valve dumps pump air overboard when the deice system is  not in use. Many control valves are built in combination with  pressure relief valves that keeps the deice system safe from  over pressure.  [Figure 15-26] All deice boot systems require a means for connecting  vacuum from the air pump to the boots when the boots are  not in use. This ensures the boots are held tightly deflated  against the aircraft structure to provide the significant change  in size and shape needed to break off accumulated ice when  the boots inflate. One single deflate valve is used on simple  deice boot systems. The deflate valve is solenoid operated.  It is located at a point in the system where when closed, air  is delivered to the boots. When open, vacuum is applied.  Often, the deflate function is built into another unit, such as  a distributor valve discussed next. A distributer valve is a type of control valve used in relatively  complex deice boot systems. It is an electrically-operated  solenoid valve controlled by the deice boot system timer  or control unit. On some systems, a distributor valve is  assigned to each set of deice boots it controls. It differs from  a control valve in that it has the deflate valve function built  into it. Therefore, the distributor valve transfers connection  of the boots from the pressure side of the air pump to the  All but the simplest of deice systems contain a timer or  control unit. This device controls the action of the distributor  valve(s) to ensure all boots are inflated in the proper sequence  and for the correct duration. Six seconds of inflation is  common to break off accumulated ice. The boot then must  be immediately deflated so that ice does not adhere to the  inflated geometry of the boot. This could cause it to fail to  deflate or break off ice when the boot is re-inflated. The  timer, or control unit, can also be made to cycle through the  inflation and deflation of all boots periodically, thus relieving  the flight crew of repetitive manual activation of the system.  The function and capabilities of timers and control units vary.  Consult the manufacturer’s maintenance information for the  performance characteristics of the timer/control unit on the  aircraft in question. The timer, or control unit, may be an independent device, or  it may be built-in as part of another deice system component,  such as a central distribution valve.  NOTE: A modern system design may use a pressure switch  to signal deflation of the deice boots. When pressure builds  in the boots to a preset amount, the switch signals the control  valve to close and connect the boots to vacuum. However,  this system retains a control unit for automatic cycling of the  system at a set time interval. Both the pressure and vacuum developed by an air pump  must be regulated for use in the deice boot system. Typical  boot inflation air pressure is between 15 and 20 psi. Vacuum  pressure is set for the requirements of the gyroscopic  instruments operated by the vacuum side of the air pump.  Measured in inches of mercury, normal vacuum pressure  (suction) is 4.5 to 5.5 "Hg. Another type of distributor valve exists that handles the  inflation and deflation of numerous sets of deice boots in a  single unit. It also connects the boots to vacuum and dumps  pump air when deice is not needed. A servo motor is used to  position the multi-position valve. These centralized units are  controlled by a timer or control unit. They inflate and deflate  all of the boots on the aircraft. The timer may be built into  the unit on some models. vacuum side of the pump once the proper inflation time has  elapsed. The valve also dumps the unneeded air from the  pump overboard. Deflate Valve Distributor Valve Timer/Control Unit Regulators and Relief Valves Figure 15-26.  A solenoid operated deice control valve. 15-18