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NORA Oilheat Technicians Manual

Did you know? The burner manufacturer determines the location of the cad cell; if for some reason an alternate location must be used, make sure that: • The cell has a clear view of the flame • Ambient light does not reach the cell • Ambient temperature is below the cell’s rating (approx 140°F, see specs) • Movement, shielding or radiation of metal surfaces near the cell does not affect cell function. • Avoid extreme changes in flame light reaching the CAD cell, as pass/fail thresholds in the control may not function properly. The cad cell is a ceramic disc coated with cadmium sulfide and overlaid with a conductive grid. Electrodes attached to the ceramic disc transmit an electrical signal to the primary control. In darkness, cadmium sulfide has a very high resistance to the passage of electrical current. As the cell is exposed to light, its resistance decreases and current is allowed to pass. For a cad cell relay to start the burner, the flame detector must sense the absence of flame and resist the passage of current. Once the burner starts and flame is established, the cell senses light, resistance drops, current passes through the circuit and the burner continues to run. If the cad cell does not sense enough light when the burner starts, the control will shut off on safety. It can not be started again until the safety switch cools off and is manually reset. Note that electronic safety switches do not have to cool down before resetting. If the flame is lost during the running cycle, very early model controls will shut the system down after the safety timing (15 to 45 seconds) is reached; later models will make one attempt to restart. Recycle type controls will shut down for 1 minute, then allow startup, for any number of startup attempts. Wiring As with stack mounted primary controls, the major manufacturers adopted uniform standards regarding wiring connections for first generation cad cell primary controls. In place of the numbered screw terminals on stack relays, cad cell relays have color-coded wire leads pre-attached to the solid-state circuitry. These wire leads from the control are connected to the wiring for the system components with wire nuts. The hot wire from the limit control is connected to the black wire of the cad cell relay. Some postpurge controls have line voltage connected to a red/white wire. The hot wire of the burner motor, igniter and oil valve are connected to the orange wire. Postpurge controls have the valve connected to the violet wire. The white wire is connected to the neutral lead along with the neutrals from the burner components. The low voltage side of cad cell relays typically has two terminals labeled T-T, which can be connected to the thermostat. The cad cell flame detector leads are connected to the F-F terminals. Figure 11-6. Operation The most common first generation cad cell primary controls you are likely to come across in the field are the Honeywell Chapter 11 Primary Controls Cad cells can only see yellow and white flames. This is why they are not used on gas controls. New low NOx oil burners burn with a transparent to blue flame and biofuels’ flames are also more blue. Manufacturers are using infrared or ultraviolet detectors to see these new flames. 11-8 Primary Controls


NORA Oilheat Technicians Manual
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