In an earlier contribution to this discussion,
Andrew Sinclair wrote:

In message , "Roger Mills (aka Set
Square)" writes

Assuming your CH is pumped, an anti-gravity valve would certainly
stop the upstairs rads from getting hot by convection when the pump
was off.
Ah ha! This might solve my problems too, since a mod to the pipe
routing in our house the upstairs radiators suck all the heat out of
the oil fired combi with the pump off with a detrimental effect on
the time to deliver hot water - is there an 'official' name for an
anti-gravity valve or is it simply a non-return valve. What would I
call it if I were to search, say, Screwfix.

I doubt whether Screwfix sell them - try a proper PM or heating engineer.
It's not the same as a non-return valve, although it also has this effect.

A non-return valve allows flow in one direction only. An anti gravity valve
has a weighted or spring-loaded flap which permits *no* flow in either
direction when closed. It requires a certain amount of pressure to open it.
This pressure is easily supplied by the pump but *not* by pure convection.

I don't understand your reference to the combi. If it *is* a combi, no heat
should go to any radiators when it's supplying hot water to the taps.
--
Cheers,
Roger
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