The way oil burners work is oil is sprayed through a nozzle at high pressure (at least 100 psig) and is ignited by an electric spark from a high voltage transformer. Once the oil starts burning there is either a photo cell to detect light from the flame or a thermostat in the stack (chimney) that detects heat from the fire and needs to see it within a certain period of time (usually 45) seconds or it will shut the burner off.

    The photo cell is of the type called cadmium sulfide and responds very well to the infrared portion of the spectrum. For residential use they are sold by at least Honeywell and White-Rodgers plus others but for oil burners have the same characteristics. If the flame is not established the circuitry on the control properly called the "Primary" will activate an electric heater on a thermal relay that will pop open after the 45 second time period. After this happens you have to reset the control (after it cools) and you have another 45 seconds.

    Newer controls may have an electronic system that gives you like 2 tries to get the oil burner started before it gives up then you must follow a procedure to clear the fault. Some commercial and new controls shut off the ignition transformer after the flame is established or a certain period of time after flame.

    The question is how does the control know that there is definitely or definitely not a flame. Well the photocell has a certain electrical characteristic called resistance measured in "Ohms". When the resistance is below a certain point there is considered to be flame, above a certain point there is considered to be no flame. So what is it?

    Thanks to one of my readers who has done the research for me. We know fairly well what those readings should be; See below

   I found a paper that listed the reading for the CAD cell.  It should read < (less than)
   1600 Ohms when burning and over 20,000 when dark. I also found out that it
   will start without the cell connected then use can use a standard alligator
   jumper to run it while testing. The furnace we have  has a resistance of
   around 3000 ohms when burning.  This may be the problem.  After moving the
   eye around, the best I could get was 2000 ohms. I tried a new eye with no
   change.  I am going to take the unit out and clean it good on the inside to
   see if soot is blocking the light on  the eye.  I suspect the value I was
   getting when I started was just barely activating the circuit.  Any light
   change at all was causing it to trip.

    Note: I have worked on burners where the best I could do is 3-5K (3,000-5,000) with a dirty air tube and it will still hold just fine. An intermittent connection to the F-F terminals or a loose socket will cause it to work fine for several cycles and then stop even though the connections look good (the burner fires just fine). This can be mistaken for a completed cycle.

    Keep in mind that one of the other functions of the "Primary" control is it will not let the burner start if there is fire present. so if you shine light on the cell or try to bypass it. The burner will keep running but it should not restart. You will notice this if you stop the oil burner and then try to restart it there will be a delay until the fire box cools down and goes dark.

    For all your oil equipment repairs, questions and answers contact us. When filling out this form please keep in mind that the oil burner and the furnace are two separate entities. I don't need details on the furnace if you are looking for parts or answers to problems with your burner. Just the brand of the burner and the parts you need. Please try the supply house list first.

    If you have air conditioning or a forced air furnace consider adding a heat pump also known as a fossil fuel kit to save you money if oil prices get high this winter (the same goes for natural gas). Sold mainly on Rheem and Ruud. but can be used on other brands of heaters.

    For a list of all files please go to the sitemap

                 This text written by: Scott Meenen * G & S Mechanical