Fuel System Combustion Air System Ventilating Air System  16-61 Figure 16-100.  A diagram of a typical combustion heater and its components. Figure 16-99.  A modern combustion heater.  Combustion heaters are similar to exhaust shroud heaters in  that ambient air is heated and sent to the cabin. The source  of heat in this case is an independent combustion chamber  located inside the cylindrical outer shroud of the heater unit.  The correct amount of fuel and air are ignited in the air-tight  inner chamber. The exhaust from combustion is funneled  overboard. Ambient air is directed between the combustion  chamber and the outer shroud. It absorbs the combustion heat  by convection and is channeled into the cabin.  [Figure 16-99] Refer to  Figure 16-100  for the following descriptions of the  combustion heater subsystems and heater operation.  The air used in the combustion process is ambient air scooped  from outside the aircraft, or from the compartment in which  the combustion heater is mounted. A blower ensures that the  correct quantity and pressure of air are sent into the chamber.  Some units have regulators or a relief valve to ensure these  parameters. The combustion air is completely separate from  the air that is warmed and sent into the cabin.  Ventilating air is the name of the air that is warmed and sent  into the cabin. Typically, it comes into the combustion heater  through a ram air intake. When the aircraft is on the ground,  a ventilating air fan controlled by a landing gear squat switch  operates to draw in the air. Once airborne, the fan ceases to  operate as the ram air flow is sufficient. Ventilating air passes  between the combustion chamber and the outer shroud of the  combustion heater where it is warmed and sent to the cabin. As mentioned, fuel for the combustion heater is drawn from  an aircraft fuel tank. A constant pressure fuel pump with  relief valve pulls the fuel through a filter. A main solenoid  valve downstream delivers the fuel to the unit. The solenoid  is controlled by the cabin heater switch in the cockpit and  three safety switches located on the combustion heater. The  first safety switch is a duct limit switch that keeps the valve  closed should the unit not have enough ventilating airflow to  keep it within the correct operating temperature range. The  second is a pressure switch that must sense pressure from  the combustion air fan to allow the solenoid to open. Fuel  Combustion air scoop Sealed fuel control assembly Regulator assembly Aircraft skin Aircraft fuel tank Fuel filter Pump Solenoid valve Damper Damper Spray nozzle Drain Filter Relief valve Pressure switch Line of flight Differential air  Combustion air  ground blower Combustion  chamber Thermal switches overheat and cycling Ignition unit Plenum Heated air Drain Drain Ventilating air gr ound blower Ventilating aft scoop Exhaust Exhaust shroud