First of all what type of boiler is it ? is ventilation needed ?
Don't just go boxing in boilers without doing the calcs to see what ventilation is required for complete combustion or they sometimes just needs vents for cooling purposes (fire risk if no ventilation), if there's a lack of oxygen - particularly with an open flued appliance then:
Incomplete combustion occurs,
Sooting up of appliance (in some cases),
Fumes entering room,
Occupiers complain of headache's, sickness etc etc etc....
Sometimes resulting in DEATH, unless you're lucky to put 2 and 2 together or go to the doctor's complaining of the above symptoms - to be told you have CO poisoning.
if you still have the installation and user manual for the boiler then you may well find that it gives details of limitations on boxing in - certainly all modern boilers come with such information.
it will tell you whether or not it is possible/inadvisable to box in and if so, how much room is required either side and at front of the boiler and if any cabinetry would require a vent.
bear in mind that the manufacturer's recommendations are only the MINIMUM requirement, you need to take into account access for repairs and servicing, ensuring there is enough room for work to be carried out.
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