Boiler Not Heating: What to Check
Boiler not heating the radiators or floors? Check the thermostat, power, the pressure gauge and the outside vent first. Low pressure, a lockout code or cold zones usually need a licensed technician.
Common reasons: boiler not heating
- Thermostat or zone valve not calling for heat
- Low system pressure (often from a leak)
- Air in the system (gurgling, cold radiators at the top)
- Pump not running
- Blocked vent or condensate drain (condensing boilers)
- Ignition or flame faults shown as error codes
Safe checks you can do yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the boiler gas valve is open (handle in line with the pipe).
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
When to call a licensed technician
- Pressure is low and keeps dropping
- Water is leaking from the boiler or relief valve
- The boiler locks out with a code
- Some zones stay cold
Showing an error code? Look it up by brand
Bosch BoilerFlowMax Combi BoilerNavien BoilerNavien Combi BoilerRinnai BoilerRinnai Combi Boiler
Detailed troubleshooting guides
- No heat or some zones cold All brands · Boiler
- Annual service checklist - Navien NHB-H Navien · Boiler
- No space heat - Navien NHB-H / NFB-H Navien · Boiler
- Relief valve discharging - Navien NHB-H / NFB-H Navien · Boiler
- Hot water is cold or turns cold - Navien NCB-H Navien · Combi Boiler
- Hot water not hot enough or too hot - Navien NCB-H Navien · Combi Boiler
- No space heating - Navien NCB-H Navien · Combi Boiler
- No water from hot water tap - Navien NCB-H Navien · Combi Boiler
Boiler not heating: questions homeowners ask
Why is my boiler not heating?
Common causes are low pressure, a zone or thermostat problem, a failed pump or an ignition lockout. The error code and the pressure gauge point to which one.
What pressure should my boiler be at?
Many residential systems sit around 12 to 20 psi when cold, but check the label or manual for your system. Pressure near zero means a leak that needs a technician.
Can I reset my boiler?
One reset is fine. If it locks out again, a licensed technician should find the cause.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.