Sizing Heating Equipment

A heat loss calculation is an estimate to help size heating equipment. The result is expressed as Energy Lost Per Time. Here in the USA, the most commonly used units are BTU Per Hour.

There are several rule-of-thumb type guesses that can be used as a (very) rough estimate. For example:

This method (1) is based the fact that most buildings have a ceiling height of 8 or 9 feet, have an average ammount of window area, and have an average insulation for the selected category. The above result is for illustration only and should be taken with a grain of salt.

Clearly, we need more information about the building than it's sqare footage before we can make a better estimate. Also, this sort of estimate does not provide any information about placement or sizing of equipment in individual areas of the building. For more details, APS has summarized an Introcution to Heat Loss Analysis.

If the heating plant also supplies the energy for domestic hot water, this capacity should also be added. A standard indirect hot water tank is typically arround 35,000 to 40,000. Depending on the situation, other methods such as system prioritization can be used to ensure sufficient domestic hot water without having to oversize the system.

Standard Methods

Some govornment agencies now require some sort of heat loss to be done in writing when new heating systems are installed. Most often, the required method for performing the calculation does not involve the complete set of formulas specified in the ASHRAE methods. Typically, these methods are a worksheet style calculation, such as the "Manual J" which captures the essential informaiton and produces reasonable approximations. Other methods employ the use of a computer and specialized software

Graphic at Left: Colorized Infrared Photo...you can "see" the heat loss.