"tarquinlinbin" wrote in message
...

Non of yer new fangled condensing
malarky here. I even spotted a relay
valve and weepline stat on one of them!.

Yep, I used to work on them. A tube of neatsfoot oil for the leather
diaphragm on the relay valve cured any gas leaks and made it run smoother.
They were purely non-electric controls with weep pipes going to the boiler
'stat, room 'stat and mechanical clock too. Some had modulating burners, in
that the stats were gradual acting and would slowly close or remain stable.
In doing so the relay valve would move up and down reducing the gas to the
burner, modulating the burner. It took many years to get modulation with
electric controls. There were some kits available to replace the leather
diaphragms with neoprene in the valves. They were very reliable, more
reliable than the electric controls. They were simple...once you understood
them. The average plumber would run a mile from them.

http://www.oldestboiler.co.uk/

view the pdf book.