This won't work if you intend to use an internal programmer because the Cylinder DHW needs to heat independent of the heating.
I'm not a plumber so you will have to ensure that plumbing things are OK such as sufficient bypass for pump overrun etc.
The MVs need to be suitable for an S plan.
1) Rewire the existing system to a Honeywell S plan if it is not already. Include thermostats and a Htg & DHW programmer. The DHW circuit will operate the cylinder. The hot water from the combi will work as normal.
2) Instead of connecting to a boiler, connect the coil of a 230v relay with a "Normally Open" (N.O.) contact rating of at least 5 amps.
3)Wire the boiler as per the manufacturer's instructions from the same power source as the S plan. 4)Connect (one of) the normally open (N.O.) contact pairs of the relay to the thermostat connections.
You may need to mount the relay in an enclosure near the wiring centre.
You can get a 230/240v relay from internet, somewhere like CPC, RS components, or Farnell components. It needs to last many years of constant use, so keep away from the cheapest of the cheap. It can have any number of contacts as long as there is at least one N.O. pair. The type that use a plug in base with screw terminals are the easiest to install, but need a larger enclosure. Don't forget to order the base with the relay.
Note that when both DHW and CH are satisfied the MVs will both close and the boiler will shut down. A suitable bypass for pump overrun will be needed.
Steve
November 2009
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