Study the code, then get started
Using the International Mechanical Code, or a relevant Code source, determine the best and most feasible location for the boiler. This will decide, for instance, whether a direct vent or a natural vent boiler can be installed. Ultimately, this will decide the efficiency of the boiler to be installed.
The International Mechanical Code is included in the References section of this article.
Determine the best possible fuel source and what will be needed to pipe that fuel source to the boiler.
After watching the tutorial video in the Reference Section, download the Speed Sheet.
Gather all the relevant information and input it into the Speed Sheet. The resulting calculation will deliver the BTU heating requirement.
With the collected information, select a manufacturer and find a corresponding boiler that meets the requirements.