This is probably something glaringly obvious so I hope that you don't
mind me posting this question.

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.

Thank you