my son moved into his first house on Monday and the CH heating only works when the water in the hot water tank is below the tank thermostat which is currently set at 90, when it reaches this he has to run the hot water to cool it down so that the CH will come back on.
He has a potterton EP2000 sp with a baxi boiler and honeywell valves.
In the airing cupboard there is a hot water tank with the thermostat that runs the heating as well, potterton EP2000 sp, a pump, two Y valves and a white 'control???' box
In the lounge there is a wall thermometer - it clicks to show under or over temperature but does not start the boiler
The boiler is in the kitchen and starts up when you run hot water off of the tank or turn the thermostat about the current temperature.
My son thinks that it is a wiring problem but he hasn't any plumbing or wiring experience.
To recap - the heating only works when the water in the hot water tank is below the temperature set on the hot water tank thermostat.