Hate to muddy the waters (and I might be wrong) but my view is that the cost of the replacement boiler is capital expenditure [unless perhaps if the cost is immaterial] and that the tax deduction could only be claimed over a number of years rather than all at once
As an analogy, if the asset in question was a replacement car, shouldn't it be treated as a capital addition with allowances spread over 8 years rather than a 1 year write off?
I'd suggest that the original poster might need to clarify this with his accountant for added assurance.