If Your Furnace is Leaking Water Do This

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A furnace leaking onto your floor is one of those problems that looks worse than it is. You spot a puddle on the floor, assume it’s catastrophic, and immediately start Googling “cost of new furnace.” Slow down. Most of the time it’s a clogged drain line — something you can fix yourself in less than an hour.

Shall we?

High Efficiency furnace/AC unit in old basement, leaking water onto cement floor.

My introduction to this particular thrill came in the form of a Saturday morning text from my tenant: “The basement floor is turning into a creek.”

Yes, I have a tenant. When I paid off my own mortage a few years ago I bought a nearby house and now rent it.

I went over and found it exactly as described — a slow, steady drip from inside the furnace, with the drip pan spilling out across the concrete. I didn’t know what to do. But I did know who to text. My HVAC guy. His reply: “Don’t hire me. You can fix this yourself.”

And that explains why he's my HVAC guy.

With that, my weekend plans shifted from getting ready for a BBQ that night to crawling around a furnace with a screwdriver in my hand and an atrophied spider in my hair.

In almost every case, especially in summer, a leaking furnace is actually your air conditioner's clogged condensate drain line. Your air conditioner produces condensation when it runs. That water is supposed to flow into a pan, then out through a drain hose and into the floor drain. When the line clogs, the pan overflows. And because the coil and pan sit on top of the furnace, the furnace gets blamed.

Here’s how it looks in action with water dripping into the bottom of the furnace:

If your furnace is leaking water onto your basement floor, try this before calling a repairperson.

Table of Contents

Immediate Steps to Take

Turn off the furnace power. Find the switch or breaker and flip it off.
Contain the water. Towels, buckets, or anything that stops the spread.
Get your tools. More towels, a bottle brush or screwdriver, and possibly a flashlight if your basement is from the era of coal lamps and scurvy.

Water has two hobbies: spreading and ruining things. Stop it before it takes up both.

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How to Fix a Clogged Condensate Drain Line

This is the part where most people panic. Don’t. Clearing a condensate line is basically unclogging a straw — a really gross straw attached to expensive machinery.

Here’s what to do:

A partly disconnected T connector on a furnace drain line.

Drain line running down side of furnace.

Trace the drain hose from the Furnace/AC unit to the floor drain. Check each connector along the way — clogs usually hide there.
Disconnect the hoses and connectors one by one. Some slide off easily. Others need twisting, pulling, and language not found in church bulletins.
Pull out the clog. Use a bottle brush, pipe cleaner, or screwdriver. Don’t push it further in — you’ll just move the problem.

You'll be very excited and proud of your clog once you find it. You'll want to show it to someone like a really good bruise.

Clogged AC condensate drain line and connector.

Scraping out clog from AC drain line with a screwdriver.

Reconnected drain lines and connectors on HVAC unit after unclogging drain line.

Reconnect everything. Push the hoses firmly back onto their fittings.
Turn the furnace back on. Watch for water draining properly into the floor drain.

That’s exactly what I did. Connector one: clear. Connector two: clear. Connector three: clear. Finally, at the very end, I found it — a T-connector stuffed with papery sludge that looked like a wet paper mâché project gone wrong.

I scraped it out with a screwdriver, brushed a spider off my forehead, and flipped the switch. Instantly, water gushed into the floor drain like it had been waiting months to perform.