I remember houses with thermo siphon domestic hot water and pumped central heating, I remember the programmer had a lever to swap from 16 option to 10 option when using thermo siphon, and remember you could not have central heating without hot water, but you could have hot water without central heating basic idea was with hot water only the pump does not run.
However I can't work out how it was wired, with the latter C plan the motorised valve acted as a relay so no back feed, but how did it work with early system?
