When a high-efficiency furnace quits on the coldest night of the year, the cause is very often outside the furnace itself: a frozen condensate line or a PVC vent pipe blocked by snow or ice. Both trip a safety switch called the pressure switch, and the furnace shuts down to protect you. Some of these you can check in a few minutes. If the furnace still will not run, call (708) 515-4822; we answer 24/7 and the diagnostic is $75 ($99 evenings, weekends and holidays).
Why high-efficiency furnaces are sensitive to cold snaps
A 90% or higher furnace pulls so much heat out of its exhaust that the water vapor in it condenses. That water drains through a trap and a small hose or pipe to a floor drain or a condensate pump. The furnace also vents through two PVC pipes, usually out a side wall: one brings in combustion air and one carries exhaust out.
If the drain freezes or backs up, or either pipe is blocked, the furnace cannot prove it is venting safely. The pressure switch stays open, the burners never light or shut off quickly, and many furnaces flash an error code.
Signs it is a condensate or vent problem
- The inducer fan starts, runs for a while, then the furnace shuts down without lighting.
- The furnace lights, runs a few minutes, then shuts off, and repeats.
- Water is on the floor around the furnace, or the condensate pump is full and not running.
- You hear gurgling from the furnace while the inducer runs.
- The problem started during a snowstorm, an ice storm or several days below zero.
- The furnace display or status light shows a pressure switch code.
What you can safely check
1. The vent pipes outside
Find the two white PVC pipes on the side of the house or the roof. Clear snow away from both openings and look for ice buildup inside the ends. Drifting snow, a plowed pile or ice from a dripping gutter is enough to block them. Do not reach far into the pipe or use tools that could crack it.
2. The condensate drain and pump
Follow the small drain hose from the furnace to the floor drain or the condensate pump. If a line runs outside, that outdoor section is the part most likely to freeze. A condensate pump that is full of water and not running needs to be replaced or cleared.
3. The filter and thermostat
A clogged filter can overheat a furnace and shut it off on a different safety switch, so replace it if it is dirty. Make sure the thermostat is set to heat and its batteries are good.
After clearing the vents, turn the furnace off at its switch for a minute, then back on. If it does not run normally after one restart, stop and call. Repeated restarts will not fix a frozen trap or a failed pressure switch.
What we check on a no-heat call
Our technicians check the vent terminations, the condensate trap and drain, the pressure switch and its tubing, and the inducer motor. Most of these repairs fall between $100 and $700, and our trucks carry pressure switches, inducers and condensate pumps, so many furnaces are running again on the first visit. You approve a flat-rate price before any work starts.
For lasting fixes, we can reroute an outdoor drain line so it cannot freeze, add a vent termination kit that resists snow, or move a termination that sits where snow drifts.
Where we see this most
We see frozen drains and blocked vents across our service area every winter, especially in subdivisions where furnaces from the 1990s and 2000s vent out a side wall near the ground. If you are in one of these towns, our local pages list typical repairs and drive times: furnace repair in Huntley, Cary, Naperville, West Dundee and Streamwood, or see all of our service areas.
Frequently asked questions
Can I pour warm water on a frozen condensate line?
On a short outdoor section of drain line, warm (not boiling) water can thaw it. Do not pour water into the furnace or the vent pipes. If the trap inside the furnace is frozen, call us.
Why does my furnace work during the day but shut off at night?
Night is when it is coldest and the furnace runs longest, so a partly frozen drain or an iced-over vent fails then. Daytime warming lets it thaw, which makes the problem come and go.
Is a blocked vent dangerous?
The pressure switch is there to shut the furnace down before exhaust can back up, so the furnace stopping is the safety system working. If your carbon monoxide alarm sounds, leave the home and call 911.
Need help now? Schedule furnace repair or call (708) 515-4822, 24/7.