boiler on, rooms not to temperature, rads now cooling

[jules42]

My boiler and heating seem to have developed a bit of a mind of their own. The boiler fires up and the rads get hot: no problem. Sometimes the boiler then fails to fire when the thermostat next calls for heating. Sometimes the whole system seems to be fine (stat calling, boiler on and gas lit) but the rads just cool down and sit at tepid.

I have had it suggested it might be the solenoid that is the problem, but would like views from folk who know more than me before I commit to the price of the part and the labour.

Boiler is an elderly (but regularly serviced) Baxi Solo

 

[bellairaphon]

Where is the boiler temp set too?

Nath

 

[jules42]

The boiler has a knob to turn to set the temperature with settings from 1 to 5: currently set of 3 (which it has been since before the problems started)