A house I used to live in had an early 1970s cast iron lump [1] for a boiler
and a mechanical time clock. One 'feature' of this system was that you
always heated the hot water when you turned the heating on. I understand
that was common in older systems.

I'm curious as to how this was plumbed. I understand about S and Y plan
systems, but I can't see how that would cause this effect. Was this set up
with the boiler output always going through the coil first? Or with a
single heating on/off valve? Or was it an S plan where the controller just
told the hot water valve to open when the heating was on?

Anyone know?

Thanks
Theo

[1] Potterton Kingfisher I think it was, although I gather there have been a
lot more recent Kingfishers since then