Why Is My Furnace Leaking Water? A Portland Tech Answers

The first step to figuring out why your furnace is leaking water is to determine which type of furnace you have.

You either have...

  • A conventional, non-condensing furnace
  • Or a high-efficiency, condensing furnace

Here’s how to tell which kind of furnace you have:

  • If you notice a white PVC pipe coming out of the furnace, you have a condensing furnace
  • If you see a large metal exhaust flue that goes up and out of the roof, you have a standard, non-condensing furnace

In this blog, we’ll explain what could be causing your furnace to leak water for each type of furnace, and how to fix the problem.

Want to a Portland HVAC pro to repair your leaking furnace? Our certified technicians are standing by, ready to help. We offer fast, same-day service and can repair any furnace type or brand.

Schedule appointment today

If you have a non-condensing furnace...

Your leak is probably caused by a defective flue pipe.

During normal operation, your furnace produces harmful gases as a result of the combustion process. The purpose of the flue pipe is to carry these harmful gases out of your home so they don’t enter your airstream.

However, if your flue pipe was not installed correctly or it is oversized, then too much air can circulate in the pipe, preventing those gases from escaping. If the gases can’t move out of the pipe into the outside air, they’ll begin to cool down, condensing into a liquid. Eventually, this liquid can drip down into your furnace, which could explain why you think you’re seeing leaking water.

Solution: Contact a professional to inspect your flue pipe. A professional can fix the slope of the pipe and make sure it is the correct size.

If you have a condensing furnace...

The leak you’re seeing is most likely caused by an issue with the condensate system. Luckily, this usually means you have a minor clog or leak.

We’ll explain. First, let’s take a quick look at why a condensing furnace produces condensation in the first place.

Unlike a conventional furnace, a condensing furnace has two heat exchangers instead of one. The second heat exchanger allows a condensing furnace to absorb more heat from exhaust gases, essentially giving you more heat from the same amount of fuel (i.e. more efficiency).

As the second heat exchanger absorbs extra heat from these exhaust gases, the temperature of the gas drops until it turns into a liquid, called condensation.

Normally, this condensation is removed from your furnace via a PVC pipe (called a condensate line) where it drips into a drain. Some furnaces use a condensate pump to help move the liquid toward the drain.

However, if there’s an issue somewhere in the condensate system, then you may see liquid leaking from your furnace.

The most common culprits of leaking condensation are:

  • A clogged or leaky condensate line
  • A broken condensate pump


Solution: Contact an HVAC technician to inspect your condensate system and fix any clogs or leaks. The technician can also inspect the condensate pump to make sure it’s working properly.

Other issues that can cause both types of furnaces to leak

The following issues can also cause leaking and can apply to both types of furnaces:

  • A leaking humidifier
  • A leaking air conditioner

A leaking humidifier

Some HVAC systems in Portland have whole-home humidifiers to help maintain proper indoor humidity levels during dry months.

If you have a whole-home humidifier (you’ll know if you see a small white box attached to your furnace/AC), it could be clogged or leaking. Since a humidifier is attached to your furnace, it could easily seem like your furnace is leaking.


Solution: Contact an HVAC pro to inspect your humidifier and fix any clogs or leaks.

A leaking air conditioner

Since your air conditioner and furnace share the same indoor component (called the air handler), sometimes it’s hard to discern which system is leaking. If you only see water leaking from your furnace after using your AC, the leaking issue is most likely with your AC—not your furnace.


Solution: The HVAC pro you hire to fix your leaking furnace can quickly determine if the problem is with the furnace or AC, and then repair the leaking system.

Want a Portland HVAC pro to repair your leaking furnace?

Schedule appointment today

Our certified technicians are standing by, ready to help. We offer fast, same-day service and can repair any furnace type or brand.

To learn more about what to expect when you hire us for a repair, visit our furnace repair page.


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