Theo wrote:
John Rumm wrote:
It was quite common to have a big cast iron lump HX with high water
content (couple of gallons). That would have 4 bosses on it - two
outputs and two returns. One set would be used for the feed and return
on the pumped circuit through the rads, and the other two would be used
for a gravity (i.e. convected circuit) to heat an indirect cylinder.

Ah, so 'gravity' meant there was no pump on the HW side, only for the
heating?

The convention circuit required that the hot water cylinder would be
placed some distance above the boiler and not too far displaced from it.
It would also typically be piped in 28mm pipe to better allow convection
flow. Hot water from the gravity output would rise up through the pipe,
and through the coil in the cylinder, the heavier cooler water would
fall and return to the boiler. Basically it sets up a thermosyphon.

That's neat. Horribly inefficient, but neat

That sounds about right - cylinder was above the boiler and about 2.5m
displaced, so that would probably work.

Some refinements of the system introduced and anti gravity valve to stop
heat being lost out of the cylinder through the boiler when it was off.
Some had a kind of thermostatic valve in the gravity loop to limit the
maximum temperature of the cylinder DHW. Some would add a motorised
valve to make it a fully controlled zone. (aka C plan)

I never investigated but I imagine there was just on/off control for the
boiler (gas valve only, since there was an always-on pilot that had a piezo
spark-button to relight it if it went out), and an additional time clock
output that was in series with a thermostat to run the pump for the heating.

How was the boiler turned off in this system, when the thermostat said the
house was hot enough and so was the water? If the heating called for heat
and the hot water was up to temp, surely that would overheat the water?

Theo


Water temperature would be limited by the boiler stat.

It was never a terribly efficient system but it was simple and reliable
with no new-fangled motorised valves to go wrong. ;-)

Tim

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Please don't feed the trolls