TM 5-4120-361-14 seconds after the fan motors. Although the compressor and fan motors operate continuously in this mode, thermostatic action of the temperature selector switch causes refrigerant to bypass the evaporator coil and stop cooling when the desired temperature setting of the temperature selector switch is satis- fied. Ventilation air will be drawn in- to the unit if the vent damper door is open. (4) Low Heat. In the low heat mode,   the evaporator fan motor operates continuously and three of the six heater elements are activated. The three heat- er elements cycle on and off under ther- mostatic control of the temperature se- lector  switch. This mode will produce half of the unit’s heating capability. Ventilation air will be drawn into the unit if the vent damper door is open. (5) High Heat. In the high heat mode, the evaporator fan motor operates continuously and all six of the heater elements are activated, but only three of the heater elements will cycle on and off under thermostatic control of the temperature selector switch. This  mode will produce maximum heating capability of the unit. Ventilation air will be drawn into the unit if the vent damper door is open. d. Evaporator Fan Speed Switch. The evaporator fan speed switch has two po- sitions,    “low”  and  “high.” The posi- tions are manually selected to control the evaporator fan speed in all operat- ing  modes. Normally the “low” speed should be selected as it is quieter and requires less electrical power; however, the   “high” speed position has three ad- vantages over   “low” which may dictate its  selection: (1) Maximum cooling capacity can only be achieved in “high.” (2) The increased flow of evapora- tor air improves air distribution in the conditioned  space. (3) Twice as much ventilation air is drawn into the unit in “high.” e. Vent Adjusting Knob. The vent adjusting knob controls the flow of ven- tilation air into the unit. The knob is connected by the damper cable to a damp- er located in the vent duct. The  knob controls the flow of ventilation air by adjusting the position of the damper from full closed to full open. Turning the knob to the right closes the damper, turning the knob to the left opens the damper. The force that draws ventila- tion air into the unit is the small pressure difference between the ambient and the evaporator fan. This is the same small pressure difference that exists between the conditioned space and the evaporator fan; the pressure differ- ence that draws return air through the inlet louver and the return air filter. When ventilation air enters the condi- tioned space a like flow of air must ex- haust from the space to prevent a pres- sure  buildup. Cracks in the space may be sufficient; but if a large flow of ventilation air is desired, some posi- tive means of air exhaust should be pro- vided. f. Inlet  Louver. The inlet louver is finger-adjustable  from full open to full  closed. Normally the louver shut- ter tab adjustment is kept at 45° posi- tion, which is essentially the same as a full  open  inlet  louver. If  the  inlet louver  is  closed,   return air will still manage to flow through it; but at a re- duced rate with high pressure drop.   The increase in pressure drop will cause ad- ditional ventilation air to be drawn in- to the unit. The “vent” mode of oper- ation is normally the only mode where closing the inlet louver (to create in- creased ventilation airflow) would be considered a real advantage. suf- ficient ventilation air flow for other modes of operation can be achieved with the inlet louver open and the damper open. The return air filter is attached to the back side of the inlet louver. A dirty filter can easily be observed by looking through the inlet louver blades. Since maximum cooling capability of the unit is obtained with maximum evapora- tor air flow,   a closed inlet louver or a dirty air filter will decrease cooling capacity, and may cause ice to form on the  evaporator  coil. g. Outlet  Louver. The outlet louver is finger-adjustable in both horizontal and  vertical  planes. This adjustment on an individual blade basis allows maximum control over the direction of outlet air. The best distribution pattern for outlet air is left to the operator’s judgment; but blades should not be ad- justed beyond 45°, as evaporator air flow  will  be  impeded. h. Condenser   Louvers. The condens- er louvers, located on the rear of the 2-2