Boiler: No heat or some zones cold
No heat or some zones cold on a boiler: The boiler fires but one or more zones stay cold, or the boiler does not fire.
Applies to: Hydronic boilers
Common causes of no heat or some zones cold
- Low system pressure
- Air in the system
- Failed circulator pump or zone valve
- Thermostat or zone control wiring
- Boiler in lockout
How to fix no heat or some zones cold: step by step
- Check the boiler display or LED for an error code
- Check system pressure (typically 12 to 20 psi cold on residential systems; confirm for the installation)
- Bleed air from radiators or purge the zone
- Confirm the circulator runs and the zone valve opens when the zone calls
- Check thermostat and zone control wiring
Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.
If the unit shows an error code, look it up: Bosch Boiler codes Navien Boiler codes Rinnai Boiler codes
Source: General HVAC service practice — verify per model. Written in our own words as a summary; always check the original manual.
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.