Furnace or AC Leaking Water? Causes and What to Do

Quick answer

Water around an HVAC system usually comes from a clogged condensate drain, a full drain pan, or a failed condensate pump. AC and heat pump coils make water in cooling mode, and high-efficiency furnaces make water while heating. Turn the system off if water is near wiring or spreading, then have it checked.

Key facts

Most common cause	Clogged condensate drain line
Makes water	AC and heat pump coils in cooling; high-efficiency furnaces in heating
Float switch	May shut the system off on purpose when water backs up
Shut it off if	Water is near wiring or controls, or spreading
Why it matters	ENERGY STAR: a plugged drain can cause water damage

Why is my HVAC system leaking water?

In most cases, water is backing up because the condensate drain is clogged or the condensate pump has failed. Your system makes water on purpose. It just isn't getting where it should go.

ENERGY STAR notes that a plugged condensate drain can cause water damage in the house and affect indoor humidity levels. That's why checking the drain is part of a typical maintenance check-up.

Where does the water come from?

Where the water comes from depends on your equipment and the season. That's why a furnace can leak in winter and an AC can leak in summer.

Air conditioners and heat pumps pull moisture out of the air in cooling mode. It drips off the indoor coil into a drain pan and out a drain line.
High-efficiency gas furnaces, also called condensing furnaces, make water while heating. It drains through plastic tubing, often to a small pump or a floor drain. Standard furnaces usually don't make water this way.
In winter, a heat pump's outdoor unit drips melted frost during defrost. That's normal, as long as it drains away.

Not sure what's wrong yet?

Start with the safe checks. Many problems are a setting, a breaker, or a filter.

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What can I safely check when my system is leaking?

Clearing a clog inside the line, or taking apart the pump, is a technician job.

1If water is near wiring, the furnace controls, or an outlet, turn the system off at the thermostat. Then turn off the power switch near the unit if you can reach it without standing in water.
2Look at the drain pan under the indoor coil, if you can see it without removing panels. A full pan means the drain is backed up.
3Find the float switch, a small device on the drain line or pan. If water has lifted it, it may have shut your system off on purpose.
4Follow the drain line to where it ends. Check that the end isn't crushed, kinked, or blocked by dirt.
5If you have a condensate pump, check that it's plugged in and not overflowing.
6Check the filter. A dirty filter can make an AC coil freeze, and melting ice can overflow the pan.
7Mop up the water and put down towels so you can see if it comes back.

What usually causes an HVAC water leak?

Most leaks trace to the drain, the pan, the pump, or ice melting off a frozen coil.

What you see	Common cause	Can you check it?
Water under the indoor unit in summer	Clogged drain line or full drain pan	You can look.
Water near a high-efficiency furnace in winter	Blocked or disconnected condensate tubing, or a failed pump	You can look.
System stopped, and water in the pan	The float switch doing its job	You can look. Clearing the clog is a technician job.
Water dripping from a ceiling or attic	An overflowing pan on an attic system	Call a technician.
Ice melting on the lines or coil	A frozen coil from low airflow or low refrigerant	Check the filter. See our frozen lines page.
Rusted or cracked drain pan	A worn-out pan	Call a technician.

When should I shut the system off and call?

Shut it off and call if water is near electrical parts, the leak is spreading, or water is coming through a ceiling.

Water is pooling near the furnace controls or an outlet.
Water is coming through a ceiling or wall.
The float switch keeps shutting the system off.
You see water inside the furnace cabinet.

Don't reset, bypass, or remove a float switch to get the system running. It shut the system off to stop an overflow.

What will a technician do, and can it be fixed?

A technician clears and flushes the drain, checks the pan, float switch, and pump, and looks for why it backed up. On high-efficiency furnaces, they also check the condensate tubing and trap.

Most leaks are drain, pump, or pan repairs, which are small fixes that protect your floors and ceilings. We fix what can be fixed — and tell you straight when it can't. You'll hear the price before any work starts.

Common questions

Why is my furnace leaking water?

If you have a high-efficiency furnace, it makes water while heating, and a leak usually means a blocked or disconnected drain tube or a failed condensate pump. If your furnace sits under an AC coil, the water may be coming from the coil instead. Either way, check the filter and drain, and have it looked at if it keeps leaking.

What is a float switch?

A float switch is a small safety device that shuts the system off when water backs up in the drain line or pan. It keeps the pan from overflowing. If it trips, the drain usually needs clearing.

Is it normal for my AC to drip water outside?

Yes, water dripping from the end of a condensate drain line outside is normal while the AC runs. That's the system doing its job. Water inside the house is the problem.

Can a clogged condensate drain cause damage?

Yes, a clogged drain can damage your home. ENERGY STAR notes that a plugged condensate drain can cause water damage in the house and affect indoor humidity levels. Standing water near your system is worth fixing soon.

Is water from my heat pump outside normal in winter?

Yes, water dripping from a heat pump's outdoor unit in winter is normal, because melted frost drains out during defrost. It's normal as long as it drains away and doesn't build into thick ice. See our iced-over page if ice is building up.

Should I turn off my AC if it's leaking?

If water is near wiring or spreading, yes. Turn it off at the thermostat. If it's a small puddle away from anything electrical, you can mop up and check the drain first. Either way, find the source before it causes damage.