source that charges the system. After opening the valve on the  container (or puncturing the seal on a small can), the center  hose connection on the manifold set should be loosened to  allow air in the hose to escape. Once the air is bled out of the  hose, the refrigerant can enter the system through whichever  service valve is opened. The sequence is the same as above  and all manufacturer instructions should be followed.  into the cabin is used to transfer the heat. Other floor or  sidewall elements simply radiate heat to warm the cabin.  Electric heating element heaters require a significant amount  of the aircraft’s generator output, which is better dedicated  to the operation of other electrical devices. For this reason,  they are not very common. However, their use on the ground  when powered by a ground electrical power source preheats  the cabin before passengers board and does not tax the  electrical system.  Oil quantity added to the system is specified by the  manufacturer. Refrigerant premixed with oil is available and  may be permissible for use. This eliminates the need to add  oil separately. Alternately, the amount of oil to be put into the  system can be selected on the servicing cart. Approximately  1 ⁄ 4 ounce of oil for each pound of refrigerant is a standard  amount; however, follow the manufacturer’s specifications.  The EPA requires certification of technicians that work with  vapor cycle air conditioning refrigerant and equipment to  ensure safe compliance with current regulations. Aircraft  technicians can obtain certification or can refer vapor cycle  air conditioning work to shops that specialize in this work.  Most single-engine light aircraft use exhaust shroud heating  systems to heat the cabin. Ambient air is directed into a metal  shroud, or jacket, that encases part of the engine’s exhaust  system. The air is warmed by the exhaust and directed through  a firewall heater valve into the cabin. This simple solution  requires no electrical or engine power and it makes use of heat  that would otherwise be wasted.  [Figures 16-96  and  16-97] A major concern of exhaust shroud heat systems is the  possibility that exhaust gases could contaminate the cabin air.  Even the slightest crack in an exhaust manifold could send  enough carbon monoxide into the cabin to be fatal. Strict  inspection procedures are in place to minimize this threat.  Most involve pressurizing the exhaust system with air, while  inspecting for leaks with a soapy solution. Some require the  exhaust to be removed and pressurized while submerged  under water to detect any leaks. Frequency of exhaust heat  leak detection can be every 100 hours. Temperatures at high altitudes in which aircraft operate  can be well below 0 °F. Combined with seasonally cold  temperatures, this makes heating the cabin more than just a  luxury. Pressurized aircraft that use air cycle air conditioning  systems mix bleed air with cold air produced by the air  cycle machine expansion turbine to obtain warm air for the  cabin. This is discussed in the section that covers air cycle  air conditioning in this chapter. Aircraft not equipped with  air cycle air conditioning may be heated by one of a few  possible methods.  Some turbine-powered aircraft not equipped with air cycle  systems still make use of engine compressor bleed air to  heat the cabin. Various arrangements exist. The bleed air is  mixed with ambient air, or cabin return air, and distributed  throughout the aircraft via ducting. The mixing of air can be  done in a variety of ways. Mixing air valves, flow control  valves, shutoff valves, and other various control valves are  controlled by switches in the cockpit. One STC’d bleed  air heat system uses mini-ejectors in helicopter cabins to  combine bleed air with cabin air. All of these bleed air heating  systems are simple and function well, as long as the valves,  ducting, and controls are in operational condition. Occasionally, the exhaust system is slightly modified in a  shroud heat configuration. For example, an exhaust muffler  may have numerous welded studs attached, which increase  heat transfer to the cabin air. Each weld point is a location  for a potential leak.  [Figure 16-98] Occasionally, an electric heating device is used to heat the  aircraft. Electricity flowing through a heating element makes  the element warm. A fan to blow air over the elements and  16-59 Figure 16-96.  The basic arrangement of an aircraft exhaust shroud heater.  Electric Heating Systems Bleed Air Systems Aircraft Heaters Technician Certification Exhaust Shroud Heaters To  cabin Heater shroud Ram air Cabin heat control Heat overboard Exhaust Engine exhaust manifold