wrote in message
...
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


Just a guess but if the valves are zone valves (two port not three port)
then you have an S-plan system and the symptoms suggest the heating valve
motor has failed, the valve has been set to manual open (actually about half
open so water passes but the microswitch is not operated) thus when the hot
water is demanded that zone valve opens and operartes the microswitch in its
head giving power to pump and boiler and heat goes to both hot water
cylinder and heating radiators.
If you are electrically competent determine which (brown)wire in the white
"control terminal box" is the supply to the motorised valve serving heating
and see if it is live when heating is called for. If it is you should be
able to hear the motor drive, if you can't the motor has in all probability
failed. Spare motors are available for a very modest sum.