Here s the principle in a perfect operational Hot Surface Ignitor (HSI) heater:
Plug the unit in.
When power is applied, the ignitor (glowbar) glows. After a delay, the motor starts to turn, the fan blows air through the combustion chamber, the fuel begins to flow, because the air pump (rotor) is directly connected to the motor shaft. Air is delivered to the nozzle adapter by the air pump. At the nozzle adapter, the passing air creates a venturi effect, sucks fuel from the tank and thereby delivers fuel to the nozzle at a precise fuel/air rate for atomization.
The photocell starts looking for a flame in the chamber
When the atomized fuel contacts the glowing ignitor, there is combustion.
Then, after 5-7 seconds the ignitor no longer glows and the flame continues to burn on it s own and it should burn until the fuel supply is gone or power is disconnected.
The photocell takes over and monitors the flame for color. Too rich/lean and the flame color changes, or any momentary flame-out and a signal is sent to the computer control board and there will be a shutdown by the electronics.
Now, these heaters are computerized and the computer wants to find a system that is operating at 100%.
IMPORTANT: At less than 100% the system shuts down for safety and environmental reasons. Simply stated- the computer is looking for a reason to shut the unit down because that is its job.
I am going to list some of the reasons that will cause a shutdown in the order of incidence, in our shop anyway.
Water in the fuel people love to buy parts but this is a free fix, just drain the tank. Shine a flashlight in the tank & look for water droplets & trash.
The pump pressure not set to specs. There is not a fudge factor here. Instructions for setting the pump pressure are at http://reddyparts.com/pressure.htm .
The air pumps on the old style heaters could be adjusted to suit the operator s nose. If the heater smells just adjust the pressure a little until it smells good. Not so with the new heaters- you adjust to suit the computer, not you.
Air leak. If there is a crack in the plastic end cover, air line, fuel line, or nozzle adapter. These conditions create a lean mixture--poor flame color
Defective or worn nozzle. Desa warns about annual replacement in their manuals. Depending on annual "hours used" this may or may not be your problem.
Defective control board and/or photocell. Eliminate all of the above reasons for your heater s failure before blaming the electronics.
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Today's Featured Article - A Cautionary Tale - by Ian Minshull. In the early 1950s my father bought an Allis Chalmers B and I used it for all the row crop work with the mangolds and potatoes, rolling and the haymaking on our farm. The farm and the Allis were sold and I have spent a lifetime working on farms throughout the country. I promised myself that one day I would own an Allis. That time event
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