Ventilation Air Pressure
As long as we are taking a quick look at proper combustion air pressure, let's peek at ventilation air pressure. The Janitrol engineers designed the heater to utilize two different sources and pressures of air for efficient operation and safety. Combustion air pressures are designed to be lower than the ventilating air pressures. This means that vent air will flow into the combustion chamber if the sealed combustion tube is compromised in any way. This safety feature keeps the by-products of combustion in the exhaust instead of the cabin. The increased flow of ventilating air helps to control the combustion process by "cooling" the combustion tube. The vent air absorbs the radiant heat as it is forced through the heater body and into the aircraft. The constant stream of vent air keeps the combustion tube (relatively) cool. Without vent air, the heater will overheat and shutdown.
Clean carbon deposit from
combustion head and remove
deposits from ground electrode.
The point here is that both combustion and ventilation air pressures are critical to the smooth operation and long life of the Janitrol heater. Of course, proper air pressures are not the only requirements for efficient operation. Proper fuel pressure and continued ignition delivery are also critical ingredients. These four conditions are required to properly operate any combustion heater.
Visual and preventative maintenance checks
Start your heater season by making sure you are current on the 100-hour inspection required by either AD#96-20-07 or the maintenance manual (24E25-1). Even if not required, this inspection is the appropriate guideline for a thorough check of the heater.
Comprehensive visual checks start with the air inlets, associated ductwork, and the exhaust outlet. During this check pay particular attention to two items. First, follow the ductwork to the combustion and vent blower motors and from the motors into their respective inlets on the heater body. Look for small cracks, restrictions, secure attachments, loose fittings, etc. This is important as any air leaks between the blower motors and the heater assembly will mean a loss of either cooling vent air (potential overheat) or combustion air (smoking exhaust or excessively rich F/A mixture).
The second item to pay attention to is the exhaust outlet. If there is evidence of soot, your heater is running rich. Remember the explanation about the excess oxygen combustion process? The heater is designed to run "lean" with a F/A ratio of 0.067. If you are getting soot accumulations, you have a shortcoming in either the fuel system or the combustion air system. Presence of soot is also an indication to look at the pressure differential sensing line in the exhaust outlet. You may need to use a wire to clear deposits on the sensing line. To blow out the line, disconnect the sense tube at the differential switch and apply filtered air to blow out the exhaust end of the tube.
Check the fuel lines and system components for any damage or indication of leakage or restrictions. Check and clean strainers, filters, and fuel drain lines as applicable. Follow the fuel lines to the heater body and check for dry rot on the grommet around the fuel feed line as it passes into the fuel solenoid housing and the heater body.
Check the electrical wiring at the heater terminal block and components for loose connections, chafed insulation, and secure attachment. Look at the blower motors and the associated wiring for secure connections and look at the radio noise filters for obvious signs of damage. Remember in later troubleshooting efforts, each blower motor plus the ignition assembly have radio noise filters wired in series to the respective units. If the noise filter does not have continuity, the assemblies will not function.
Remove and inspect all brushes for excessive wear. Most technicians use the "one half rule" to determine if it is time to replace brushes. If one half of the original length has worn away, replace the brushes. Replace brushes in pairs. Take a look at the armature for proper coloration (medium to dark brown) and no damage to the commutator is allowed. Inspecting, and replacing as necessary, the brushes will help prolong the service life of the motor.
Check the freedom of movement of the blower fan wheels and inspect them for obvious signs of damage. After you look at the terminal block and wiring harness, find the data plate and see if there is any shrinkage or "cooked" edges that might indicate overheating. Possible culprits in this situation may include blocked or leaking ductwork, fuel solenoid stuck in the open position, or insufficient ventilation air.
Inspect the ignition assembly and the ignition lead. Burning or discoloration of the ignition lead or any of the components of the ignition assembly indicates arcing. Remove and replace as necessary to prevent any damage to the heater and ensure the unit delivers the required continuous spark.
Remove, inspect, and clean the spark plug. Look for evidence of arcing and, also, look at the condition of the electrode and build up of carbon deposits on the spark plug (see plug pictures). The maintenance manual outlines an appropriate test for the spark plug. Do not test the spark plug on the heater body with any other object to avoid personal injury or damage to the ignition unit. Before re-installing the spark plug, clean the electrode and the core with a cotton swab and some appropriate solvent to remove all fingerprints. The oils on your fingers will cause tiny arcing on the surface of the plug and may degrade the performance of the unit. Remember to verify the spark gap.
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