INDIANAPOLIS GUIDE

When a high-efficiency furnace shuts off in a cold snap

The furnace runs, then quits, then tries again, and the house slides toward the outdoor temperature. On a condensing furnace the clue is often water: a plastic drain, a trap, or a small pump, plus white or gray intake and exhaust pipes through the wall. A cold Indianapolis night is enough to freeze a line that was routed through a crawlspace, a garage, or along an outside wall. National Weather Service normals put the January low near 21°F, so a night in the teens is ordinary winter here, not a freak event. This page is that lockout. It is not a guide to rebuilding the drain.

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

What a condensing furnace has to drain

A high-efficiency furnace cools the exhaust enough that water drops out of it. That water is the condensate. A U.S. Department of Energy measure guideline on these furnaces says that in a cold climate the condensate should drain into the house sanitary drain, not outdoors, and that a condensate pump can lift it to a higher drain when a floor drain is not nearby. If you see plastic pipe and a pump or a hose headed toward a floor drain or a laundry sink, you are looking at that system.

The same water can stop the furnace. Installation instructions for a common condensing furnace say a plugged condensate drain or a failed pump may cause the furnace to shut down. You do not have to know which sensor opened. You do have to say whether the unit tries to start, quits, and tries again, and whether you see water where a hose should be carrying it away. The ignition guide covers a furnace that never lights. Stay here when the failure shows up with the drain, the pump, or a cold snap.

Where the line freezes around Indianapolis

The Department of Energy guideline says draining outdoors is the wrong plan in a cold climate, and a line that freezes can mean a service call to restart the furnace. Do not install heat tape tonight, and do not pour antifreeze, bleach, or hot water into the trap. Freeze protection belongs to the installation, not a Saturday pour-in.

A pump that has quit is a related failure. The reservoir fills, the furnace locks out, and water may be on the floor. Look from a dry spot. If you would have to stand in water to touch the pump or the furnace switch, leave both alone and shut the system off at the thermostat if you can do that from a dry floor. The leaking water guide covers the puddle itself. Tonight, the lockout and the freeze are the heating problem.

Snow and ice on the sidewall vent

Condensing furnaces usually exhaust through plastic pipe on a side wall, often with a second pipe for intake air. Snow, ice, a drifted pile, or a nest can block that opening. A control troubleshooting chart lists a restricted vent among the reasons a limit or pressure circuit opens, with the blower running while the switch is open. Indianapolis January snowfall in the climate normals is 8.8 inches on average, and a drift against a low vent does not need a record storm. Clear loose snow you can reach with your feet on the ground. Do not climb, and do not use a flame, a heat gun, or boiling water on the plastic pipe.

A blocked exhaust is also a carbon monoxide problem if the furnace is forced to keep trying. If an alarm sounds, or if people feel a headache, dizziness, or a flu-like illness that eases outdoors, get everyone outside and call emergency services. Do not clear the vent and go back to sleep. The carbon monoxide guide is the order of operations. A gas smell is separate: leave, avoid switches, and from outside use the emergency number on the gas bill. Many Indianapolis addresses are on Citizens Energy Group, and Citizens tells customers to make that call from outside.

Checks that stay outside the cabinet

With no gas smell and no alarm, look once. Is the thermostat on heat, above the room temperature, with the fan on auto? Is the filter so packed that it is bowed? Replace it only if it slides out and you have the right size. A starved furnace overheats and shuts down, which can look like a drain problem when it is an airflow problem. The cold-air guide is that blower-running case.

Count the attempts: an inducer, a pause, a failed light, and a shutdown is different from a blower that simply runs cold. One reset of a button you can see without removing the burner door is worth mentioning. A string of resets hides the lockout.

Photograph the pipes outside, the pump if you can see it without moving hoses, and any water on the floor. Do not disconnect tubing to prove the trap is full. Do not bypass a pressure switch, a float, or a door switch so the furnace will run. The do-it-yourself page draws that line. If the house is getting cold, the no-heat checklist covers safe backup heat and the point where you take people somewhere warm.

What the technician is sorting out

Ask what they found and whether the line needs a different route so the next cold snap does not repeat the night. A furnace that runs only because a switch was jumped is not repaired. Freeze protection on a line that can freeze is their work, written down, not a heat-tape experiment after they leave.

Say whether anyone smelled gas or the carbon monoxide alarm chirped. Say whether the pipes outside were buried. Ask what the visit includes before anyone talks about replacing the furnace. Repeated lockouts can become a replacement conversation later, on a calm day. The repair or replace page is that conversation, and the cold-air guide lists the Citizens Energy Group and AES Indiana credits that apply only when a real change-out is on the table. Tonight is a restart and a drain, if that is the finding.

After it runs again

When heat returns, watch one full cycle from a supply vent and from the furnace, without opening the door. You want burners that stay lit, a blower that starts and stops on its own, and no new stream of water onto the floor. If the furnace quits again within a few minutes, write how many minutes. Short cycling after a thaw is still a finding.

Look at the outside pipes in daylight. If the opening is barely above the snow line, say so at the next visit. The fall heat check is when a clear drain and a clear vent should be confirmed, before the next night in the teens. Put it on the calendar while you still remember which hose goes where.

If water keeps coming with the furnace off, it may not be condensate. Use the leaking water guide to separate a pump from the water heater. Do not leave a locked-out furnace resetting itself over a full pump.