Potterton Netaheat 16-22 HW doesn't work without CH?

I have had this problem with our Potterton Netaheat 16-22 Boiler for a while now. It is fine during the winter but a pain during the summer. The HW will not work/heat up the water in the house unless the CH is on. Obviously it gets ridiculously hot during the non-winter months and is a complete waste. If the CH is not on, all I get is cold water. Even with the nob on the boiler to 5 or MAX, nothing heats up the water in the house. Is there a separate thermostat for the water that could be broken? Am I missing something obvious?

Doubt it is related but we also sometimes get house-shaking, loud squeal/moan when the hot water is on (mainly shower). It only happens when you first turn the hot water (usually) tap on and stops soon after you turn the hot water off. If you wait 30 seconds and turn it back on, all is fine.

Any help or advice would be appreciated.

Charles
April 2010