Heater not turning on: what to check on a boiler
If the heater does nothing when the house gets cold, the cause is usually upstream of the burner: power, the thermostat call, or a safety lockout. Work through them in that order.
Start with the cheapest explanation: nothing is asking for heat
The most common reason a heater does not turn on is that it was never told to. Set the thermostat to heat, raise the target a few degrees above the room temperature, and wait a couple of minutes. A blank display usually means dead batteries or no power to the thermostat.
Wait before you conclude anything. Many boilers run a short start-up routine before the burner lights, and some hold off if the water is already warm enough.
Check the switch and the breaker that feed the boiler
Most boilers have a plain on/off switch on or near the unit, often on the wall of the utility room or at the top of the basement stairs. It looks like a light switch, and it gets flipped off by accident more often than you would expect, especially after someone has been cleaning or storing things nearby.
If the switch is on and the boiler is still dead, look at the breaker labeled for the heating system. Switch it fully off and back on once. If it trips again, stop. A breaker that will not stay on is telling you there is an electrical fault, and that is a job for a licensed HVAC technician, not for you.
A lockout is the boiler protecting itself
Modern boilers shut the burner down and lock out when something is off: a failed ignition attempt, a limit switch that tripped, low water, or a blocked vent. Many show a code or a blinking light, and the manual usually explains what each one means.
Most units have a single reset button. Pressing it once after a lockout is normal. Pressing it again and again is not. If the boiler locks out a second time, something real is wrong, and every extra reset just stresses the same safety circuit. Write down the code you see and call for service.
Gas and safety come before any troubleshooting
If you smell gas, or a carbon monoxide detector goes off, do not keep checking anything. Leave the house, then call your gas utility or 911 from outside. Do not try to find the source and do not relight anything yourself.
If the boiler uses a standing pilot and the pilot is out, follow the relighting label on the unit only if there is no gas odor at all. If it will not stay lit, that is a technician's repair.
Why the heater might be on but you still get nothing
Sometimes the boiler is running and the house is still cold. That points past the burner, to the circulator pump, a zone valve that did not open, or air trapped in a radiator loop. If you can hear the boiler fire but the pipes stay cold, read the companion guide on a heater that will not turn on in part of the home.
| What you see | Most likely area | Your move |
|---|---|---|
| Thermostat screen blank | Batteries or thermostat power | Replace batteries, check the breaker once |
| Boiler dead, no lights | Service switch or breaker | Confirm switch is on, one breaker reset |
| Code or blinking light | Safety lockout | One reset, note the code, call if it repeats |
| Burner fires, pipes stay cold | Pump, zone valve or trapped air | See the in-home guide |
| Gas smell or CO alarm | Safety emergency | Leave and call 911 or the gas utility |
What not to do
- Do not hold the reset button down or press it repeatedly to force a start.
- Do not bypass a safety switch or jumper a control. Those exist because the alternative is unsafe.
- Do not open electrical compartments or touch wiring. Electrical work on a heating system is for a licensed technician.
When it is time to call
Call a licensed HVAC or heating technician if the boiler locks out twice, a breaker keeps tripping, the display shows a code you cannot clear, or you have any doubt about gas. In cold weather, do not wait a week to find out whether it fixes itself. It rarely does.
Frequently asked questions
Why will my heater not turn on even though the thermostat is set to heat?
The thermostat only sends a request. If the boiler has no power, is locked out, or is waiting on a safety switch, it will ignore that request. Check the service switch, the breaker and any code on the boiler display before assuming the thermostat is faulty.
Is it safe to keep pressing the reset button?
Press it once after a lockout, which is normal. If the boiler locks out again, stop and call a licensed technician, because a repeat lockout means a real fault such as ignition, a limit switch or low water.
What should I do if I smell gas near the heater?
Leave the house right away and call your gas utility or 911 from outside. Do not turn anything on or off, and do not try to find the leak yourself.
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.