When I was a young lad I can remember running gravity flow and return pipes from a solid fuel boiler vertically into the roof space of a bungalow and then dropping to an indirect cylinder in the airing cupboard, I can remember that we used to vent both F &R but I have forgotten any other criteria for this to work.
This is purely to satisfy my own memory loss when thinking of the 'old days'

Our very own retired master plumber Scruff will be along later. He probably made his out of lead

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As the flow rises it pushes the cold down giving you the circulation.

On the ladder system you will have several flows and one common return.

Quite often the vents would go through the roof and turn down over the tiles

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Very simple system without pumps relying on hot water rising. Even though pumps have taken over that same circulation acts around our radiators and in hot water cylinders. Don't remember vent pipes going on roof tiles though!

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Was quite common to have the old steel vents poking through the roof
Always used to do main flow and return in lead to take care of expansion, god knows how much that would cost now
Do not forget you have to have the heat loss to make above work ,so pipework and cylinder coil took this into account and was a lot bigger

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Common practice on larger systems, to take the vent pipe through the roof.

The reason hot water rises is, it becomes less dense the hotter it gets.

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That explains why I can do the newspaper crossword when on holiday and at home not a chance!!

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I was joking about the primary's being in lead

Respect to you old plumbums

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Doesn't work with Blondes!

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stratification mon!!!(say in a Caribbean accent)

Thankyou all for your welcome replies,.... pipethreader