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NORA Oilheat Technicians Manual

Mounting thermostats • Servicing or installing a thermostat is a job for clean hands. Do not mar or soil wall surfaces. • Be absolutely sure that all wires are connected to their proper terminals and that all connections are tight. If a color code is being used, be certain that it is followed. • If mercury switches are used in the thermostat, be absolutely certain the thermostat back plate and/or the thermostat itself are level. • All excess wire should be pushed back into the hole in the wall, and the hole should be plugged with putty to prevent cool air drafts from affecting the thermostat performance. Limit controls A limit control is a temperature or pressure actuated switch. Limit controls are generally divided into two groups: the high limit or safety controls and the low limit or operating controls. Limit controls are of either the direct acting or reverse acting type. Direct acting controls break (open) their contact on temperature rise while the reverse acting controls make (close) their contacts on temperature rise. Remember, in electricity, open means that there will be no flow of electricity and closed means electricity can flow through the switch. The high limit is a safety device that turns off the burner should temperatures get too high within the furnace or boiler, or steam pressure become excessive in a steam boiler. This control should be line voltage, wired in series with the primary control so that it can turn off only the oilburner, never the circulator or system fan. The circulator or fan must stay on to remove the excessive heat produced. A low limit or operating control is a limit that is used to control the operation of the burner and blower or circulator. Warm air limit controls Warm air limit controls protect the furnace heat exchanger from excessively high temperatures and operate the blower. Both the high limit control and the fan control may be operated by a bimetallic element inserted in the plenum, or through an electronic control panel. The line voltage high limit control in series with the primary control may employ either metal to metal contacts or a mercury switch to make or break the circuit. Fan and limit controls are usually combined into one housing. In this instance, the helix type bimetallic element operates both the fan control switch and the high limit control switch. Figure 12-8 shows a combination fan and limit control. A single dial as shown in Figure 12-9 (old 12-8 Limit Controls and Thermostats Remember, in electricity, open means that there will be no flow of electricity and closed means electricity can flow through the switch. Figure 12-8: Combination fan and limit control Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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