If an actual balanced flue, generally it is 600mm, if you mean a modern fanned flue boiler it tends to be 300mm, but as said, check the specific MI's if available.
Are you talking about a boiler installed in a basement, where the flue will have to discharge above ground level? If not, I would like to know who makes a boiler which is only 300mm high? Most wall mounted boilers will have their flues at least 1500mm above the ground.
Why would most wall boiler have to be fitted at 1500mm.
Quite common to have a raised ground level outside especially where steep slopes involved and houses get stepped into it
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