heating probs?

Well the boiler works - so if it stops working under some conditions then there's something going wrong external to the boiler.

Your question could be a little clearer - you don't mention the make or model - you don't actually say what happens when CH and DHW are being called or just DHW on its own.

The timer programmer feeds the cylinder stat when DHW is required and that should fire up the boiler until the cylinder stat operates.

The timer programmer feeds the room stat when only CH is required and 230V is sent to the MPV to drive it fully over to the CH port - also making a microswitch which sends a fire up signal to the boiler on the Orange wire. The room stat controls the on/off. The boiler only fires up when the MPV is in the CH position - not before.

When both CH & DHW are called for the MPV motor begins to move but operates a microswitch at the mid position causing the motor to stall. Both room stat and cylinder stat provide control until one of them operates and the situation reverts to either CH only or DHW only.

An explanation with drawings is here...

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

If you have somebody select CH only you should hear the motor driving the valve fully over - if you don't hear it perhaps the motor is defective.

Good luck...

Peccavi
April 2010