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NORA Oilheat Technicians Manual

Figure 12-27: Double-pole, single throw (DPST) relay extensively on forced hot water systems to provide for multiple zones. In order for a switching relay to use a low voltage switch (thermostat) to control the line voltage loads (the circulator), it must contain a step down transformer (from 120 to 24 volts), a relay or relays and the necessary line and low voltage connections. Most of today’s switching relays are double-pole, double-throw (DPDT), but relays can be found in several configurations. This is the oldest type and can control only one load without exceeding the contact current capacity of the relay. Figure 12-27 shows a double-pole, single-throw (DPST) relay that allows for two devices to be switched at the same time. A good application for this type of relay is where the switching relay turns on the circulator and primary control. Figure 12-28 shows a DPDT switching relay for use with a low voltage, low (operating) limit control. A typical use for this control is to control an additional zone with a combination control package that will provide for high limit, low limit and circulator operation. Figure 12-28: DPDT switching relay 12-20 Limit Controls and Thermostats Figure 12-29: Panel used with circulators Figure 12-30: Zone valves panel Multiple zone switching relays can also be used to reduce costs and simplify wiring when several zones are used. Figure 12-29 shows a panel used with circulators and Figure 12-30 shows a zone valves panel. Note that both panels can work with all makes and wiring configurations of thermostats, zone valves and circulators. Hydro air fan controls Figure 12-31 shows a wiring diagram for a typical hydro air fan control. A hydro air system uses boiler water pumped through a fan coil for heating. The fan coil is mounted in an air handler along with the air conditioning coil from the compressor. With these units you can use hot water to heat the air flowing through the same ducts used for summer cooling. The controls are Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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