How to Size a Hot Water Boiler


Every aspect of installing a custom boiler must be sized appropriately for the house it will service. This includes the size of the boiler, which must comply with local and state codes, use readily available fuel sources, and ultimately compensate for the heat loss for the house in question.

The calculations required are complicated and cumbersome, yet many HVAC companies have attempted to simplify this task by creating a "cheat sheet." Air Conditioning Contractors of America, on which Energy Star bases its Quality Installation program, has developed a Manual J 8AE Speed Sheet Excel spreadsheet which many contractors use daily to calculate the necessary BTU (British Thermal Unit) requirement for a given installation.

Things You'll Need

  • Pencil
  • Paper

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.

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