"Ian Middleton" wrote in message ...

I had fiited a similar arrangement in my last house. The relay is to switch
on the boiler when the CH is called for by theromstat/timer.

Original setup was gravity HW + pumped CH and thermostatic rad valves and
had a timer where you moved notches to select times and a switch to select
off, HW or HW&CH. I found the control very poor, house too hot or if trying
to do HW and CH from cold house would not heat up till HW was getting hot.

snip

It had crossed my mind that this might be some sort of hangover from a
previous gravity-fed system. Surely though when converted to fully
pumped S-Plan you would rip all the extra wiring out?