If a user, replace like for like. A telephone call to the boiler manufacturer will yield a quotation for a replacement, for installation and the witness testing for burst indemnity insurance purposes prior to putting the boiler back into use.
If a boiler manufacturer, then sizing algorithms should be on-board inside the company's procedure and design protocols for the original poster and others in the company to use.
In respect of the latter, a generalised enquiry via CR4 is singularly inappropriate.
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