combie wiring

Hi, does anybody know how to wire a combie, using the combie heating side to do the heating and a tank of hot water, and the instant (mains water) side to do just the kitchen tap.
It is a Potterton Performa 28 and using a Siemens heating pack (3 port, timing clock RWB9, room stat & cylinder stat)
Have got the heating side up and running and doing the water tank, but not done the mains tap.
Have got the L,N,E coming from the wiring block going to the boiler but they are only live when the timer tells it to be. I assume there has to be constant live for the hot tap to be able to fire up at any time.
Any help appreciated
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In an earlier contribution to this discussion,

The heating side of your boiler is, in effect, powering a Y-plan system - so the wiring for that side will need to be more or less like Y-plan in http://content.honeywell.com/uk/homes/systems.htm [The pump may be different and is probably integral with - and controlled by - the boiler, so you can probably ignore this bit].
As I understand it, combis mormally have a permanent live and switched live connection. You will need to connect the permanent live to your mains supply - i.e. from the fused spur or whatever which powers the whole heating system. The live boiler connection in the Y-plan diagram needs to be connected to the boiler's switched live terminal.
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Set Square
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Set Square wrote:

However, many newer combis require a volt free contact pair which you don't get with a Y-Plan with 3-way valve but do with S-plan with 2-port zone valves. If this applies to your boiler then a mains coil relay is required.
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On Wed, 10 Dec 2003 18:34:19 +0000, BillR wrote:

All boilers (except for one) I know of either require a connection to the mains to demand heat or they require that you connect to terminals together (sometimes a low voltage pair - hence the term 'volt free') .
I strongly recommend that you opt for S-plan (see main FAQ). This is much less troublesome, easier to work around when there is a fault, simpler to wire and possible to extend later on.
I have installed several combis as hybrid systems they work well (why wouldn't they) used in this manner.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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In an earlier contribution to this discussion,

The problem is - that according to the initial post - he seems to have already obtained and installed the bits relating to a Y-plan system. I assume that if the boiler requires volt-free contacts, he'll have to use the boiler demand from the 3-port valve to drive a relay.
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Set Square wrote:

I'd wholeheartedly second that. I had a Y plan for 15 years and eventually got so fed up with it I changed it to S plan.

Thats exactly what Ariston tech support told me when I enquired about putting their combi on an existing Y plan system. They even had a go at creating a circuit diagram which they sent to me.
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