Two kinds of boiler no-heat
Either the boiler isn't firing at all, which points to power, controls, ignition or fuel, or it fires but the heat doesn't reach the radiators, which points to circulation, air or valves.
Look at the boiler while the thermostat calls for heat: is the burner lit? Is the circulator pump humming? Is the pipe leaving the boiler hot?
Hot-water boiler: where the heat stops
| What you find | Likely cause |
|---|---|
| Burner off, no lights | Power, switch, fuse or control |
| Burner tries, then locks out | Ignition, flame sensor, gas or oil supply |
| Boiler hot, pipes to radiators cold | Circulator not running, or a zone valve stuck |
| Radiators warm at the bottom only | Air in the system (bleed radiators) |
| Low pressure on the gauge | Leak or failed fill valve (pressure guide) |
Steam boiler: no-heat checks
- Sight glass shows water (low water shuts the burner off)
- Low-water cutoff hasn't tripped (some have a reset)
- Pressuretrol isn't set too low
- Main vents and radiator vents aren't stuck closed
- The emergency switch at the basement stairs is on
Circulator pumps
On hot-water systems, the circulator moves heated water to the radiators. A failed capacitor, seized impeller or bad relay stops circulation while the boiler stays hot.
A pump that's hot to the touch but silent, or humming without flow, is a common winter failure, especially after a summer idle.
How a boiler technician diagnoses it
- Confirms the thermostat call reaches the boiler control.
- Checks firing: ignition, flame proving and fuel supply.
- Checks circulation: pump, relays, zone valves.
- Checks pressure, water level and air.
- Fires the system and verifies heat at each zone.
Don't add water to a hot, empty steam boiler
If a steam boiler's sight glass is empty and the boiler is hot, shut it off and let it cool before adding water. Cold water on a dry, hot cast-iron boiler can crack it.
Boiler no-heat FAQs
Why does my boiler work for hot water but not heat?
On boilers with an indirect water heater or combi design, domestic hot water can have priority. A heating zone valve, circulator or diverter valve is likely at fault.
Can a thermostat stop a boiler from heating?
Yes. A failed thermostat, wiring or zone control won't send the call for heat.
Next steps
Ready to get it fixed? Call (888) 555-0144 to reach a licensed provider in your area. Related guides:
Busy winter markets: boiler repair in New York · boiler repair in Massachusetts · boiler repair in Pennsylvania · boiler repair in New Jersey · boiler repair in Connecticut · boiler repair in Illinois · all states.