1 hvacr317 - refrigeration operating controls 1 thermostats operating controls 1 thermostats

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HVACR317 - Refrigeration

Operating Controls 1Thermostats

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Overview of Thermostats

• It is the most basic control device for the Air Conditioning system. If the customer turns down the thermostat, the A/C should come on.

• In most split systems (a 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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Overview of Thermostats

• The thermostat is the most frequently changing device in the HVAC industry; but all do the same thing – turn a unit on and 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 are Two-State Devices

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Characteristics of Thermostats• A thermostat is a switching device. • It 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 Styles of Thermostat

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Characteristics of Thermostats• Direct Digital Controls (DDC) are most

often used in large offices, or in 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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Sensing Elements 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.

• There are now three types of sensing elements that are used in thermostats. They are bi-metal, remote bulb, and solid state.

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Bi-Metal

• Characteristics:− 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

• Characteristics, cont’d.− This movement of the metal and

the contacts can move a mercury bulb or bring a magnet on the contacts together.

− The thermostat must have a way of making a good connection with the contacts. This is called “snap acting.”

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Bi-Metal Elements

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Remote Bulb

• Characteristics:− 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 on

a diaphragm, and this in turn closes or opens a set of contacts.

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Solid State

• Characteristics:− 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.