Central heating pump and 3 way motorised valve problem

14 Apr 2011
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United Kingdom
i'm hoping there will be someone here who can help me with 2 problems i have.

The first problem is that my central heating and domestic hot water system are approx 30 years old. I had until recently an SMC controller pump which acted in 2 ways - it allowed water to run straight thorugh (when not powered) to the central heating pump which was below it and secondly when the SMC controller pump switched on it still allowed water straight to the central heating pump but also diverted water off to the side which went into my cylinder etc. I have recently removed the SMC controller pump because i have had a lot of trouble with it and getting replacement pumps was difficult and expensive. So what i have now is the following - Water from the boiler arrives in airing cupboard and i have fitted a conventional central heating pump, below the pump is a three way motorised valve, the valve diverts water to either the cylinder or the radiators or both. The sytem works fine when i operate the 3 way valve manually because its not wired in yet (see problem 2) however i have an issue with the electrics.

Because my system used to have 2 pumps i have 2 live feeds in the airing cupboard, which live feed do i connect the new pump to so that it runs when i select either hot water or central heating? At the moment i have it connected to the hot water feed so if i want central heating i have to have hot water too. If i want central heating the pump wont energise so i have to have central heating and hot water together to get the pump to run.

Second problem

I have a 5 way wiring block in the airing cupboard and it is wired in the following way.

Terminal 1: Central heating pump (new pump) neutral
Terminal 2: Nothing connected but used to be a live for old Central heating pump when 2 pumps were fitted
Terminal 3: Cylinder thermostat neutral
Terminal 4: Cylinder thermostat live
Terminal 5: Live feed for new cental heating pump but used to be live feed for SMC controller pump (DHW)

How would i wire in the 3 way valve? The valve is a drayton mid position valve model MAI/679-3.

The valve has 5 wires coloured white, grey, orange, green/yellow, blue.

From my research i think i figured out the orange wire shares the same live feed as the pump, and the blue shares the neutral from the pump too. Green/yellow is earth but where do i connect the wires and on what terminals on the wiring block?
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The orange wire provides a call for heat which would normally go to the pump and boiler!

Thats when the system is properly wired as a "Y" plan.

Later boilers control the pump directly to provide an over run after firing.

you old system was called an smc controller system with a smc controller clock which uses a relay to power boiler. As you have now converted to a y-plan you will need to alter wiring centre & change programmer
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Before you go too far, I must draw your attention to your cylinder thermostat.
You quote 2 terminals one live and one neutral.
Firstly if there are only two terminals available, then it would need replacing with one with three terminals. This would act like a two way switch, with one 'live' input to the common terminal from HW ON switch at programmer, but two outputs (not at the same time)
One output from the ' not satisfied' terminal would go to boiler/pump and when the cylinder is up to temperature, it breaks with this terminal and makes with the 3rd terminal. A wire from this terminal will lead to the grey wire of the valve.
As the cylinder stat is a switch and has no 'load' it does not have a 'neutral'.
A switch with two terminals would have a 'live' and a 'switched live'
So you need to check the cylinder stat. you may have three terminals but only using two of them.

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