In article
, Lee
Nowell writes

Connect the micro switches from all motorised valves together in
parallel and to the demand on the boilers

Finally, we have 2 boilers. Can they be wired together in parallel or
do we need any relay etc.?

Thanks again for all your help.

Lee.

You can wire demands in parallel in a simple system but where you have
separate pipe circuits with eg separate pumps then you have to find a
way of combining the demands (to feed the boiler 'call for heat') while
keeping the pumps separate (or you will turn on the pump for a circuit
that is meant to be off). This can be done with relays.

I've uploaded a couple of sketches which give the basics of how this is
done: http://i26.tinypic.com/2yzejko.jpg (first sketch).

Secondly, it wouldn't be that clever to run 2 boilers to service a
demand from a single small zone. To overcome this problem you can use
relays to create a demand to the second boiler only when a number of
conditions become active ie when condition (or demand) 1 AND condition
(or demand) 2 are met. I've shown a basic example of this in the second
sketch. Another option would be to only fire the second boiler when
there is a certain level of demand AND the temperature outside is lower
than a certain level, ie. when full power is really needed.

A clever 2 boiler design would also have provision for swapping the
control of the boilers round so that they aged at the same rate and an
option to remove a faulty boiler from the circuit.

Some manufacturers have helpful notes on the design of 2 boiler sytems,
here's a link to info from Keston which is for their pre-fabricated rigs
but the information should read across to other designs too:
http://www.keston.co.uk/multi-rig.html
http://www.keston.co.uk/downloads/manuals/rigcntrl.pdf

Or google for "dual boiler" +heating +system +design ( -coffee -espresso
)
--
fred
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