I'm installing the electrics on a S Plan Plus system
Is there a standard wy of wiring this up? Is there a way of wiring it up without using relays?
The problem is that if you follow the diagram on the link, but install three pumps, then all the pumps would be started together when only one zone is required.
Surely, you just wire each zone's pump in parallel with the zone valve's motor feed - and use the volt-free contacts to control just the boiler.
Do you mean that you have zone valves PLUS 3 pumps, or just 3 pumps and no zone valves? The latter is like the Grundfos pump plan. Grundfos' controller manages to avoid having to have relays by using some devices I failed to indentify. If you could get sort of AC diodes (i.e. passed AC in one direction only) they'd do the trick :-)
Otherwise as far as I can see you need relays.
Only thing I can see with that is that the pump will start running before the valve opens - will that cause any problems (normally the thermostat opens the valve, and then the switch closes starting the pump).
Also you can't do any sort of pump overrun this way.
But to get round this would be complicated and probably involve relays.
It probably doesn't hurt to run the pump stalled for the few seconds it takes to open the zone valve.
Pump over-run is a bit of a problem. If the boiler needs it, you will need a by-pass circuit anyway. You can probably wire it so that (say) the DHW pump is powered either by the cyl stat or by the pump over-run switch - but this
*may* require a relay. I need time to think about it!
I know many go blank just thinking of using them, but try and think of them as a group of individual switches being operated by separate people on the command of another.
Always assuming the valve opens. Think about failure modes, failed valve motors are fairly common.
I designed and installed a 4 independant zone, 2 pump system here with pump over run. Half a dozen double pole change over relays in the box. The overrun, though works, is not ideal.
If I was to do this again I'd start with a PLC and do all the logic in firmware. One day I may well rip out the relays and put in a PLC.
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