The boiler temperature control... 

Setting the control knob of the front of the boiler:

 

I've come to notice that many people are perplexed about the function of the temperature control knob on the front of their boiler, as it isn't clear what effect it has. Hence this page to hopefully explain.

This control knob adjusts the temperature of the water coming from the boiler. This water is used by the heating system controls for two different purposes. 

Firstly, it is sent directly to the radiators when the central heating is ON, so this is the temperature to which the radiators will heat. If you find your radiators are scaldingly hot, turning this control on the front of the boiler to a lower setting will reduce the temperature of the rads.

Secondly, this hot boiler water is used to heat the hot water cylinder. Here it gets more complex. There is usually a thermostat attached to the side of the hot water cylinder. This would normally be set to about 50 degrees C and is designed to turn the boiler OFF when the cylinder reaches this temperature, in order to save fuel. 

The complexity arises from the fact that the boiler control needs to be set to a higher value than the hot water cylinder thermostat, or the boiler water will never raise the cylinder water to the setting on the cylinder thermostat. Hopefully that makes sense! If the boiler temperature control is set too low, the hot water cylinder thermostat will never be satisfied and will continuously 'call for heat' (as we say in the trade) so the boiler will continuously cycle ON and OFF. This wastes fuel and heat so paradoxically, for economy it is better to set the boiler temperature control higher than lower. 

If you boiler seems to be for ever turning itself ON and OFF, try turning the control to a higher setting (or the cylinder thermostat to a lower setting). This may well fix the problem.