Quick answer
A furnace condensate pump usually stops for one of four reasons: the float is stuck in slime, the motor has failed, the check valve or discharge tube is blocked or kinked, or the pump has lost power at an unplugged cord or a tripped GFCI. Clean the reservoir with warm water and vinegar first. A pump that hums or stays silent with a full tank needs replacing, typically $250–$450 installed.
If your furnace condensate pump is not working, the water has nowhere to go, and a high-efficiency furnace makes several litres of it on a cold day. The pump is the small plastic box on the floor beside the furnace with a clear tube running up and away. Its only job is to lift condensate to a drain the furnace cannot reach by gravity. When it quits you get either a puddle or a furnace that refuses to run, depending on how the pump was wired.
The four reasons a condensate pump stops pumping
- Stuck float or slime in the reservoir. Condensate grows a jelly-like biofilm, and summer AC water makes it worse. The float that switches the motor on gets glued down, so the tank fills and the pump never starts.
- Failed motor. These are small motors sitting directly above acidic water. A pump that hums without moving water, runs hot, or stays silent with the float lifted and power confirmed is finished.
- Clogged check valve or kinked discharge tube. The check valve at the pump outlet stops water running back down the tube. When it sticks shut, or the vinyl tube is pinched behind a storage box, the motor runs and the level never drops. When it sticks open, the pump restarts every minute or so as the same water falls back.
- No power. The pump plugs into an ordinary 120-volt receptacle. Someone borrows the outlet for a shop vac, or a GFCI receptacle elsewhere in the basement trips and takes the pump with it.
Why a full pump can shut the furnace off or blank the thermostat
Most condensate pumps have a second, higher float wired as an overflow safety switch. Its two thin leads are meant to be connected in series with the furnace's 24-volt thermostat circuit. When the reservoir fills past its normal level the switch opens, the furnace loses its call for heat, and a thermostat that takes its power from the furnace can go dark.
That is the system working as designed: no heat is a nuisance, but it beats condensate running across a finished basement floor. If the thermostat is dead and the pump tank is full to the brim, suspect the pump before the thermostat. Where the safety leads were never connected, the furnace keeps running and the tank spills instead, which is the situation covered in why a condensate pump overflows.
Check this yourself first: clean and test the pump
- Confirm power. Plug a lamp or phone charger into the pump's receptacle, press reset on every GFCI outlet in the basement, laundry room and garage, and check the panel.
- Turn the furnace off at its switch, unplug the pump and slide the inlet tubes out so you can lift it away. Empty the reservoir into a bucket.
- Take the motor and lid assembly off the tank (a few clips or screws) and scrub the tank, float and inlet with warm water and a bottle brush. Fill it with equal parts white vinegar and water, leave it 15 to 20 minutes and rinse.
- Move the float up and down by hand. It should travel freely and the switch should click.
- Pull the check valve from the outlet, rinse it, and blow through the discharge tube to confirm it is clear. Follow the tube to its far end and straighten any kinks.
- Reassemble, plug in and pour water slowly into the tank. The motor should start before the tank is full, empty it in seconds and stop.
If the pump passes that test, the stopgap worked and the furnace will restart once the safety float drops. With a dead pump overnight, you can unplug it, direct the furnace drain tube into a bucket set lower than the drain outlet, and empty the bucket at bedtime and again in the morning. Do not join the safety switch leads together to force the furnace to run.
When the pump needs replacing, and where it is allowed to drain
Replace the pump when the motor hums or stays silent with a clean tank, a free float and live power, when the reservoir is cracked, or when it has needed cleaning twice in one season. A new condensate pump installed is $250–$450. Where the real blockage is upstream inside the furnace, condensate drain cleaning or trap replacement typically runs $150–$350. The price is confirmed in writing before work starts, and the pricing page shows the other common repair bands.
The discharge has to end at a proper drain: a laundry tub, a floor drain or a drain standpipe, with an air gap so nothing can siphon back. Running the tube outdoors through the wall is a common shortcut and a poor one in Ontario, because the tube freezes in January and the pump pushes against ice. Check the furnace manual and your municipality's rules before sending condensate to a sump pit. A neutralizer cartridge on the line is worth adding where acidic condensate runs through older metal drain piping.
When a dead condensate pump needs a technician today
- Water has reached the furnace cabinet, the control board or a finished floor.
- The pump is clean and powered, still will not run, and the house has no heat in freezing weather.
- The pump works but the furnace still shows a pressure-switch or drain fault, which points to a blocked trap inside the furnace.
- The pump trips its GFCI every time it starts. The motor is leaking current and should not be reset again.
ZK Mechanical replaces failed pumps, re-routes discharge tubes to a proper drain and wires the overflow switch so a future failure stops the furnace instead of soaking the floor. Our condensate pump replacement page sets out what that visit includes. For a furnace that is down because of its drain, call (647) 801-1252 for furnace repair or book online.