Marginal Cad Cell Resistance Causing Intermittent Lockout
A homeowner reports that a Beckett AFG oil burner locks out intermittently, usually after the boiler has run for 10–15 minutes. The burner restarts after pressing reset, then may run for days. A technician connects a meter to the cad cell leads and reads 1,800 ohms with the flame established and 30,000 ohms with the burner off. The primary control is a Honeywell R7184. The cad cell window has a thin, even film of soot; the cell is aimed at the flame but the retaining clip is slightly loose. Nozzle is 0.85 GPH 80°B at 140 psig; smoke 0, CO2 12.0%, draft −0.04 in. w.c.; the flame looks steady through the sight port, though slightly orange at the tips. The transformer spark is bright and the electrodes are correctly gapped. The control’s LED is steady while the burner runs and flashes after lockout.
1. Interpret the 1,800-ohm reading. Is it acceptable, and how does it explain the intermittent lockout?
Read the worked answer
Most oil primary controls, including the R7184 series, require the cad cell resistance to be below about 1,600 ohms to prove flame reliably. A reading of 1,800 ohms is above that threshold, so the control is seeing a marginal flame signal. The 30,000-ohm off reading shows the cell is not shorted.
The soot film blocks light, and the loose clip can let the cell shift as the burner heats and expands. When resistance drifts above the control’s threshold, the control shuts off the burner and locks out, even though a flame may be present. This explains the intermittent pattern: it runs until alignment or heat pushes the signal out of range.
2. List the corrective steps in order, and explain when replacing the cad cell or control is justified.
Read the worked answer
Correct in this order:
- Clean the cad cell lens with a soft, lint-free cloth; do not scratch it.
- Reposition the cell to sight the flame through the retention head opening, and tighten the clip.
- Recheck resistance with flame on. A clean, well-aimed cell typically reads 400–1,000 ohms.
- Check the cad cell leads and connections for corrosion or loose terminals.
- Verify flame quality: orange tips can indicate slight rich operation, though smoke 0 and CO2 12.0% are acceptable. Confirm draft and nozzle size.
Replace the cad cell if resistance remains above 1,600 ohms after cleaning and alignment. Replace the primary control only if a known-good cad cell produces a low resistance (e.g., under 1,000 ohms) and the control still fails to hold flame, or if the control does not respond correctly to the cell. Do not replace the control first; the cell is the cheaper and more likely cause.