Twin engine and a few single engine aircraft heat the cabin area with a combustion heater. In most general aviation aircraft, up to 50,000 Btu is produced by this heating method. Unlike most single-engine aircraft that use a heat muff around the engine exhaust, combustion heaters are similar to a house furnace. A combustion heater is an airtight burner chamber with a stainless-steel jacket. Fuel from the aircraft fuel system is ignited and burns to provide heat. Ventilation air is forced over the airtight burn chamber picking up heat, which is then dispersed into the cabin area.
How is a Heater Controlled?
When the heater control switch is turned on, airflow, ignition, and fuel are supplied to the heater. Airflow and ignition are constant within the burner chamber while the heater control switch is on.
When heat is required, the temperature control is advanced, activating the thermostat. The thermostat (which senses ventilation air temperature) turns on the fuel solenoid allowing fuel to spray into the burner chamber. Fuel mixes with air inside the chamber and is ignited by the spark plug, producing heat.
The by-product, carbon monoxide, leaves the aircraft through the heater exhaust pipe. Air flowing over the outside of the burner chamber and inside the jacket of the heater absorbs the heat and carries it through ducts into the cabin.
As the thermostat reaches its preset temperature, it turns off the fuel solenoid and stops fuel flowing into the burner chamber. When ventilation air cools to the point that the thermostat again turns the fuel solenoid on, the burner starts again.
How Safe are Combustion Heaters?
This method of heat is very safe. An overheat switch is provided on all combustion heaters, which is wired into the heater's electrical system to shut off the fuel in the case of malfunction. In the unlikely event that the heater fuel solenoid, located at the heater, remains open or the control switches fail, the remote fuel solenoid and/or fuel pump is shut off by the mechanical overheat switch, stopping all fuel flow to the system.
Unlike heat muff style cabin heaters used on most single engine aircraft, combustion heaters do not have carbon monoxide poisoning as a major concern. Combustion heaters have low pressure in the combustion tube, which is vented through its exhaust into the air stream. The ventilation air on the outside of the combustion chamber is of higher pressure than inside and ram air or pressurization increases the higher pressure on the outside of the combustion tube. In the event a leak would develop in the combustion chamber, the higher-pressure air outside the chamber would travel into the chamber and out the exhaust.
There have been occurrences when heater exhaust could be smelled in the cabin. Loose panels or door and window seals are likely allowing engine or heater exhaust from outside the aircraft to enter the cabin. There is also the possibility that foreign objects, such as bird nest material or paper logging against the burner chamber may produce fumes when heated.
Overhaul — When and What?
According to the FAA, time on a combustion heater is based on 50 percent of tach time unless a dedicated heater hour meter is installed. Janitrol "S" series heaters require overhaul at 500 hours according to Janitrol; it is recommended all of the accessories be overhauled as well. Overhaul costs vary depending on the combustion tube condition. The externally mounted accessories (such as ignition, blower motor, fuel solenoid, fuel pump, etc.) are all additional overhaul costs associated with the "S" series heater, no "ADs," pressure decay test at each 500-hour overhaul.
"B" series Janitrol heaters vary in required overhaul time at either 1,000 or 1,500 hours with an "AD" requiring mandatory pressure decay test every 100-hour inspection or two years. South Wind heaters, having 2-inch diameter exhaust, require overhaul every 1,000 hours, requiring a pressure decay test at each overhaul according to the "AD". South Winds, with a 1 1/2-inch diameter exhaust, require overhaul at 1,000 hours, and thereafter each 500 hours, unless a new combustion tube is installed. Most accessories are included with the overhaul since they are mounted on the unit. The remote fuel regulator/shut off is a separate item.
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