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NORA Oilheat Technicians Manual

Leveling Post (2) Chapter 12—Limit Controls and Thermostats 12-5 are usually two disconnect switches. The first is called a customer or emergency switch and it is normally located at the head of the basement stairs or at the entrance to the heater room. The second switch is a serviceman’s switch and may include a fuse. This switch is located on or near the furnace or boiler. Thermostats Principles and design A thermostat is a mechanical or electronic switch that automatically opens or closes a circuit as room temperature changes. The thermostat’s purpose is to start the burner and/or circulator or blower when the temperature is below the established setting, and to shut them off when the heat demand is satisfied. Thermostats must be extremely sensitive to temperature changes. In older thermostats, a bimetal element warps or unwinds in response to temperature change to open or close a switch. In the solid-state thermostat, the room temperature changes the resistance of an electronic device that will act in various methods to open and close circuits. The majority of thermostats installed in the field still use a bimetal element and mercury switch to function. The following text covers this type of thermostat, Figure 12-3 shows the most common type in use. The replacements are mostly electronic and will be discussed later. The bimetallic element comprises two dissimilar metal strips, bonded together, which expand or contract with a change in temperature at different rates of speed. This difference in expansion Figure 12-3 Mounting Slots Figure 12-4: T-87 thermostat rate will cause the bonded bimetallic element to bend or warp with temperature changes. By bending or moving when heat is applied to it or taken away, it creates a mechanical force that flips a mercury switch to make or break a pair of switch contacts. Making or breaking a contact means closing or opening a circuit. Remember, as with all mercury switches, such thermostats must be installed level. Figure 12-4 shows how to level the T87 thermostat. 3 wire thermostats vs. 2 wire thermostats Very old style thermostats needed three wires to operate. When replacing an old three wire thermostat with a new two wire, eliminate the red wire. Today there are some three wire thermostats that operate some zone valve motors and dampers. A circuit is necessary to drive the valve or damper open and another circuit must drive it closed. The switching action of these thermostats is single pole, double throw as opposed to a single pole, single throw switch for the two wire circuits. Thermostats of this type are never connected to primary controls. Chapter 12 Limit Controls/Thermostats Mounting Slots Be Sure Wall Hole is Plugged R Terminal Screws W Plumb Line Spirit Level Spirit Level Plumb Line Diamond Guides (2) Mounting Opening for Slots Thermostat Wiring


NORA Oilheat Technicians Manual
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