Hi all,

Does anybody happen to know how you can have hot water just to fire the boiler and not the pump (as there is only one valve on the hot water tank side. gravity system), but the central heating to fire the boiler and pump ?
Think is maybe something to do with the valve wiring ??

Can you describe your layout in more detail, or post a photo of the boiler/valve/pump layout?

A valve in the HW gravity side sounds odd.

That still doesn't make much sense really.  What's opening the valve? Us there a cylinder stat?

The extra white wire is simply a changeover contact, along with the orange and grey.

Photos would help.

Sorry cant get any photos untill next week, yes its controlled with a 2 channel programmer a tank stat, also a roomstat for ch.  But as there is only a valve on the hot water pipe, as soon as that valve opens it fires the boiler and the pump meaning all the radiators get red hot and no control as its down to the tank stat. As its a gravity system it wants the valve to open and circulate naturally round the tank, with no pump.

This diagram seems right if the valve had this extra white wire ?!
http://www.google.co.uk/imgres?imgurl=http://www.timstephenson.me.uk/files/2012/...

You are trying to make your system fit a "C Plan" which is quite a rare setup (even though the site you link to says it is a common system).

If you truly do have a C Plan, then you should of had the 6 wire valve installed, and not the 5 wire.  You could either swap the head with a new one (no plumbing involved, but expensive), or add a relay to take place of the changeover contacts.  Perhaps not particularly straight forward for a DIYer.

HOWEVER, if you have the wrong valve fitted, and are missing the white wire, I cannot see how the pump would fire up unless the time control and stat called for it.  So your rads would not get hot just because the HW was on....

There must therefore be another incorrectly wired part, possibly when the valve was swapped.  Has the pump and boiler been wired into the same terminal?