INDIANAPOLIS GUIDE

Water at the furnace or the air conditioner

A puddle in the basement feels like an emergency because it is one. The heating and cooling equipment is a common source, and it is not the only source. In summer the indoor coil wrings water out of humid air. In winter a condensing furnace makes water from the exhaust. Both send that water through a trap, a hose, or a small pump, often to a floor drain or a laundry sink. This page is how to tell those leaks from a water heater or a pipe, and what to do before the water reaches storage, a panel, or the bottom of the furnace.

By Mike Maziarz · Sources checked October 12, 2026 · How we write these guides

Find the water before you name the part

Note whether the water appears only while the system runs. Cooling-season water that starts when the air conditioner starts is the coil doing its job and the drain failing to carry it. Heating-season water that starts when the furnace fires is the condensate trap, the pump, or a humidifier on the duct. Water that is there at 3 a.m. with the thermostat off has another source. That timing is the whole diagnosis you can do without tools. Start at the highest wet spot, the pan or the hose, rather than the widest puddle on the floor.

If you would have to stand in water to reach the furnace switch or the panel, do not wade in. Shut the system off at the thermostat if the hallway is dry. A gas smell still means leave and use the emergency number on the gas bill from outside. Many Indianapolis addresses are served by Citizens Energy Group. A carbon monoxide alarm still means everyone goes outside and emergency services are called.

Summer: the cooling coil, the pan, and the pump

Humid Indianapolis air is why the coil makes so much water in July and August. The humidity guide explains that job. The water should leave. A basement air handler usually has a trap and then either gravity to a floor drain or a condensate pump that lifts the water to a sink or a standpipe. A closet or attic unit may have a pan under it. When the line clogs with slime, the pan fills.

A Building America guide, citing the residential code, describes a water-level detection switch that shuts the equipment off before a secondary pan overflows, and a switch on the primary drain that does the same if the drain blocks. If the air conditioner quit and the pan is full, that switch may be the reason the house is getting warm. Do not bypass it to finish the day. Bypassing it turns a stopped system into a ceiling stain or a wet furnace. The emergency cooling page is what to do about the heat while the pan is dealt with.

Look at the pump reservoir if it is in the open. A full tank, a pump that hums and does not empty, or a tube that popped out of the floor drain is a clear note. You may set that loose end back in the drain if you can do it from a dry floor without opening the pump. Do not cut the line or pour bleach or vinegar into a trap you cannot see through. A clogged line overflows into the cabinet. The not-cooling guide covers ice. Say if you have ice and water together.

Winter: the furnace trap and a frozen line

A condensing furnace makes water whenever it heats. Installation instructions say a plugged drain or a failed pump may shut the furnace down, and that the line should end indoors so it does not freeze. A U.S. Department of Energy guideline says cold-climate condensate belongs in the house drain, not outside. If the furnace quits on a cold night and you find water, or a hose stiff with ice in a crawlspace or garage, the frozen condensate guide is the heating side of this puddle.

Do not pull the trap apart and do not pour hot water or antifreeze into it. Do not use a flame on plastic pipe. Shut the furnace off at the thermostat so it stops making water, if you can do that from a dry floor. One reset after a technician clears the drain is their step. A night of resets while the pump overflows is how the basement gets worse.

A humidifier on the duct can leak in winter too. If the water is at the trunk rather than the trap, turn the humidifier off at its own switch if you can reach it dry, and say so.

A water heater or a pipe is a different call

Basement furnaces in older Indianapolis houses often stand next to the water heater. A leak from the tank, from the temperature-and-pressure valve, or from the lines on top of the heater is plumbing, even when the water runs toward the furnace and makes it look guilty. Feel nothing with your hands if the floor is wet and electrical gear is in it. Look. If the heater jacket is wet and the air handler is dry, you do not need a heating technician first.

The water-heater notes on the Indianapolis plumbing guide are the matching page for that tank: water heater leaks. A wet supply line or a joint that drips with the heating system off belongs with a leaking pipe, not with a condensate pump. Those are separate guides. They are not a dispatch desk, and neither is this site.

What you can do before the water spreads

Turn the system off at the thermostat so the coil or the condensing furnace stops making water. Turn the fan off too. Towel up water you can reach from a dry edge, and get boxes off the floor. Do not vacuum a puddle that has reached a receptacle or the furnace cabinet. If you cannot shut the breaker off without standing in the water, wait.

Do not open the coil door, drill a pan, or raise the furnace on bricks. Write the time you shut the system off and whether the water slowed. If it does not slow, look again at the water heater and the pipes.

The do-it-yourself list stays short here: thermostat off, a tube end that obviously fell out of a drain, photos, and a dry path for the person who comes next. Everything that involves the trap, the pump motor, the float switch, or the coil is the visit.

What to tell the technician

Say summer or winter, furnace or air conditioner, and whether a pump, a gravity drain, or a humidifier is in the picture. Say whether the system shut itself off. Say how far the water went: pan only, floor around the unit, or toward the water heater and the panel. Say what you already turned off. A technician who knows the door never came off will trust the rest.

Ask them to clear the cause, not only to dry the floor, and to say whether the float switch actually stops the equipment. Ask what they will test after the repair: a cooling cycle that makes water and then drains it, or a heating cycle that does the same. A pan that is empty only because the system is still off is not a finished repair. The seasonal maintenance page is where a flushed trap belongs twice a year so this page is not an annual event.

Photograph damaged drywall or a wet ceiling before anything is moved. Ask whether the quote includes the drain and the pump or only a mop-up. The cost factors page covers what is included, without prices. A clogged drain is not, by itself, a reason to replace the whole system.