Key Takeaways
- Check the thermostat mode, setting and batteries first.
- Furnaces have a service switch that looks like a light switch; it's easy to flip off by mistake.
- A badly clogged filter can make a furnace overheat and shut itself down.
- On an oil burner, press the reset button once only. Repeated resets can be dangerous.
1. The Thermostat
Make sure it's set to Heat, not Cool or Off, and that the set temperature is a few degrees above the room. If the display is blank or dim, replace the batteries. Programmable thermostats sometimes revert to a schedule after a power blip, so check the program too.
2. The Service Switch and Breaker
Every furnace has a power switch nearby, often on the side of the unit or at the top of the basement stairs. It looks exactly like a light switch, and people turn it off by accident all the time. Check it, then check the breaker for the furnace in the panel.
3. The Filter
A clogged filter starves the furnace of air. The heat exchanger gets too hot, a safety limit trips, and the burner shuts off, sometimes after running for just a few minutes. Pull the filter and hold it to the light. If you can't see through it, replace it.
4. The Door Panel
Most furnaces have a safety switch that keeps them from running when the blower door is off or loose. If someone changed the filter recently, make sure the panel is seated firmly.
5. The Gas Supply
Check that the gas valve on the line to the furnace is open (the handle in line with the pipe). If other gas appliances like the stove aren't working either, the issue may be the gas supply to the house.
If you smell gas, don't try to restart anything. Leave the house and call 911 or your gas utility from outside.
6. The Condensate Drain
High-efficiency furnaces, the ones with white plastic vent pipes, produce water. It drains through a small tube, often to a condensate pump. If that line clogs or the pump fails, a float switch shuts the furnace off to prevent a flood. Look for water around the furnace or a full pump reservoir.
7. Vent Pipes Outside
Those same high-efficiency furnaces usually vent through the side of the house. After a Connecticut snowstorm, drifts can block the intake or exhaust pipe and the furnace will shut down. Clear the snow from around the pipes, gently, and the furnace may restart on its own.
8. Oil Furnaces: Tank and Reset
Plenty of Hamden homes still heat with oil. Check the tank gauge first; running out is more common than people think, especially early in the season. If the burner has locked out, the oil furnace reset button is the red button on the burner control.
Press an oil burner's reset button once. If it doesn't start, don't keep pressing it. Each attempt can pump oil into the combustion chamber, which can cause a dangerous puff-back or fire. Call for service instead.
Read the Error Light
Many modern furnaces have a small window with a blinking light on the control board. The pattern of blinks is a code, and the legend is usually printed on the inside of the panel. Note the code before you call; it speeds up the diagnosis.
When to Stop and Call
- You've done the checks and it still won't run
- It starts, then shuts off again after a few minutes, over and over
- You hear banging, screeching or grinding
- A carbon monoxide alarm goes off: get everyone outside first
- The oil burner has locked out more than once
Common causes at that point include a dirty flame sensor, a failed igniter, a bad inducer motor or pressure switch, or a control board fault. Our heating repair team diagnoses and fixes them. If you still have no heat, Hamden, CT homeowners can reach us on the HVAC line.
Most no-heat calls in the first cold week of the year could have been caught by a fall tune-up. See our fall heating checklist.
