similar to the thermostatic control for the refrigeration
plant. The type 1 system requires single-pole
thermostats, but type 2 and type 3 systems can use
two-position dual controls (2PD). The cooling switch
would then be connected in the normal manner with the
heating switch inoperative.
The type 2 system is most commonly used to make
living and working spaces more habitable and for
various types of weapons systems that require cooling.
These systems often use a common cooling coil serving
several different spaces. Since load changes seldom
occur simultaneously, electric or steam reheaters are
installed in the cooling air ducts. The cooling
thermostats of the various spaces are connected in
parallel so that any one of the thermostats may open the
cooling coil valve.
Suppose three spaces are being cooled by a common
coil. Space B in figure 5-6 has a load change and spaces
A and C do not. With the coil operating to take care of
space B, these spaces would become too cold for
comfort. To prevent this condition, the thermostat would
close the heating switch and energize the reheaters for
spaces A and C.
The type 3 system is identical to the type 2 system,
except that a humidistat is wired in parallel with the
thermostatic heating switch. This type of system is used
mostly in weapons and electronic spaces. The
humidistat is set for the relative humidity desired. In
most installations, it is only necessary to prevent the
humidity from exceeding 55 percent. Where the
humidistat is installed, an increase in temperature
beyond the thermostat setting will close the thermostat
cooling switch. An increase in relative humidity beyond
the humidistat setting will close the heating switch and
energize the reheaters.
Proper attention to the planned maintenance system
often exposes developing troubles in time to take
corrective action. Since most breakdowns occur at the
most inopportune times, periodic checks and
maintenance will help to avoid malfunctions.
The 2PD control system can easily be checked out
in a reasonably short time. The checkout should be made
at least every 3 months or more often if necessary.
Inspections and checks should be made at the beginning
of, and midway through, the cooling season and heating
You should inspect the sensing elements and
remove any dust accumulations. Remove dust and dirt
from thermostatic sensing elements with a soft brush.
Use air to gently blow off any dust on the sensing
elements in humidistats. The air will not damage the
element but will remove any problem-causing dust.
Magnetic valves should be checked for operation.
Be sure that they open and close completely.
Set points of the thermostats and the humidistats
should be checked with a calibrated thermometer and a
reliable humidity indicator.
When servicing the two-position control system,
look for three possible sources of trouble:
l The sensing element and its associated
Figure 5-6.A typical air-conditioning system.