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NORA Oilheat Technicians Manual

spark across the electrodes that ignites the oil is present whenever the burner is running. Interrupted duty ignition means the spark comes on for a short time at the beginning of each burner operating cycle and is turned off once flame is established, or for a fixed time—which may include a period of time after trial for ignition, known as a ‘Sparkout’ period—to insure a stable flame is fully established. Interrupted duty ignition is preferred because it makes for quieter combustion, increases the life of the ignition system, uses less electricity, and generates fewer nitrogen oxide (NOx) emissions. Types of primary controls There are two basic types of primary controls commonly found in the field today: • Thermal-mechanical primary controls • Cad cell primary controls Thermal-mechanical controls, commonly called “stack switches” or “stack relays”, utilize bi-metals to detect flame and to mechanically open and close electrical contacts to provide burner control. They have spring loaded reset buttons and a Figure 11-4: Low voltage safety switch Basic Concept of How a Safety Switch Operates thermal safety switch that must cool before the control is allowed to be removed from lockout. Through the 1960’s, stack relays were the industry’s standard primary control and Chapter 11—Primary Controls 11-5 could continue to pump fuel into the unit, creating a hazardous condition. When the primary control shuts off the burner because no flame is detected, it is referred to as being “off on safety,” “in safety” or “in lock out.” Once the primary control goes off on safety, it must be manually reset before it can send power to the burner again, Figure 11-4. The amount of time that elapses from the start of a cycle in which flame is not detected, to the time the control goes off on safety is referred to as “trial for ignition” or “safety timing.” Slight variations from an individual control’s stated safety timing often exist due to manufacturing tolerances, voltage variations, and temperature changes. Ignition modes Oilheat systems operate in one of two ignition modes: intermittent duty or interrupted duty. Intermittent duty ignition means that the Did you know? Primary controls that are designed for interrupted duty ignition can be adapted for intermittent duty ignition, but primary controls designed for intermittent ignition cannot be used for interrupted ignition. Bimetal Heater Wires Contacts Insulators Contact Blades Spring Spring Reset Button Wires Chapter 11 Primary Controls


NORA Oilheat Technicians Manual
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