Designing a commercial boiler replacement where you were not the original designer or installer might have some hidden dangers that could haunt you for years. The project is going to be a series of assumptions, educated guesses, design skills, and some good old-fashioned luck. The boiler is only part of the heating system, albeit an integral part. Always assume that the existing boiler is installed incorrectly.
When designing a replacement low pressure steam boiler, it should be sized according to the existing connected load. The connected load includes all piping, radiators, and heat emitters. Unlike the steam boiler replacement, a replacement hydronic boiler system will be sized according to the building heat loss.