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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.