Central Heating only problem

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
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In an earlier contribution to this discussion,

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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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.
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On Thu, 27 Jan 2005 21:50:50 +0000 (UTC), "John"

Thanks for this the valves all seem to be working - we can't find a wiring diagram for the controller so can't check the wiring.
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Update- son has just checked
the valves he has are two open shut valves (both appear to be working) not y valves.
The thermostat downstairs seems to open and shut the valve but it doesn't trigger the boiler and pump on.
The honeywell room thermostat in the lounge has 2 x 2 (2 larger wires which contain a red and a black in each) be he can't see where these should connect in the airing cupboard.
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There should be a control terminal box in the locality of the valves where the main interconnections are made. IIRC you said there was in an earlier post. Both Honeywell water and heating valves should be connected into this via five wire flex. Each valve will have a opening signal to its motor via a (usually brown) wire and if the valve is opening then it must be present ok. Also each valve will have a neutral (blue), an earth (green/yellow), a permanent live feed for controlling the boiler and pump (grey) and a switched or controlled signal wire (orange). The greys will be joined together and connected into the live supply, the oranges will be connected together and joined to the wire(s) feeding the pump and boiler. The valve heads contain microswitches which close their contacts when the valve is fully open. The grey and orange wires are simply connected to these. With a suitable test instrument (and we don't need a clamour about not using neon screwdrivers here thanks folks) check if the live feed is present to the grey wire to the valve serving the heating and if it is, continue checking to see if it arrives at the orange wires in the terminal box. Note here that manually operating the valve with the trigger will not cause the microswitch in the unit to operate so energise it via the programmer and room thermostat. I suspect from afar that you may have either a cocked up wiring scheme of one of the wires described is not making contact somewhere along the route. possibly one of the greys or oranges is broken off under a screw in the terminal. Honeywell valves don't usually suffer from microswitch failure but the same cannot be said about a lot of their competitors. I come across a LOT of wiring centres/junction boxes which do not conform to the industry standards but you may find it helpful to google for honeywell or S-plan and have a look at a wiring diagram for how it should be wired
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Refer to the S-plan wiring on the Honeywell link posted by Set Square.
If the valves are moving, the fault is probably with the micro-switch (2 grey wires from terminal 1 live to microswitches, 2 orange wires from switches to terminal 10 on the Honeywell schematic; terminal 10 provides a live feed to the pump and boiler). Your terminal numbers won't necessarily correspond to the arrangement shown.
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In an earlier contribution to this discussion,

From your initial post, I assumed it was a Y-Plan system. I now see that it is an S-Plan system. Have a look at the S-Plan wiring diagram at http://content.honeywell.com/uk/homes/systems.htm
You will see that the stats control the opening and closing of the valves, and that each valve has an additional set of contacts which close when the valve opens and switch the boiler and pump on. It's the wiring to and from these additional contacts which you need to check very carefully. Both valves should have a permanent live feed to their grey wires, and both orange wires must be connected to "switched live" on the boiler (via terminal 10 in the junction box if the standard diagram has been followed).
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Set Square
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