Download the ACCA spreadsheet and watch the linked video tutorial on how to use the spreadsheet.
Complete a comprehensive heat loss calculation to determine the needed British Thermal Units to supply the necessary heat for the location of the house in question. The spreadsheet is self-explanatory for the measurements and determinations required to complete it accurately.
Determine the best location for the boiler installation by thoroughly understanding the International Mechanical Code's sections on Combustion Air requirements and Boilers. This will lead to specific choices for the venting requirements of the boiler.
Determine the best possible fuel source for the boiler, the final step in the selection process.