Quick answer
A condensate pump overflows when water arrives faster than it leaves: the float is stuck, slime has jammed the reservoir, the motor has failed, or the vinyl discharge tube or check valve is blocked or kinked. Set the thermostat to Off, empty the reservoir and check the tube and the plug. A replacement pump installed runs $250–$450, and its overflow safety switch should be wired in.
If your condensate pump is overflowing, one of two things has happened: the water cannot get out, or the pump does not know the water is there. Either way the reservoir fills while the AC or furnace above it keeps sending more. Stop the source first by setting the thermostat to Off, then deal with the pump.
The four faults that make a condensate pump overflow
- Stuck float. The float inside the reservoir rises with the water and trips the motor switch. Slime or scale glues it to the bottom, so the tank fills and the motor never gets the signal.
- Slimed reservoir. Condensate carries dust and algae, and in summer it grows a jelly that blocks the pump inlet and the outlet barb. In Halton and Hamilton, hard-water runoff from a furnace humidifier adds scale on top of it.
- Failed motor. A pump that hums without moving water, runs hot to the touch, or stays silent over a full tank with a live outlet is finished. These motors are replaced, not rebuilt.
- Blocked or kinked discharge. The 3/8-inch vinyl tube that carries water to the drain gets pinched behind stored boxes, crushed under a staple or plugged with slime, and the check valve at the pump outlet can stick shut.
Check the simplest cause before any of those. The pump plugs into a wall receptacle, and a plug knocked loose, a tripped GFCI or a breaker left off produces exactly the same puddle.
The overflow safety switch that is often left unwired
Most condensate pumps have a second float switch set higher than the first, with two thin low-voltage leads coming out of the pump body. Wired in series with the thermostat circuit, it shuts the AC or furnace off when the reservoir is about to spill. A dead pump then gives you a house that stops cooling, not a wet floor. On many installations those two leads are coiled up and taped to the side of the pump, connected to nothing.
Look at yours. If the leads end in bare or capped wire, the safety is not in use, and the equipment will keep producing water over a failed pump until someone notices. Wiring it in is a short job during any service visit, and it matters most where the pump sits on a finished floor or above a ceiling.
The same pump serves the furnace and humidifier all winter
A condensate pump is not a summer-only part. A high-efficiency furnace condenses water out of its exhaust every time it fires, and that mildly acidic condensate runs to the same pump from October to April, along with the drain water from a bypass humidifier. A pump that overflowed in August will do it again in January, when the consequence is a furnace that shuts down on a backed-up drain.
Twelve months of duty is why these pumps wear out sooner than the equipment they serve. The heating-season failure, where the furnace locks out, is covered in why a furnace condensate pump stops working.
What to do right now, and when to call
- Set the thermostat to Off so no more water is made.
- Unplug the pump and empty the reservoir with a sponge or a wet vacuum.
- Follow the vinyl tube from end to end and straighten any kink. Flush it with water; if nothing passes, the tube or the check valve is blocked.
- Wash the reservoir and float with warm water and a little vinegar, and make sure the float moves freely.
- Plug the pump back in and pour water slowly into the reservoir. It should start before the tank is full and empty it in seconds.
Leave the system off and call for service when:
- The pump stays silent or only hums over a full tank with power at the outlet.
- Water has reached the furnace cabinet, the control board or a finished ceiling below.
- The pump empties, then the reservoir refills from the tube each time, which means the check valve has failed.
- It overflows again after you have cleaned it.
What replacing a condensate pump costs
Replacement is like for like: a 120 V plug-in unit with the same or a greater lift rating, since the pump has to push water up to the height of the drain. Reusing a slimed tube on a new pump invites a repeat, so the tube and check valve go in new with it.
- New condensate pump, installed: $250–$450.
- Drain cleaning or trap replacement, when the pump is fine and the line is the problem: $150–$350.
- Diagnostic visit: $90–$150, applied to the repair if you proceed.
ZK Mechanical does condensate pump replacement and drain repairs as part of AC repair in Burlington, Oakville, Milton, Hamilton and the west GTA. If the water is at the furnace and there is no pump in sight, start with why an AC leaks water. Call (647) 801-1252 or send the details through the contact page; a photo of the pump's label tells us which unit to bring. Drain and leak questions are collected in the cooling section of the answers hub.