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NORA Oilheat Technicians Manual

Chapter 12 Limit Controls and Thermostats Chapter 12—Limit Controls and Thermostats 12-3 There are a large variety of limit controls, thermostats, and switching relays used in oil-fired heating systems. At first glance, it can be confusing. Remember, these devices are just switches that turn things on and off. Some of these switches are turned on and off by the warping or flexing action of bimetal blades. Some use magnets and springs. Some are controlled by a fluid that expands and contracts quickly. Some are line voltage and some are low voltage. Some need transformers to change the voltage so that low voltage switches can control line voltage loads. Some controls now use solid-state microprocessors and other electronic devices. No matter what they look like, limit controls are just switches. You may want to review the section on switches in the basic electricity chapter before you go any further. Electrical control circuit The hot (L1) electric power wire for the basic oilburner circuit begins at the service panel fuse, or circuit breaker, travels to the main switch, then to a junction box that is usually located on the ceiling near the burner. Many states and local codes require a thermal or Firomatic® switch at this junction box. From the junction box, the hot line (which is a black wire) runs to the serviceman’s switch, through the high limit Figure 12-1: Basic oil burner control circuit Basic Oilburner Circuit Showing Switches, Motor, and Ignition Transformer L1 L2 Fuse Emergency Switch 1 Line Switch 2 High-Limit Primary Control Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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