Furnace condensate drain: where the water goes and what to check
Your high-efficiency furnace makes water on purpose. When the drain line backs up, the furnace can shut down or leak onto the floor.
Your furnace drain carries real water, and it needs a clear path
A furnace condensate drain is the small plastic line that carries acidic water out of a high-efficiency furnace. Only condensing furnaces have one. If yours has white or gray PVC exhaust pipe and no metal flue, you almost certainly do.
The water is not a defect. A condensing furnace pulls so much heat from the exhaust that the vapor turns to liquid. That liquid has to leave the house somehow.
Where the water goes after it leaves the furnace
Inside the cabinet, the water collects in a trap and then runs into a drain line. From there it usually takes one of three routes.
| Route | What you will see | What can go wrong |
|---|---|---|
| Straight to a floor drain | A plastic line ending over a drain | Line sags, kinks or clogs with sludge |
| Into a condensate pump | A small box with a tube leading up and away | Pump loses power, float sticks, outlet tube clogs |
| Into a laundry tub or standpipe | A line draped into a sink | Line slips out or backs up with lint |
Many systems share this line with the air conditioner coil above the furnace. That means a clog can show up in summer even though the furnace is the part that is idle.
What you can safely check yourself
Stay on the outside of the cabinet. Look at the visible line from the furnace to wherever it ends. Is it attached at both ends? Is it sloping down the whole way? Is there standing water in a low spot?
If you have a pump, check that it is plugged in and that its outlet tube is not pinched. Then note whether there is any water on the floor and when it appears. That detail helps a technician a lot.
Signs the drain is the problem
- Water on the floor near the base of the furnace.
- The furnace starts, then shuts off, and a pressure or drain safety switch is blamed.
- A gurgling sound from the line.
- A musty smell near the furnace in winter.
Many condensing furnaces have a safety switch that stops the burners when the water backs up. That is the furnace protecting itself, so do not bypass it.
Leave the trap and the inside of the cabinet to a pro
The trap sits right beside the burner and the exhaust path. Opening the cabinet to clear it exposes you to live wiring and combustion parts. A licensed HVAC technician can clear and flush it safely.
Do not cap, plug or pinch the line to stop a drip. The water has nowhere else to go, so it will back up into the furnace.
Keeping it clear
A yearly tune-up should include the drain. Ask for it by name. If you have a pump, ask them to test the float too. Nothing here is worth a DIY chemical treatment in a line you cannot see into.
One safety note. If you smell gas or a carbon monoxide detector sounds at any point, leave the house and call your gas utility or 911 from outside. Do not look for the cause.
Frequently asked questions
Does every furnace have a condensate drain?
No. Only high-efficiency condensing furnaces make condensate. Older mid-efficiency furnaces vent hot exhaust through a metal flue and have no drain line.
Why does my furnace shut off when the drain is clogged?
Condensing furnaces have a safety switch that stops the burners when water backs up. It protects the furnace and the floor, so it should not be bypassed.
Can I clean the condensate trap myself?
The trap sits inside the cabinet next to the burner and exhaust. Leave it to a licensed HVAC technician. You can check the visible outside line and a pump's power.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics