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NORA Oilheat Technicians Manual

Chapter 11 Primary Controls Did you know? Cad cell resistance looking into an inactive chamber should be at least Did you know? Cad cell primary controls, also called “cad cell relays,” were introduced in the early 1970’s. Modern oilheat systems operate more cleanly, more efficiently and much more economically than systems designed when stack relays were the standard. Chapter 11—Primary Controls 11-7 is used, connect the red wire to Terminal R, white wire to W and black wire to B. Operation of a stack relay If all components are functioning properly, the control is in standby mode until: • Line voltage is applied through the limit control to Terminal 1, AND a call for heat from the thermostat completes the circuit between W and B, or T-T. • Then Terminal 4 will be powered and Terminal 3 will be powered a split second later. • The transformer will generate ignition through the electrodes. • The motor will turn the fuel pump and burner fan. If there’s a delayed oil valve installed, it will open based on its delay timing. • Once flame is established and heated flue gases pass over and expand the bimetal detector element, the safety switch heater will be de-energized, preventing the unit from shutting off on safety. • The ignition circuit will be de-energized, shutting off the transformer if the unit operates on interrupted ignition. • The oilburner will then continue to run until either the thermostat is satisfied, opening the low voltage circuit, or the limit control is satisfied, opening the line voltage circuit. Either of these actions causes the primary control to cut power to Terminals 3 and 4, safely shutting down the oilburner. The primary control then returns to standby mode. Safety check It is important to check the safety timing of primary controls every time you perform a service call. To check the safety timing: • Turn off the service switch at the unit. • Remove the stack switch from the flue pipe or block the flow of oil by installing your pressure gauge in the nozzle port. • Turn on the service switch. • Make sure that both the line and low voltage circuits are complete. • The control should energize and shut off on safety in approximately 75 seconds. • If the control goes off on safety within 120 seconds, clean the bi-metal and reinstall it. • If the control does not go off on safety, replace it and be sure to check the timing on the new control. Cad cell primary controls Any system still operating with a stack relay is probably a good candidate for replacement with a high-efficiency oilheat system. While the basic functions of cad cell primary controls are the same as those of stack relays, they feature a much quicker reaction time through visual, as opposed to thermal, flame detection. The cad cell primary control circuit is made up of two components: the flame detector and the primary control. Flame detection The flame detector consists of a cad cell, a holder to secure it in place, and a wiring harness to connect it to the primary control. The detector is installed in the air tube of the oilburner where it can view the flame. 50,000 ohms.


NORA Oilheat Technicians Manual
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