Figure 4-43.Typical forced warm-air control system.
Limit Control.The limit control is a device that
responds to changes in air temperature (in a warm-air
heating system), to changes in water temperature (in a
hot-water heating system), and to changes in steam
pressure (in a steam-heating system). The limit control
has two distinct functions. The first function is to
control the operation of the fire so the temperature and
pressure of the heating plant never exceeds safe
operating limits. This function is distinctly for safety
The second function of the limit control is to limit
the temperature and pressure of the heating system for
better temperature regulation in the building. This
function is particularly useful in controlling coal-fired
heating systems where the coal bed continues to give
off heat when the stoker motor stops. By lowering the
setting of the limit control, however, it is possible to
prevent an excessively hot fire that would continue to
throw off excessive amounts of heat after the
thermostat has been satisfied.
automatic control units, such as the thermostat, limit
control, fan control, and many others, must respond to
temperature changes. Actually, these are the
instruments that use a temperature change to cause the
electrical contacts inside each unit to open and close.
The opening and closing is an indicating signal that is
transmitted to the primary control for specific action,
such as starting or stopping the operation of the heating
Bimetallic Strip.Some automatic control units
are equipped with a switch that contains a straight
Figure 4-44.Interior view of a primary control.