Why is My Furnace Leaking Water in the Middle of Winter?

You went to change your filter and stepped in a puddle. Here is why your heating system is a water factory—and how to stop the hidden leak.

It’s late February. Your furnace is still working hard to keep the Utah chill at bay. You walk into your mechanical room to change the filter, and you step in a puddle.

Your first thought is usually confusion: "Wait, this is a heater. Why is there water?"

If you have a modern, high-efficiency furnace (likely, if your home was built or updated in the last 15–20 years), your heating system is actually a water factory. And if the plumbing attached to it fails, it can silently flood your basement or mechanical closet, rotting baseboards and growing mold behind the scenes.

Here is why your furnace is wet—and how to stop the hidden leak.

1. The Science: Why Does a Furnace Make Water?

Old-school furnaces were simple: they burned gas, sent heat into your house, and shot the hot exhaust gases out the chimney. They were also wasteful.

Modern "High-Efficiency" furnaces (90% AFUE or higher) squeeze every ounce of heat out of that gas. They have a second heat exchanger that pulls heat from the exhaust gases. In this process, the exhaust cools down so much that it turns from steam back into liquid water (condensation).

• The Stat: A high-efficiency furnace running all day during a cold winter snap can produce up to 5 to 6 gallons of water every single day.

• The Hazard: This condensation isn't pure water; it is slightly acidic. If it leaks, it eats away at metal, wood, and drywall much faster than a standard plumbing leak.

2. The Failure Points: Where the Water Escapes

That acidic water should drain safely away through a PVC pipe. But sludge, age, and cold weather often get in the way.

The Clogged Trap

Just like your bathroom sink, your furnace has a "P-trap" inside. Over time, this fills with sediment and slime from the combustion process. If it clogs, the water backs up inside the furnace until it overflows out of the cabinet and onto the floor.

The Condensate Pump

If you don't have a floor drain right next to your furnace, you likely have a small white box (a condensate pump) sitting on the floor next to it. It collects the water and pumps it up and out of the house. These pumps are mechanical; eventually, they burn out. When they do, the tank overflows, and those 5 gallons of water keep coming.

The Frozen Exit

The drain line often runs to the exterior of the house. In a cold February snap, the tip of that pipe can freeze shut. With the exit blocked, the water has nowhere to go but back into your basement.

3. The "Silent" Damage

The real problem with furnace leaks is that they are slow, steady, and often hidden in unfinished rooms.

• Baseboard Rot: The water often seeps under the wall of the mechanical closet and soaks the carpet or baseboards in the finished room next door. You might not notice until the baseboard turns black or the carpet smells musty.

• Rust: Because the condensation is acidic, it can rust the bottom of the furnace cabinet itself, leading to expensive HVAC replacement.

• Mold: A warm mechanical room combined with a constant slow leak creates the perfect mold nursery.

4. Your Inspection Checklist

Go downstairs today and take a quick look at your unit.

• Check the Floor: Is there any moisture, hard water stains, or rust around the base of the furnace?

• Check the Pump: If you have a condensate pump (the little white box), is it humming? Is it covered in dust or grime? Pour a cup of water into it to make sure the float switch kicks on and pumps the water out.

• Check the Clear Tube: Look at the clear plastic tubing running from the pump. Is it brown, black, or filled with algae? That blockage is a flood waiting to happen.

What to Do If You Find a Leak

If you find water flooding out of your furnace, you actually need two different professionals.

1. Call an HVAC Pro: To repair the furnace, unclog the internal trap, or replace the broken condensate pump.

2. Call Apex Restoration: If the water has soaked into drywall, seeped under flooring, or caused mold growth, that is our job. We don't fix the furnace, but we fix the damage it caused. We can extract the water, dry the wall cavities, and ensure mold doesn't take over your basement.

Don't ignore a small puddle in the mechanical room. It’s a warning sign.

If you see water damage spreading, call Apex today at (801) 513-1137.