Control wiring

22 Mar 2004
Reaction score
United Kingdom
I am having problems with the control wiring in my central heating system. The boiler is a Baxi solo 40, controlled from a Drayton Tempus 7 programmer, a Horstmann Z322XL 3 port valve, with a cylinder and room stat.

The problem I'm having is the central heating will not come on without the hot water being selected on the timer. I can have hot water only, or both, but not central heating only.

It is wired at present with the HW On going through the cylinder stat to the orange wire on the 3 port. This also goes to the switched live on the boiler.

The CH On goes through the room stat to the white wire on 3 port.

Blue on 3 port goes to supply neutral.

Now I think the problem may be the grey (although I'm not sure), as it is connected to the normally open terminal on the cylinder stat.

I'm not sure how the 3 port valve works but it seems it must have some microswitches in a certain configuration that should make the orange wire live when either hot water or central heating is selected. I think the problem may be the grey wire, as in the present configuration it will only go live if the hot water is selected on the programmer. Should I connect it to the HW Off terminal on the programmer?

Any help will be appreciated :D
Sponsored Links
Who me Kev? I haven't got one remember, just gravity flow on that wood burner. Keep it simple & no maintenance. In fact there was a power cut last night but my wood-burner and gravity-fed rads just kept on going. :)
Sponsored Links
Still snowed in here.
John! Had same problem! power on grey wire was present when CH demanded, but no power on orange wire. Valve moved to correct position.
Replacement actuator head cured problem. Since then have dismantled old actuator and indentified one of the two microswitches not functioning. Desoldered and dismantled microswitch for close examination. cleaned up contact points. Using garage test rig to simulate controller and stats (1 way and 2 way switches, bulb to represent boiler/pump) it functions correctly.
:rolleyes: :rolleyes: :rolleyes:
Shaggy is correct but I thought you might like to know why. The white wire drives the valve to mid position when CH is needed. At this point an internal switch takes the valve motor off white and onto grey, except for a tiny holding current coming through a resistor from white.

A demand for No-HW from the controller coming through grey can now drive the valve all the way over to CH only. The boiler goes off while this is happening because the HW demand that powered it has gone. Once the valve reaches its end stop, another internal switch connects white to orange and feeds the CH demand BACK to HW and so to the boiler.

Incidentally, you can extend the useful life of some of those valves by programming HW and CH to work at different times. If you run both at once, the valve must move as the room stat cycles on and off. With CH only, the valve moves just once and stays there until the next time HW is demanded.
Thanks for the info. I'll run a cable in later from the programmer and hopefully that should fix the problem. I'm sure the 3 port valve should be ok as it's brand new.
Yoo're slipping Felix, you didn't mention the diode!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links