1. Fresh-air inlet 2. Drain tube 3. Overhead vent blower 4. Overhead fresh-air duct 5. Cabin-air exhaust 6. Fresh-air outlet 7. Defroster outlet 8. Cabin-heat outlet 9. Heater and defroster air controls 10. Muffler and heat shroud 11. Air box, cabin-heat control valve 12. Defroster valve 4 12 10 8 8 7 9 3 5 6 11 6 2 1 1 6 Fresh air Defroster air Heated air Figure 16-98.  An exhaust manifold with its shroud removed showing numerous welded studs used to increase heat transfer from the  exhaust to the ambient air going to the cabin. Figure 16-97.  The environmental system of a single-engine Piper aircraft with an exhaust shroud heating system.  Regardless of age or condition, aircraft with exhaust shroud  heating systems should contain a carbon monoxide detection  device in the cockpit. the aircraft’s engine(s), although it does use fuel from  the aircraft’s main fuel system. Combustion heaters are  manufactured by a few different companies that supply the  aviation industry. Most are similar to the description that  follows. The most up to date units have electronic ignition  and temperature control switches.  An aircraft combustion heater is used on many small to  medium sized aircraft. It is a heat source independent from  16-60 Combustion Heaters