Boiler runs but some radiators or zones stay cold

Safety: Turn off power to zone valves before manual operation. Protect floors from water when bleeding radiators.

• Bleed affected radiators: Starting with the highest radiators, open bleed valves until air stops and water flows steadily.
• Check zone valves: Verify that thermostats are calling for heat and that corresponding zone valves open (you may feel or hear them move). Manually open valves if they are stuck and replace if necessary.
• Inspect pump performance: If multiple zones are weak, the Dettson system circulator may be undersized or failing. A technician can measure differential pressure and replace the pump if needed.
• Adjust balancing valves: Ensure balancing valves are not overly restricting flow to cold zones.