Burner Short-Cycles With a Good Cad Cell
A forced-air oil furnace has a 24 V control circuit with a thermostat, a high-limit switch, a fan-and-limit control, and a primary control with a cad cell. On a call for heat, the burner starts and runs for about 4 minutes. The flame then goes out while the blower continues to run. After 30 seconds, the burner restarts without a manual reset, then repeats. The technician measures 26 V between R and C at the transformer, 24 V across the thermostat when it is open, and 0 V when it is closed. The cad cell reads 900 ohms with flame and 45,000 ohms with no flame. The primary control is rated to trip at 1,600 ohms. When the burner is off, the technician measures 24 V across the high-limit switch; when the burner is on, the voltage across the same switch is 0 V. The furnace filter is clean, and the blower is running at the correct speed.
1. What does the cad cell resistance tell you about the flame detector and primary control? Explain why the cad cell is probably not the cause of the short cycling.
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The cad cell is doing its job. A flame-on resistance of 900 ohms is below the primary control 1,600 ohm trip point, and 45,000 ohms flame-off is well above it. If the control were losing flame signal, the burner would lock out and require reset, not restart after 30 seconds. The fact that the burner restarts on its own points to a safety or operating control opening and reclosing, not a failed cad cell. Intermittent cad cell wiring or a dirty lens can still cause problems, but the measured values and restart pattern make the high limit a stronger suspect.
2. Interpret the voltage readings across the high-limit switch. What is the most likely fault, and how would you confirm it?
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In a series 24 V circuit, an open switch drops the full control voltage across itself. The 24 V across the high-limit switch when the burner is off means the switch is open; 0 V when the burner is on means it is closed. That is consistent with the high limit opening on temperature rise and then reclosing. To confirm, measure plenum temperature at the limit with a thermometer and check continuity of the limit after it cools. Do not bypass the limit. Check the limit setting against the furnace rating, and compare the measured temperature rise with the data plate.
3. If the limit is opening because plenum temperature is excessive, what could cause this and what corrections are appropriate?
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Excessive plenum temperature can come from low airflow, overfiring, or a limit that is set too low or located poorly. Check the filter, blower wheel, blower speed, and duct static; verify the temperature rise is within the furnace rating. Verify pump pressure and nozzle size so the input is not above rating. If airflow and input are correct and the limit still opens at normal plenum temperature, replace the limit with the specified temperature and differential. Correct the cause before replacing the control; a new limit will simply open at the same temperature.