Planning a Forced-Warm-Air Heating System

ASHRAE publications (such as recent editions of the ASHRAE Guide) summarize design procedures for planning several types of forced-warm-air heating systems. The following six elements are common to each planning procedure:

1. Calculate the heat loss for each room or space, and from this data determine the total heat loss for the structure.

2. Determine the required furnace-bonnet capacity from the total heat loss of the structure.

3. Determine the location of the diffusers (warm-air outlets) on the building plan.

4. Calculate the required Btu/h delivery of each diffuser.

5. Locate the position of the feeder ducts on the building plan.

6. Calculate the size of each feeder duct on the basis of the total Btu/h delivery it must supply.

Other elements in the design of a forced-warm-air heating sys­tem will be specifically oriented to the characteristics of each system. For example, it is recommended that the maximum length of an extended plenum (nonexistent in a forced-warm-air perimeter system) not exceed 35 ft. The locations and types of warm-air out­lets and return-air inlets selected for the system will also depend upon the type of forced-warm-air heating system used in the structure.

Useful and authoritative sources of information for installing warm-air heating systems are provided by the Air-Conditioning Contractors of America (ACCA) and the National Fire Protection Association (NFPA).

Proprietary systems should always be installed according to the manufacturer’s instructions. Any variation from these instructions increases the probability of error and future problems in the system. It may also void the equipment warranty.


Local codes and ordinances always take precedence over installation instructions provided by manufacturers. When there is a conflict, consult the local regulatory body for a resolution.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers