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Why Is My Gas Furnace Leaking Water? A California Tech Explains

By Your Advisor 1/15/21 1:27 PM Time to read:

If you’ve recently noticed water leaking from your furnace, you’re probably wondering why, and more importantly, how to fix it. 

The answer to both of these questions depends on whether you have a high-efficiency (condensing) furnace or a conventional (non-condensing) furnace.

  • If you have a conventional (non-condensing) furnace, your system will have a metal vent pipe, and your furnace is likely leaking due to:
    • An improperly designed flue pipe
    • Damaged/malfunctioning humidifier
  • If you have a high-efficiency (condensing) furnace, your furnace will likely have a white PVC vent coming out of the top or side. This type of furnace is likely leaking because of:
    • A clog in the drain pan or condensate line
    • Damaged condensate pump 

Below, we’ll look at why both a conventional and a high-efficiency furnace may leak, as well as the steps you can take to stop leaking and prevent it going forward. 

Reasons why a conventional (non-condensing) furnace can leak

As we mentioned above, your conventional furnace is likely because of the:

  • Flue pipe
  • Humidifier

Leakage from the flue pipe

Flue pipes are designed to carry the gases your furnace produces out of your home before they have time to cool down and condense into water. 

However, if your flue pipe is damaged or was installed incorrectly (no slope or too large), it may prevent those gases from exiting your home before they cool down. If your flue pipe isn’t designed correctly, those gases can condense inside the flue pipe and drip back down towards the furnace.

If you think this may be the issue you’re experiencing, reach out to a professional. They will be able to check out your furnace and determine if your flue pipe needs replacing. 

Leakage from the humidifier

Your furnace may have a humidifier attached to it, which adds moisture to your home’s air.

However, if your humidifier is clogged or damaged, the water circulating in it can leak out and pool around the bottom of your furnace, causing the leak you’re noticing. 

If you think it’s your humidifier (rather than your actual furnace) that’s leaking, reach out to a technician. They should be able to quickly determine what the issue is and unclog or fix your humidifier. 

Reasons why a high-efficiency (condensing) furnace can leak

Unlike conventional furnaces, high-efficiency (condensing) furnaces are actually meant to produce condensation (hence why they’re called condensing furnaces). 


High-efficiency furnaces have 2 heat exchangers rather than the 1 heat exchanger a conventional furnace has. 

Instead of traveling through 1 heat exchanger and then out through the flue pipe, gas in a condensing furnace travels through 2 heat exchangers, which means it has time to cool down by the end of the heating cycle. 

In a perfect world, this gas will condense and drain out of your home via the condensate drain.

However, if there are clogs along the condensate line or an issue with the condensate pump, it can cause condensation to spill out onto the floor. 

Issues along the condensate line include:

  1. A blocked condensate drain trap- Your drain trap is in place to prevent gases from escaping through the drain line. Over time, it can become clogged and prevent condensation from draining and exiting your home. Before calling a professional, check your drain trap. If it’s clogged, you can try to unclog it using a shop vac. However, if this doesn’t work, you should reach out to a professional for help. 

  2. Clogged condensation line- The condensation line is the part of your system that connects the drain trap to the outside of your home. If there is a clog in any part of this line, condensation can back up and overflow onto your floor. Because clogged condensation lines are a little more difficult to fix, our advice would be to call a professional rather than to attempt this fix on your own.

  3. Condensate pump issues- The condensate pump is a small pump attached to the drain line of your furnace. This pump works to push water into a drain or outside.. However, if this pump is damaged or malfunctioning, the water may leak and spill out onto your floor. You’ll probably be able to tell if this is your issue because if your condensate pump is leaking, water will usually leak from the pump itself. You’ll need to call a professional to come and fix your condensate pump. 

Fix your furnace leak with Burgeson’s

If you’re still having issues with a leaking furnace, reach out to us. We understand that dealing with a leaking furnace can be stressful, but it doesn’t have to be. We have over 72 years of experience helping homeowners in California repair their heating systems, and we’d be happy to help you too. When you’re ready, you can request to schedule an appointment by calling us at 909.792.2222.

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