Stacks weren't always built into the engine house; some were separate, sited away from the boiler house and connected to it by a long flue - this was usually done for reasons of getting sufficient updraught. See the recent postings on this site about Cornish design engine houses - they come in all shapes and sizes!!!!
I was in the CSL today and got sidetracked, as one does, and started reading
Cornish Stacks and Engine Houses, an article by J.H. Trounson in the 1982 Trevithick Society Journal.
He opens:
One of the first questions frequently asked is 'Why were the stacks so frequently built as a part of the engine house itself?'.The reason is that this construction saved a considerable amount of masonry and therefore cost, and the two being combined made for a stronger structure in as much as one supported and 'stiffened' the other.
Although the tall beam engine houses frequently had their stack built at one corner, in other cases the stack was free standing and sometime"s quite a distance from the house. The reason for this is not clear, drawings still exist of some of these buildings designed by the same consulting engineer who in one case might build house and stack combined, and the next time buiIt them separately. One can only concllude that it depended on the whim of the engineer.
I can scan and upload the article if anyone is interested. It's mainly to do do with construction and unusual stacks.
The past is a foreign country: they do things differently there.