I have a source of potable well water that is pumped to a storage tank ~ 5m up in a barn next to my house. There is no loft space for a storage tank in the house. I would like to use the well water for the HW supply to the downstairs bathroom.

Given the low pressure could a vented type HW cylinder (+ expansion bottle) be used as an unvented cylinder? This would save having to run a vent line all the way back into the barn.

I would obviously be careful to make sure that an air inlet is provided on the outlet if the HW cylinder needed to be drained.

If a vent is needed could I use 15mm rather than 22mm?

If I've understood correctly he's asking if this is ok and if so what type of cylinder would be needed (eg one approved for vented or unvented?). No pumps involved.