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. MAINTENANCE 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 season. 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 mechanism Figure 5-6.—A typical air-conditioning system. 5-12