Gravity primaries

[Mike2007]

Its common for multi fuel stove makers to state 28mm copper on the primary flow and return for their appliance. I have seen 22mm primaries in many houses that have been plumbed over the decades!
Yes I know you just follow manufacturers instructions, but wonder why others have "got away" with 22mm copper pipework on gravity circulation?

 

[Corgigrouch]

Simple.... as you have found, 22mm will work but 28mm works better... Larger diameter pipe means less drag on the water flow. Better flow allows you cylinder to heat quicker and 28mm is easier to bleed the air out.... Think about it, you cannot force the air out with a pump on gravity...