CH & HW wiring plan questions

14 Dec 2010
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United Kingdom
Hi all,
Sorry for the long post
I have a few questions/queries regarding how my HW & CH are wired and how it works.
Firstly I have the following components:-
Potterton Prima 50B boiler (pump overrun connected)
SMC Comet 2-130 pump
Switchmaster 400 timer/programmer (HW or HW & CH)
Switchmaster midi mid position valve
Honeywell L641A cylinder stat
Wickes room stat
10 way wiring centre

Wiring centre connected as follows:-
5-Boiler switched red wire from mid pos valve
6-Room stat switch yellow wire to mid pos valve
7-Cyl stat satisfied to white of mid pos valve
8-Room stat feed
9-Boiler pump control
10-Cyl stat call to orange of mid pos valve

I have recently changed the room stat to a Heatmiser PRT/HW programmable stat, I removed the room stat feed from terminal 8 and fed direct from terminal 3 to give me a permanent supply to Heatmiser, I then linked Live in to terminal 1 on stat and terminal 2 is call for heat from stat. (only CH controls used at present)The problem is I need to leave the HW permanatly on on old switchmaster 400 for CH to work through stat. This is first query.
Second query is the wiring doesn't match any S, Y, or W plan I have seen before, paperwork says Ken Grey wiring diagram, is this recognised or a one off independant?
Also my mid position valve has 6 wires where as most have only 5, why is this?
Would I be better to rewire the whole system to a recognised plan eg Y to make it easier for identifiaction and fault finding???
Would I be better to re cable

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You cant wire your valve as a "mormal" Y, S or W plan, all of which are honeywell systems that have been adopted by the industry, in general.
The wiring looks ok for independent HW and CH, and if I'm correct there must be a fault somewhere - you'd need to be checking voltages at connections in junction box as and when various permutations of HW and CH are selected. I wouldn't be surprised if the valve is faulty as they are pretty old now, and obsolete.
Have you not connected the Heatmiser thermostat feed to the ‘HW On’ from the programmer (terminal 3), hence you have to have HW on for the CH to work? It sounds like the limitation you may have is that the programmer can only be HW or HW + CH, as per a gravity fed system. You need a separate permanent live for the Heatmiser. You should be able to arrange that from the programmer using the wire that feeds terminal 8.

The valve looks like it is wired correctly, as per information on this web page:
It seems the reason for the different wiring to the normal Y plan is that the Switchmaster is not a spring return valve, so it has an additional connection for HW On that connects to the Tank stat.
Thanks twgas and mikely for your replies.

I have checked out the whole system today and everything appears to be switching and calling when it should. When I took the 3 port valve apart for testing one of the plastic lugs which hold the spring steel retaining clip over the motor is missing so doesn't hold properly (the relay smells old and a little electrically burnt so I'm guessing is past its best), when powered back up it did about 3 rotations before deciding which position to stop at. When turning the cyl stat down to confirm switching (it was very stiff) and was not a positive click when turned up or down, so guessing is also past its best.

twgas- I disconnected the stat feed (from term 8 in wiring centre)from the programmer and fed the Heatmiser with perm live from wiring centre, linked perm live to term 1 (Heatmiser) then output from term 2 to term 6 in wiring centre.
It looks to me like the mid position valve and tank stat may be on its last legs, and to top it all the top flange of the pump has started to weep (I can't tighten the nut any more). I am guessing I need to spend a little money on the system and replace a few parts (it has been in since 1989 so isn't doing too bad).
From your guys experience are Danfoss components any good? Plumbfix are doing a contractor pack of Progammer, mid position valve, room stat and cyl stat for about £59.[/u][/b]
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Not had much experience of Danfoss, it does seem cheap though for that package. Steer well clear of Sunvic valves, known for playing up after a couple of years use. If its your own place i'd spend the extra and get Honeywell.

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