Thermostats, Pressure Switches, and Other Electric Control Devices Electricity for Refrigeration, Heating and Air Conditioning 7th Edition Chapter 12 Thermostats, Pressure Switches, and Other Electric Control Devices
Thermostats, Pressure Switches, and Other Electric Control Devices Upon completion of this chapter the student will be able to: • Explain the purpose of a transformer in a control circuit • Size a transformer for a control circuit • Troubleshoot and replace a transformer in a residential air conditioning control circuit • Explain the basic function of a line and low-voltage thermostat in a control system • Identify the common types of thermostats used in the industry • Draw schematic diagrams using line and low-voltage thermostats and operating and safety controls • Install line and low-voltage thermostats on heating, cooling, and refrigeration equipment • Correctly set the heating anticipators and cooling anticipators, if adjustable, on a residential low-voltage control system
Thermostats, Pressure Switches, and Other Electric Control Devices Upon completion of this chapter the student will be able to: • Explain the modes of operation and be able to correctly set or program a clock thermostat • Explain the function and operation of pressure switches • Install and correctly set the pressure switches in control systems used as operating and safety controls • Troubleshoot pressure switches • Understand, install, and troubleshoot the following controls in control systems used in the industry: • 1) humidistat • 2) oil safety switches • 3) time-delay relays • 4) time clocks • 5) solenoid valves
Key Terms • Anticipators • Clock Thermostat • Differential • High-Pressure Switch • Humidistat • Line Voltage Thermostat • Low-Pressure Switch • Low-Voltage Thermostat • Oil Safety Switches • Pressure Switches • Range • Snap Action • Solenoid Valve • Staging Thermostat • Thermostat Controlling Element • Time Clock • Time-Delay Relay • Transformer
Transformers • The transformers is a heating or cooling system provides the low-voltage power source for the control circuit. • Most residential and small commercial installation use a 24-volt control system. • The transformer for a residential unit is used to convert line voltage to 24volts. • Some commercial and industrial high-voltage equipment uses transformers that drop the line voltage from 240 or 120 volts.
Operation • Transformers are stationary inductive devices that transfer electric energy from one circuit to another by induction. • The transformer has two windings, primary and secondary. • An alternating voltage is applied to the primary winding of a transformer and induces a current in the secondary winding.
Sizing Transformers • Transformers are like many other electrical components. • They are not 100% efficient. • There is a loss between the primary and secondary windings. • This loss is considered when sizing transformers for a certain job. • Transformers are rated by their primary voltage, secondary voltage and voltamperes (VA). • System equipment must be considered in transformer sizing along with the transformer rating.
Thermostats • The temperature in any structure, regardless of its age, location or design can be maintained at comfortable levels with a thermostat. • Thermostats are designed and built in many different forms and sizes to meet the applications required in the industry
Applications • The basic function of a thermostat is to respond to a temperature change by opening and closing a set of electric contacts. • Many different types of thermostats are used in the industry to perform a variety of switching actions. • A heating thermostat closes on a decrease in temperature and opens on an increase in temperature.
Controlling Elements and Types of Thermostats • Two types of thermostat controlling elements are commonly used. • The controlling element of a thermostat is the part that moves when a change in temperature is sensed. • The bimetal thermostat is commonly used to control the temperature of air in an air-conditioning or heating application.
Remote Bulb Thermostat • The power element, which is the bulb, and the diaphragm are connected with a section of small tubing. • The bulb is filled with liquid and gas and then is sealed. • The pressure exerted by the diaphragm on the mechanical linkage will open and close a set of contacts. • As the bulb temperature changes, so will the pressure exerted on the diaphragm. • If the temperature of the bulb increases, so will the pressure.
Bimetal Thermostat • The heart of most types of thermostats is a bimetal element. • The element get its name from the fact that it uses a bimetal to cause the movement that open and closes a set of contacts. • A bimetal is a combination of two pieces of metal. • The metals are welded together. • Each metal has a different coefficient of expansion • If the temperature of these two metals is increased, one will become longer than the other because of the different expansion qualities.
Line Voltage Thermostat • The line voltage thermostat is designed to operate on line voltage. • This type of thermostat is used to open or close the voltage supply to a load in the system. • The line voltage thermostat lacks many good qualities that are obtained with low-voltage thermostats.
Low-Voltage Thermostat • The low-voltage thermostat is used to control systems with a 24-volt supply. • The low-voltage thermostat is used on all residential heating and air-conditioning systems and many commercial and industrial systems. • The low-voltage thermostat can be used for heating operation, cooling operation, automatic operation of fans, manual operation of fans, and automatic changeover from heating to cooling. • The difference between a line voltage and a low-voltage thermostat is in the sizing of the bimetal elements.
Anticipators • Anticipators used to give a more evenly controlled temperature range. • There are two types of anticipators: heating and cooling.
Heating Anticipator • A thermostat that has no means of heat anticipation will allow a wide swing from the desired temperature, especially on forced-warm-air systems. • The temperature difference between the opening of the thermostat and the time when the warm air is no longer being delivered to the room is called overshoot.
Cooling Anticipator • The cooling anticipator operates somewhat differently from the heating anticipator. • This type of anticipator is also known as an off-cycle anticipator. • The cooling anticipator adds heat to the bimetal on the off cycle of the equipment because of its parallel connection in the circuit. • When the thermostat contacts are close, the current takes the flow of least resistance, which is through the contacts rather than the cooling anticipator. • On the “off” cycle the current passes through the anticipator and the contactor coil.
Thermostat Installation • The installation of a thermostat is simple because all the thermostats are marked with identifiable letters, although the letters used are not consistent from manufacturer to manufacturer. • When installing a thermostat, choose a location that will be most suitable for maintaining the correct temperature in the desired area. • Thermostat installed in applications that require control of the temperature of some enclosed areas should be installed where the correct temperature can be sensed by the thermostat without interference from some other sources of heat or cold.
Staging Thermostats • A staging thermostat is designed to operate equipment at different times with respect to the needs of the structure. • The staging thermostat has more than one contact and opens and closes at different times with regard to the conditions of the area being controlled.
Staging System • Many heating and cooling systems are operated in stages because the load in some structures fluctuates a great deal. • A heating or cooling system that has been designed to operate on two different capacity levels is a staged system. • Staging systems offer many advantages because of their efficient operation. • The staging thermostat permits the equipment to operate at half of its capacity.
Operation and Types • A staging Thermostat is designed to be used on a system that has two stages of heating, cooling or both. • The staging thermostat operates on the differential in temperature between stages. • Staging thermostat can be obtained in a variety of stage configurations. • Common staging thermostats used in the industry are the one-stage heating thermostat with two-stage cooling, the two-stage heating thermostat with one-stage cooling, and the two-stage cooling thermostat with the two-stage heating.
Heat Pump • A heat pump is a refrigeration system that heats or cools by reversing the refrigerant cycle for the heating operation and then operating conventionally for cooling. • Most heat pumps use a staging thermostat to operate a set of supplementary heaters. • The first-stag thermostat operates the compressor. • The second stage operates the supplementary heat when the first stage cannot handle the load.
Clock Thermostats • A clock thermostat is used to control temperature of a structure, allowing the customer to lower or raise the temperature control point of at least one period in 24 hours. • With energy conservation and high utility bill playing a large part in air-conditioning and heating systems, clock and programmable thermostats are becoming increasingly popular.
Pressure Switches • A pressure switch is a device that open or closes a set of contacts when a certain pressure is applied to the diaphragm of the switch.