1. The Humidifier is Leaking

If you have a humidifier, this is easily the most likely issue. There are several potential causes of a humidifier leaking when it is running.

Most solutions, or at least diagnoses, are DIY.

The water line is loose or broken. Turn off the water. If the leak stops, tighten the fittings, add Teflon tape or look for a crack. If the line is cracked, the fitting was probably over-tightened.

The pad isn’t absorbing water due to a build-up of minerals. If it is hard and crusty, change it.

The drain is clogged. Some humidifiers are supplied too much water, or, like steam humidifiers, the have to be flushed periodically. In either case, the humidifier has a drain. Remove the drain fitting, and using a wire or small screwdriver, try to dislodge the clog.

  1. Your Heat Exchanger is Cracked

The heat exchanger is at the core of your furnace. It transfers heat from the combustion chamber to the ductwork carrying air to the rooms in your home.

As we noted, water is a byproduct of combustion. Normally, the moisture leaves the furnace and your home through the vent (and the condensate drain in a high-efficiency furnace).

If the heat exchanger is cracked, the moisture might leak out of the furnace. Has your carbon monoxide detector gone off? That’s another clue the heat exchanger is cracked – and deadly gases are leaking from it. If so, turn off the furnace and call a furnace company.

DIY Solution? Not this one. The furnace must be largely disassembled to get to the heat exchanger. Most pros don’t even do this. They recommend furnace replacement instead.

Why? Because a cracked heat exchanger usually happens later in a furnace’s life – after 10 years or longer. While heat exchanger warranties are either 20 years of Lifetime, the labor to replace the part is not covered by the warranty. Surprise, surprise!

The labor cost to replace a heat exchanger is $800 to $1,500 depending on the furnace and the repair company’s pricing.

That money is better spent on a new furnace, in our opinion. If this is the problem, and you don’t want to get stuck with the repair vs replace dilemma again, consider a Goodman furnace. The top Goodman furnaces come with a lifetime furnace replacement warranty on the heat exchanger. Goodman will give you a new furnace if the heat exchanger fails.

Final Thoughts

Even if you decide to call an HVAC repair company, knowing what is wrong is to your advantage.

For example, let’s say you believe the condensate drain is blocked. If the tech says your furnace is leaking because the heat exchanger is cracked, you can ask about the condensate drain (or the dehumidifier, etc.). The “cracked heat exchanger” is one of the most common furnace scams. Sometimes the exchange is broken, but often that’s a ruse to get you to buy a new furnace.

Knowledge is power! We trust this guide will assist you in diagnosing the problem and solving it. But if you have to call an HVAC company, the information will help ensure a proper diagnosis and repair.

If the information helps, please consider using our social media share buttons to pass it along. And feel free to leave a comment too.

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