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HVACR317 – Core for Refrigeration
Operating Controls 1Thermostats
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Thermostats
• The most basic control device for the Air Conditioning system.
• This is where it all starts – Customer turns it down, A/C should come on.
• In most split systems (Split system is where there is an air handler inside and a condenser outside connected by refrigerant piping) the thermostat is 24 volts.
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STATS ARE 2-STATE DEVICES
TEMPERATURE PRESSURE
HEATING CLOSES ON TEMPERATURE FALL
COOLING CLOSES ON TEMPERATURE RISE
HIGH PRESSURE SWITCH OPENS ON PRESSURE RISE
LOW PRESSURE SWITCH OPENS ON PRESSURE FALL
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Thermostats
• The thermostat is the most frequently changing device in the HVAC industry.
• But they all do the same thing – turn it on, and turn it off at the proper temperature.
• Basic function of the thermostat is to respond to a change in temperature by opening or closing a set of contacts.
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Thermostats
• A thermostat is a switching device. • Thermostats can be line voltage,
control voltage, or direct digital.• Line voltage are most often used in
electric heat applications as well as refrigeration.
• Control voltage thermostats are used mainly in residential or small office applications.
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VARIOUS THERMOSTATS
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Thermostats
• Direct Digital Controls (DDC) are most often used in large office, commercial or public buildings. If time allows at the end of this term we will talk more about DDC.
• For the purpose of this AC 422 term we will be discussing Control Voltage (low voltage thermostats).
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Thermostats
• There are now three types of sensing elements that are used in thermostats.
• The controlling part of the thermostat is the part that moves or causes the contacts to close when the thermostat senses a change in temperature.
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Thermostat Controlling Elements• Bi-metal:
– Contain two pieces of dissimilar metals that are welded together.
– These metals expand or contract at a different rate.
–When heated or cooled they bend and move the contacts closer together or further apart.
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Bi-Metal Elements
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Thermostat Controlling Elements• Bi-metal:
– This movement of the metal and the contacts can move a mercury bulb or bring a magnet on the contacts together (see figure 12.10 in your electrical book).
– The thermostat must have a way of making a good connection with the contacts. This is called “snap acting”. (see figure 12.8 in your electrical book).
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Thermostat Controlling Elements• Remote Bulb:
– Has a bulb filled with a liquid and/or gas.– The liquid and gas expand, travel down a
capillary tube.– The expanded material puts pressure
onto a diaphragm and this in turn closes or opens a set of contacts.
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Thermostat Controlling Elements• Solid State:
–Most digital or programmable thermostats use a solid state control.
– Uses a material that changes resistance based on temperature.
– Knowing the resistance the thermostat can judge (and display) the correct temperature.
– Longer and more accurate life span, no moving parts.