Furnace Leaking Water? Winter and Summer Causes in Minnesota Basements

Finding a puddle under the furnace is unsettling, especially in a finished basement. The good news is that most furnace leaks come from a short list of sources, and several are simple fixes. The bad news is that water and furnace electronics don't mix, so it's worth finding the cause quickly.

Where the water comes from depends a lot on the season. In a Minnesota winter, it's usually the furnace's own condensate system or the humidifier. In summer, it's usually the air conditioner coil that sits on top of the furnace. Here's how to narrow it down.

First: protect the furnace and the floor

• If water is near the furnace's electrical parts or pooling under the cabinet, turn the furnace off at the thermostat, then switch off the service switch on or near the furnace. It looks like a light switch.
• If you have a whole-house humidifier, turn off its water supply. There's usually a small valve on the water line feeding it.
• Soak up the water with towels or a wet-dry vacuum so you can see where new water is coming from.
• If you smell gas, hear a carbon monoxide alarm, or see water near the gas valve, leave the house and call 911 or your gas utility from outside.

In very cold weather, don't leave the house without heat for long. Find the source, then call for service if it's not a simple fix.

Why high-efficiency furnaces make water

If your furnace has white plastic vent pipes, it's a high-efficiency condensing furnace, usually 90 percent efficient or better. It squeezes so much heat out of the exhaust that water vapor in the combustion gases condenses into liquid. On a cold Minnesota day, when the furnace runs for hours, that adds up to a lot of water.

That water is supposed to drain through a trap and a small hose to a floor drain or a condensate pump. As Bryant notes in its guide to furnace leaks, condensation is normal in these furnaces, but water pooling around the furnace is not. When you see a puddle, something in that drain path has failed.

Winter leaks: the usual causes

These are the leaks we see most during heating season in Twin Cities homes.

• A clogged condensate trap or drain hose. Sediment and slime build up in the trap over time and water backs up. This is the most common cause, and clearing the trap is part of every furnace tune-up.
• A failed condensate pump. In finished basements and homes without a floor drain near the furnace, a small pump pushes the water to a drain. If the pump dies, its float sticks or it loses power, the reservoir overflows. If you hear it humming without pumping, or it's full and silent, that's your leak.
• A frozen drain line. If any part of the condensate line runs outside or through a cold space like an unheated garage or crawlspace, it can freeze solid in a cold snap. The water backs up and often shuts the furnace down too.
• A leaking humidifier. A cracked housing, a loose water line, a stuck water valve or a clogged humidifier drain can drip water onto or into the furnace. Our guide to whole-house humidifiers covers how they should drain.
• A disconnected or poorly pitched vent pipe. The plastic exhaust pipe carries some condensation back toward the furnace. If a joint has come apart or the pipe slopes the wrong way, water can drip out at the joint instead of draining properly.

Older 80 percent furnaces and chimney water

A standard-efficiency furnace with a metal flue pipe into a chimney shouldn't produce condensate water at all. If you see water there, the source is usually something else: a humidifier, the AC coil, or condensation in the flue and chimney.

That last one is common in older Twin Cities homes. When a house switches from an old furnace to a high-efficiency model, the water heater is often left venting alone into a big masonry chimney sized for both appliances. The smaller flow of exhaust cools in the oversized chimney and condenses, and water can run back down to the base of the chimney or the water heater vent connector. It's worth having a technician look at that, because it can also mean the water heater isn't venting properly.

Summer leaks: it's usually the AC

If the water shows up when the air conditioning is running, the cause is almost always the indoor AC coil that sits on top of the furnace. That coil pulls moisture out of the air and drains it through its own pan and line. A clogged drain line, a cracked pan or a failed condensate pump will overflow.

A coil that freezes from low airflow or low refrigerant can also dump a lot of water when it thaws. Our post on frozen AC coils explains why that happens and how to thaw it safely. Water during cooling season isn't a furnace problem, but it can still soak the furnace below it, so don't ignore it.

What you can safely check

• Follow the small drain hose from the furnace to the floor drain or pump. Make sure it isn't kinked, pulled loose or sitting above the drain opening.
• Look at the condensate pump. Is it plugged in? Is the reservoir full? Does it run when you pour a little water in?
• Look at the humidifier and its water line for drips, and check whether the humidifier's own drain hose is clear.
• Check the furnace filter. A badly clogged filter can freeze the AC coil in summer and makes everything work harder.
• Look at the plastic vent pipe joints near the furnace for drips or separation.

Don't open the burner compartment or take apart the trap assembly on the furnace itself. If the leak is coming from inside the cabinet, it's time for a technician.

When it's more serious

Most leaks are drain problems. A few aren't. Rust at the bottom of the furnace cabinet, water coming from inside the burner area, or water stains on the control board point to a problem that needs a closer look, and on high-efficiency furnaces, an internal leak can damage the control board or the inducer motor. Rarely, water inside a furnace is tied to a failed heat exchanger. Our post on what a cracked heat exchanger means explains why that's a safety issue.

If your furnace is leaking and has also started shutting off, cycling or throwing error codes, our posts on short cycling and what to check when your furnace stops working cover the next steps.

Get the leak fixed before it becomes a bigger repair

Water and furnaces are a bad combination, and a small drip can turn into a ruined control board or a damaged floor. If you can't find the source, or it comes back, schedule a furnace repair visit or call 763-428-3677. Office hours are Monday through Friday, 7:30 a.m. to 4:30 p.m., and after-hours emergency service is available. An annual maintenance plan visit keeps the trap, drain and pump clean so most of these leaks never start.