What the symptom tells us

If the boiler heats up quickly but the heat does not reach the radiators, the problem is usually in the water circulation.

What to check first

  • If the circulator is working.
  • If the supply and return valves are open.
  • If there is air in the network.
  • If the filter is clogged.
  • If any zone solenoid valve remains closed.

Important indicator

If the supply immediately after the boiler is very hot but the return remains cold, the flow is likely very low or zero.

Example

The boiler quickly reaches 80°C and stops, while the radiators are cold. We check the circulator, filter, and valves first, not the burner.

Why it cycles on-off frequently

Without proper circulation, the boiler cannot transfer heat to the network and heats up quickly.