Been looking at the heating/hot water wiring, no particular reason
other than the boiler seems to take up to 5 mins to fire when CH/HW is
activated. System working OK other than that.

Looks like a fully-pumped S-Plan with Zone Valves on the 22m returns
rather than the 28mm flow, boiler is a Potterton Kingfisher II 30KW,
also got tank, room and frost stats. Pump is wired parallel to boiler
so not separately controlled or anything. Rather old installation,
Zone Valves look ancient and I would guess the boiler must be pushing
20 years old, all open vented.

Wiring is a real mess; doesn't use a standard wiring centre but
instead three double pattresses with covers, wires go everywhere.
Rather than the standard S-plan wiring for some reason there seems to
be some sort of 240v relay (?), also for some reason the white wires
for both zone valves are used, as are the CH/HW off terminals in the
programmer, and the CH off terminal in the CH stat.

Can't really make head nor tail of the wiring especially as there is a
relay to confuse things, any bright sparks (or plumbers ;o)) seen
anything like this? Why would you use a relay (assuming that's what it
is) in a CH/HW circuit?

TIA,

Andy