The controls used in a warm-air heating system will depend upon a number of factors, including: (1) the method of air circulation used (e. g., forced or gravity), (2) the size of the structure, and
(3) the method used for supplying the fuel (automatic or manual) to the fire. In other words, for all intents and purposes, a control system is custom designed to fulfill the requirements of a specific heating system.
All gravity and forced-warm-air furnaces in central heating systems are controlled by a thermostat located in one of the rooms. If the heating system is zoned, two or more room thermostats are used. In forced-warm-air systems equipped with automatic burners or stro — kers, the thermostat will also control the operation of these units.
Each furnace should be equipped with a high-limit control to shut off the air or gas burned when plenum air temperatures exceed the furnace manufacturer’s design limits. The high-limit control is automatic and will switch on the burner again as soon as the air temperature in the plenum has returned to normal (i. e., reached a level below the manufacturer’s design limits). The high-limit control is frequently designed to operate in conjunction with the fan (blower) switch in forced warm-air furnaces.
Other controls used in warm-air heating systems are described in Volume 2—Chapter 4, “Thermostats and Humidistats,” Chapter
5, “Gas and Oil Controls,” and Chapter 6, “Other Automatic Controls.”