Classifying Warm-Air Heating Systems

A warm-air heating system is one in which the air is heated in a fur­nace and circulated through the rest of the structure either by gravity or motor-driven centrifugal fans. If the former is the case, then the system is commonly referred to as a gravity warm-air heating system. Any system in which air circulation depends primarily on mechanical means for its motive force is called a forced-warm-air heating system. The stress on the word “primarily” is intentional because some grav­ity warm-air systems use fans to supplement gravity flow, and this may prove confusing at first. In any event, one of the oldest forms of classifying warm-air heating systems has been on the basis of which method of air circulation is used: gravity or forced air.

Forced-warm-air heating systems are often classified according to the duct arrangement used. The two basic types of duct arrange­ments used are:

1. Perimeter duct systems

2. Extended-plenum duct systems

A perimeter duct system is one in which the supply outlets are located around the perimeter (i. e., the outer edge) of the structure, close to the floor of the outside wall or on the floor itself. The return grilles are generally placed near the ceiling on the inside wall. The two basic perimeter duct systems are:

1. The perimeter-loop duct system (Figure 6-1)

2. The radial-type perimeter duct system (Figure 6-2)

An extended-plenum system (Figure 6-3) consists of a large rectan­gular duct that extends straight out from the furnace plenum in a

Classifying Warm-Air Heating Systems

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Classifying Warm-Air Heating Systems

Classifying Warm-Air Heating Systems

Straight line down the center of the basement, attic, or ceiling. Round supply ducts connect the plenum to the heat-emitting units.

These and other modifications of duct arrangements are described in considerable detail in Chapter 7 of Volume 2, “Ducts and Duct Systems.”

Classifying Warm-Air Heating Systems

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