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.
C&D Associates Inc. heaters (FAA/PMA direct replacements for Janitrol and South Wind) call for overhaul at 2,000 hours with no ADs, but there is a recommended pressure decay test each 250 hours after reaching 2,000 hours.
Does an AD Affect My Heater?
All aircraft with a Janitrol heater of the "B" series (1 1/2-inch exhaust) have airworthiness directives. There are no ADs at this time on the "S" series heaters installed in the Baron manufactured prior to 1973 or the Travel Air. It would be prudent, however, for the owners of these airplanes to periodically perform a pressure decay test to check for leakage.
What ADs Affect My Aircraft?
AD 2004-21-05 affects heaters identified as Janitrol or Jan-Aero Devices of the "B" series design, installed in most Pipers except early models of Twin Comanche and early Aztec. This AD requires mandatory inspection and a pressure decay test for leakage every 100-hour inspection or two years. The purpose of the pressure decay test is to test for cracks or leaks in the combustion liner. Overhaul even with a new ceramic style combustion tube cannot eliminate the AD from a Janitrol heater.
AD 81-09-09 affects most South Wind heaters and mandates 250-hour inspections and overhaul with a pressure decay test at each 1,000 hours.
What Inspections Can I Do?
Pilots and owners operating under FAR 91 can perform checks needed to assure proper operation of the heater prior to flight. Ensure that the combustion and ventilating air inlets, fuel drain line, and exhaust outlet, are not damaged and free from obstructions.
Adjust the temperature control for a low setting and perform an operational check. (Use a thermometer [0 to 300 F] in the defrost outlet to determine the temperature range from low to high.) After the heater has begun to burn in low, allow it to run a minimum of two minutes to ensure proper operation.
Move the heater control to mid-range and verify that the heater produces medium heat. Once mid-range has been verified, move the heat control to high. A high temperature of 200 F in the defrost outlet is reasonable. Set the thermostat to low, allowing the heater to run in this setting for two minutes to purge the combustion chamber prior to shutdown.
If the heater does not work when the system is turned on, there is a simple preliminary check. Verify the heater circuit breaker is set or is not activated. With the heater switch on, confirm there is airflow coming from the exhaust of the heater, and at the same time smell the exhaust for a fuel smell.
No airflow from the exhaust pipe may result from a bad blower or no power to the heater control switch, which may be caused by an activated circuit breaker.
If there is airflow from the exhaust accompanied by a fuel smell but no burn, the spark plug, glow plug, or ignition may have failed. With airflow from the exhaust but no fuel smell, it is possible that failure of the thermostat, fuel pump, or fuel solenoid valve could be suspected.
Knowing these indications reduces troubleshooting time required to have the system back up and running. (A more complete troubleshooting guide can be found at www.aircraftheater.com.)
What Is the Most Common Cause of Heater Failure?
In Janitrol product line, the most common cause of a heater failing to produce heat is the spark plug. When a heater burns fuel, it leaves a coating on the plug. This coating creates a corrosive effect through the summer months when the heater is not used, etching the ceramic insulator of the spark plug. The next season, the spark will follow the path of least resistance down the etched ceramic, which will cause the plug to misfire. Fuel burn will be incomplete, causing low or no heat. Because of the shiny protective coating on the ceramic, they can only be cleaned by wiping with a nonabrasive cloth. Never sand or bead blast, as it will remove this protective coating. It is recommended that the spark plug be replaced each heating season and the gap be checked.