How to Fix / Troubleshooting

Safety: Turn off power before working on valves or pump. System water may be hot.

Check system pressure: Ensure pressure is within the recommended range. Low pressure reduces circulation.
Bleed radiators: Remove trapped air starting from the highest radiators.
Feel flow and return pipes: With the furnace running, the flow pipe should be hot and the return warm. If both are only slightly warm, circulation may be poor.
Check zone valves: If fitted, ensure motorized valves are opening. You may feel or hear them move when heating is called. A stuck valve head may need replacement.
Powerflush (professional): If radiators are hot at the top but cold at the bottom, sludge may require professional cleaning.

Furnace runs but radiators remain cold or only lukewarm.

Possible Causes