In short
A condensate trap is the small water-filled trap on a condensing furnace's drain. It lets the water the furnace makes run to the floor drain while keeping exhaust gas in and the inducer's suction intact. When it plugs with slime or freezes, water backs up and the furnace locks out on a pressure-switch code or leaks. Cleaning or replacing one runs $150–$350.
Every high-efficiency furnace makes water, and the condensate trap is the small plastic part that gets that water out without letting exhaust out with it.
What a condensate trap does on a high-efficiency furnace
The condensate trap works like the trap under a sink: a pocket of standing water seals the pipe. Condensate from the secondary heat exchanger and the vent pipe collects at the inducer, runs into the trap and spills out the other side toward a floor drain or a condensate pump. The seal matters because that part of the furnace is under suction or pressure from the inducer. With no water in the trap, air gets pulled up the drain and holds the condensate back, or flue gas escapes through the drain into the basement.
The condensate is mildly acidic. That is why the trap and tubing are plastic, and why the water is piped to a drain instead of being left to run across the floor.
How much water a furnace makes, and why Ontario winters freeze it
A condensing furnace makes water every minute the burners run, which adds up to several litres a day in a cold snap. All of it leaves through one small trap and a length of tubing. In most GTA basements that tubing runs a short distance to a floor drain and stays warm.
Trouble starts where the furnace or its drain line sits in an unheated crawl space, garage or attic: the line freezes, and a frozen drain behaves exactly like a plugged one. The steps for that are in what to do if a furnace condensate line is frozen.
How a condensate trap fails
A condensate trap fails by plugging. Over a few seasons a film of slime, rust flakes and debris from the vent settles in the bottom of the trap until water can no longer pass.
- Gurgling or sloshing from the furnace when the inducer starts.
- Water backing up into the inducer housing, then a pressure switch code and a lockout.
- Water on the floor around the furnace, or dripping inside the cabinet toward the control board.
- A furnace that runs for a few minutes, stops, and restarts after sitting.
- A trap that dried out over the summer and gurgles on the first start-up in fall.
Water at the furnace has other possible sources, including the humidifier and the air conditioner's drain, and why a furnace is leaking water sorts them out. The air conditioner's evaporator coil has a trap on its own drain for a similar reason: it keeps blower air from blowing out of, or being sucked in through, the drain line.
What cleaning or replacing a condensate trap costs
Condensate drain cleaning or trap replacement typically runs $150–$350 in the GTA. On a maintained furnace it rarely needs its own visit: cleaning, flushing and priming the trap is a standard step in a furnace tune-up, which runs $120–$250.
If there is water on the floor now, switch the furnace off at its power switch and mop up so the board and blower stay dry. Leave the trap assembled unless the owner's manual shows how it comes apart, since a trap put back wrong or left empty will stop the furnace. For a leak or a lockout, book furnace repair at (647) 801-1252 or tell us where the water is showing up.
Questions people ask about Condensate trap
Can I clean a furnace condensate trap myself?
On some furnaces, yes: the owner's manual shows a trap that unclips, rinses out with warm water and goes back filled. Turn the power off first and note where each hose connects. If the trap is buried behind the inducer, or you are unsure of the hose routing, leave it for a tune-up. A furnace should never be run with the trap missing.
Why does a condensate trap need to be primed?
A trap only seals when it holds water. After a summer of sitting, or after cleaning, it can be empty, and on the first start the inducer pulls air through the dry trap instead of letting condensate drain. The furnace may fault before it makes enough water to refill it. Priming means pouring water into the trap until the seal is restored.
Does a mid-efficiency furnace have a condensate trap?
No. A mid-efficiency furnace keeps its exhaust hot enough to leave through a metal vent or chimney without condensing, so it has no condensate drain. Water near one of those furnaces comes from somewhere else: the humidifier, the air conditioner's drain pan, or condensation running back down a cold or oversized chimney, which needs checking.