A cad cell that has failed will send your waste oil burner into lockout even though it is burning fine. The test is quick: measure resistance across the "F" terminals on the primary control with the cell in the dark and again in the light. It should read 30K ohms or higher in the dark and below 1600 ohms in light. If it does not, the cell is bad. Everything below is drawn from section 9.3.3 of the EnergyLogic heater manual and NuEra Heat's own service notes.
Written and reviewed by the NuEra Heat service team
Written from sections 7.4.3, 9.3.3 and 9.3.6 of the EnergyLogic heater installation and operation manual, the Troubleshooting and Testing service document, and our own service notes. NuEra Heat has been selling, servicing, and stocking parts for waste oil heating equipment for over 30 years across Alaska and the Pacific Northwest. About NuEra Heat — questions on any step, call 1-800-347-9575.
The cad cell — also called the photo eye or flame detector — on your EnergyLogic waste oil heater or boiler senses the flame by detecting light in the visible spectrum. It is a safety device. The primary control watches its resistance and keeps powering the burner during a call for heat only while it sees a flame. The normal flame sensing range is less than 1600 ohms; above 1600 ohms the controller concludes there is no flame and stops the burner as a safety precaution.
That matters because if the flame goes out inside the burner, the cad cell is what shuts down the other systems — including the pump — to prevent excess fuel from being pumped into the burn chamber. Jumping the "F-F" terminals on the control module bypasses that protection and lets the pump keep running, which is why it is done only briefly for priming. Never run the burner for an extended period with F-F jumped.
The classic symptom is a burner that trips into thermal lockout while it is actually burning. Confirm it by watching through the peep hole as the machine starts: if you see a flame present and then the machine resets itself, suspect the cad cell or photo eye.
Before you condemn the part, clean it. A dirty photo receptor effectively hides the flame from the cad cell and tricks the machine into thinking it is not burning. Use light pressure and a soft cloth — and no solvents on the lens. If it is clean and still failing, move on to the resistance test.
Tools required: flat-blade screwdriver and a 120VAC capable digital multi-meter set to ohms.
Put plainly: with the burner cord unplugged and the transformer closed, the cell is in the dark and resistance across F-F should be very high — around 30K ohms, or reading as an open circuit. Expose the cad cell to daylight and that resistance should fall below 1600 ohms. A cell that fails either half of that swing needs to be replaced.
A qualified service technician can also verify the cell against a real flame during burner checkout:
A good cad cell narrows the problem to the rest of the ignition and control chain. The primary control has its own test: disconnect one wire from the F-F terminals, jumper T-T, and the burner should run for several seconds and then reset — that is the limited recycle mode working correctly. Then jumper T-T, jumper F-F once the burner starts, let it run a minute, and pull the F-F jumper; again the burner should run briefly and reset. If the burner does not operate at all, unplug it and check the connections in the burner wiring box before replacing the control.
The terminal layout and wire colors behind those checks are on the furnace wiring diagram page. Related reading: the Cad Cell Testing blog post and Nozzle Maintenance.
NuEra Heat stocks 100% of the parts for EnergyLogic and BlackGold equipment, including cad cells, ignition transformers, primary controls, and electrodes — if yours fails the test, we can ship one today. Use the interactive explosion charts to find the component on a diagram and jump to it in the store, or browse the support library for manuals and troubleshooting documents. Full context on the equipment itself is on the waste oil heaters guide.
Remember that these machines must be serviced annually or every 1,000 hours, whichever comes first. Free phone tech support is available at 1-800-347-9575.
Source: EnergyLogic Heater Installation and Operation Manual, sections 7.4.3, 9.3.3 and 9.3.6 — download the manual (PDF).
