Quick answer
If your furnace condensate line is frozen, switch the furnace off, find the frozen section where the drain passes through a cold space, and thaw it with warm towels or a hair dryer, never an open flame. To get heat back tonight, direct the drain into a bucket indoors. Then have the line rerouted, re-sloped or protected so it cannot freeze again.
A frozen furnace condensate line shuts the heat off on the coldest night of the year, which is exactly when it is most likely to happen. The furnace itself is usually undamaged. Water that should be draining away has turned to ice in the pipe, backed up into the furnace and tripped a safety. Thaw it, give the water somewhere to go, and the heat comes back.
Why a frozen drain stops a high-efficiency furnace
A condensing furnace, 90% AFUE and higher, pulls so much heat out of its exhaust that the water vapour in it condenses. That adds up to several litres of water a day, and more in a cold snap when the furnace runs for long stretches. The water collects in a trap inside the cabinet and leaves through a plastic drain line.
Where that line runs through an unheated garage, a crawlspace or an exterior wall, or ends outdoors, it freezes from the cold end back. Water then fills the trap and the collector box, the inducer fan can no longer pull the right vacuum, and the pressure switch opens. The burners will not light, and the board flashes a pressure-switch code. If your furnace drains into a small pump, the pump's thin discharge tube can freeze the same way and trip the pump's overflow switch.
How to thaw a frozen condensate line safely and get heat back tonight
- Turn the furnace off at its service switch.
- Follow the drain line from the furnace and find the cold section. Ice usually sits where the pipe enters the cold space or at the outdoor end; the pipe feels hard and very cold, and a clear tube will show the ice.
- Wrap that section in towels soaked in hot tap water and replace them as they cool, or warm it with a hair dryer on a low setting, moving it along the pipe.
- Put a pan or dry towels under the furnace end. Water will come out when the plug lets go.
- Never use a torch, a heat gun or any open flame. PVC pipe and vinyl tubing soften and scorch, and a furnace cabinet is no place for a flame.
- Do not pour boiling water into the furnace or chip at the ice. Both crack fittings.
A bucket gets the heat back on when the ice will not clear or the frozen section is buried in a wall. Find a joint in the drain line downstream of the trap, inside the heated space, where the tube pulls off a fitting or a slip coupling comes apart by hand. Point the furnace side into a bucket or a low tote.
Leave the trap itself connected and full of water. The trap is a water seal that keeps exhaust gas from being pushed out through the drain, and a furnace must not run without it. Restore power, let the furnace start, and check the bucket every few hours, because it fills faster than most people expect. Condensate is mildly acidic, so empty it into a laundry sink or floor drain and rinse the floor if any spills.
Permanent fixes for a condensate drain that freezes
- Reroute the line to an interior floor drain or laundry standpipe so that no part of it leaves the heated space.
- Add a condensate pump where there is no drain nearby, and run its discharge tube through heated space. A new pump installed is $250–$450; see condensate pump replacement.
- Correct the slope. The line needs a continuous fall with no sags, because standing water in a belly is what freezes first.
- Shorten and insulate any run that has to cross a cold area.
- Fit self-regulating heat cable rated for plastic pipe, plugged into a GFCI-protected receptacle, where a cold run cannot be avoided.
- Clean or replace a slimed-up trap so water leaves quickly. Drain cleaning or trap replacement runs $150–$350.
Why older Burlington and Hamilton houses get frozen drain lines
The pattern is usually a retrofit. Houses built with a chimney-vented furnace, which made no condensate, had no drain planned near the furnace. When a condensing furnace went in, the drain line was run to wherever was convenient, and in a 1950s or 60s bungalow or a century farmhouse that can mean through a crawlspace or out through the rim joist. Furnaces installed in garages, and in additions built over crawlspaces, have the same exposure.
A drain that froze once will freeze again at the same temperature. If it happens during a deep cold snap, expect a repeat the next night unless the line is bypassed or fixed.
Call a technician if
- There is water inside the furnace cabinet or dripping from the burner compartment. A soaked pressure switch, inducer or control board has to be checked before the furnace runs.
- The furnace still locks out after the line is clear. The trap, the pressure-switch tubing or the switch itself may be blocked or damaged; why a furnace pressure switch fails covers that.
- The ice is in the plastic exhaust pipe outside and not in the drain. That is ice building up on the furnace exhaust, a venting problem.
- A fitting has cracked, or the frozen section cannot be reached.
Rerouting a condensate drain is priced in writing once the route has been seen; the diagnostic visit is $90–$150 and comes off the bill if the work goes ahead. Call ZK Mechanical at (647) 801-1252 for furnace repair at any hour, or describe the drain run to us and we will tell you what the fix involves. Other winter no-heat problems are in the heating answers hub.