Pressure falls below .7 bar when heating is not on and therefore does not operate. When valve is opened to re-set pressure to 1 bar and then heating comes on and will stay on until timer swtiches off. Once off pressure drops to below .7 bar and will not operate until re-pressurised.

Steve Barum, December 2009

Thanks for your response. There is no hot water tank. This is a combi boiler and water heats up as it is used, but will not when pressure falls below .7 bar.
When heating is on everything is ok and stays on. It is only when timer turns heating off then the pressure drop occurs.

Stevebarum, December 2009

You have either got air in the system or a leak some where within the heating circuit which could be in the heat exchanger. You don't make clear if you have a hot water tank or direct hot water. If you do have a tank and the weep is inside it the only sign could be a dripping header tank. This is a bit of a guess but good luck.

Brian, December 2009