Oil Burner Guide

The cad cell, also known as the cadmium sulfide cell, is a major component of your oil burner system.  It is one of the primary safety control systems and is more important than most people realize.  Its main purpose: detect the presence of the oil burner flame.

This specific mechanism on your oil burner consists of three parts.  They include the actual cad cell itself, a holder, and a cord which serves to connect the cad cell with the the primary control.  It is placed inside of the air tube of your oil burner where it can easily see if the flame is operational or non existent.

A cad cell is produced from a ceramic disk made out of cadmium sulfide.  There is a conductive grid which covers the surface of it.  Also attached on the surface are electrodes and the cell itself is sealed in a glass to protect it from harm.  When in the darkness the cad cell has a huge tolerance to the an electrical current’s flow.  But, it has a very low resistance when operating in the presence of visible light.

Most heating technicians recommend that the cell cell resistance be about 300-1000 ohms for optimal operation of most oil burners.  Which this number may change slightly it should definitely not be more than 1600 ohms.  If any resistance over 1,600 ohms is seen then there is most likely a problem that should be looked at.

Understanding How Cad Cells Operate

Before we go over some basic troubleshooting methods its important that you first understand and appreciate how this control works.  First, the cad cell must have a fairly high resistant across the F-F terminals before it will try to commence the operation of your oil burner.  Again, this control does not sense light that is visible.  Should this device detect a light then the relay its attached to in the main control will not start the oil burner.

Once the oil burner is on, running, and operational with the flame on then the cad cell has to have a low resistance in order for it to prevent the primary control from igniting the safety and locking out.

Potential Cad Cell Problem #1

Its a cold winter night and the oil burner will just not start.  You think it may be the cad cell.  What do you proceed to do?

Initially, you want to go to the thermostat and double check that its calling for heat.  Make sure there is voltage going to this specific control as well.

Next, remove one of the leads on the cad cell from the attached FF terminals.  Does the oil burner start right away?  If it does not then its possible that the cad cell is sensing external light inside of the housing.  Or, quite possibly, the wires are pinched in some fashion.  There are also instances where the assembly has been shorted or the cell itself it defective.  If you can repair any of these items then by all means do so.  If not then calling a heating technician is advised.

Potential Cad Cell Problem #2

Does the oil burner start but not too long afterward locks out and you suspect that its the cad cell.  It is highly likely that the control did not sense, or see, the high resistance needed to start the oil burner.  Nevertheless, while in the lock out timing period it did not detect the low resistance needed on the cad cell to continue operation of your oil burner.  Remember that in order to prevent a lock out the resistance has to be no more than 1,600 ohms.

Here is how you fix this particular problem.  Start your oil burner.  Before the safety lock out timing period ends disconnect cell wires from the FF terminal on the main control. Then, using a piece of wire, or a 1,500 ohm resistor, jumper the FF terminal.  Doing so will allow the oil burner to continue to run so that you can inspect the cad cell resistance while its currently in its run cycle.  Now, while the burner is running proceed to connect the ohm meter across the cad cell leads.  Depending on who is the manufacturer of your oil burner will determine how much resistance you need to measure on the cad cell in order to find the root cause of the problem and take subsequent action.

Cad Cell Is Vital for Safety

Keep in mind that while the cad cell can be a pain at times it is a vital component of any oil burner.  It has proved to be extremely reliable and is a huge safety check that you should be really glad you have.  Without it unburnt fuel could collect over time without you knowing it and cause a huge explosion in the future if left unchecked for too long.