Hi,
Have a look at the guidance on what is deemed a repair
BIM46915 - Specific deductions: repairs and renewals: what is a repair: improvements
Replacing windows is considered a revenue expense, BIM46925 advises that the use of modern materials does not necessarily mean work is an improvement.
One example of this is double glazing. At one time, replacing single glazed windows with double glazing was an improvement. Over time, double glazing became the industry norm.
This meant that replacing single glazing with double glazing ceased to be an improvement and capital expenditure, and became allowable expenditure for tax purposes as it was simply replacing like with currently available like.
Please see
BIM46925 - Specific deductions: repairs and renewals: what is a repair: changing technology
This would equally apply to central heating. Capital expenditure produces an ‘enduring asset’ for the property; revenue expenditure does not.
An example of capital expenditure would be adding a conservatory to the property, or installing central heating where there was none before.
Replacing an old boiler with new modern one is a revenue expense, in the same way as windows/doors etc.
Thank you.