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No heat or some zones cold
What is going on
The boiler fires but one or more zones stay cold, or the boiler does not fire.
Likely causes
- Low system pressure
- Air in the system
- Failed circulator pump or zone valve
- Thermostat or zone control wiring
- Boiler in lockout
Check in this order
- 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
Applies to: Hydronic boilers
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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.