In an earlier contribution to this discussion,
wrote:

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

Not sure about the *two* Y-valves, but ignoring that it sounds like he's got
a Y-Plan system with a 3-port mid-position valve which directs the flow from
the boiler either to the radiators or to the coil in the hot cylinder or to
both at the same time.

Have a look at Y-Plan in http://content.honeywell.com/uk/homes/systems.htm
and see if it matches what he's got.

From the symptoms, it's a pretty safe bet that it's a wiring fault. With
this setup, the cylinder stat *must* be the change-over type, and the NO
contact must be connected to the grey wire of the 3-port valve. For good
measure, there also needs to be a Hot Water Off connection to the
programmer.

When the hot water demand is satisfied, the boiler is turned on by a switch
in the 3-port valve's actuator. If there's no feed on the grey wire, you get
no boiler - simple as that. [When the HW is being heated, the boiler if fed
through the tank stat's NC contact - which is why he needs to keep the HW
below the stat setting to get any CH].

[If the HW OFF connection is correct, but the cylinder stat isn't, the CH
should work if he turns HW off at the programmer. Has he tried that?]
--
Cheers,
Set Square
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