i have recently fitted a new hot water tank thermostat and a pcb on the boiler, the boiler now fires up as and when it feels like it ? can anyone help ?

paul, May 2010

This might help

http://www.gasheating.co.uk/Mid-Position-Valve.html

Peccavi, May 2010

A boiler-engineer will be able to help you - and you do not mention the make or model or style of your boiler.

Notwithstanding a frost stat the timer programmer is the primary control device - the tank stat controls hot water firing and the room stat controls central heating firing.

You do not mention the condition of these controls when the boiler fires up erroneously. Neither do you say if the controls work correctly at any time.

Peccavi, May 2010