I need to fix up proper controls on a CH system that has separate pumps driving CH and DHW. At the moment there is no stat on the HW cylinder so $DEITY knows how it's wired. (The wiring seems to be crammed into a back box behind the programmer so I haven't investigated yet, but my money is not on it having proper interlock between CH and DHW systems and the boiler.) The pipework layout makes conversion to a Y plan or a packaged Grundfos Pump Plan (tm) a very unattractive option so I need to arrange proper temp control on the DHW and interlock with the boiler.
I know I can do it with 2 relays with their contacts wire-ORed to drive the boiler Switched-Live:
Programmer Room stat ---- ---- L ---o o-----------o o--------+--------+ | | CH pump (>) | RLA | | | N ------------------------------+--------+
Programmer Cyl stat ---- ---- L ---o o-----------o o--------+--------+ | | DHW pump (>) | RLB | | | N ------------------------------+--------+
L ----------------+--------------+ | | o | o | A | B | o | o | | | +-------+------+ | --------- | SL | | boiler | | N | ---------- | N ------------------------+
- but ISTR seeing somewhere a single relay solution. I'm damned if I can see how that would work but willing to be proved wrong :-)
Incidentally has anyone grokked how the Grundfos pump plan wiring centre does it? IIRC there's no relay in the box so presumably it's some sort of triac cct with wired-OR to the gate.