1 HVACR317 – Core for Refrigeration Operating Controls 1 Thermostats Operating Controls 1 Thermostats

2 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. 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.

3 STATS ARE 2-STATE DEVICES TEMPERATUREPRESSURE HEATING CLOSES ON TEMPERATURE FALL COOLING CLOSES ON TEMPERATURE RISE HIGH PRESSURE SWITCH OPENS ON PRESSURE RISE LOW PRESSURE SWITCH OPENS ON PRESSURE FALL

4 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. 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.

5 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. 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.

6 VARIOUS THERMOSTATS

7 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). 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).

8 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. 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.

9 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. 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.

10 Bi-Metal Elements

11 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 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). 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 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).

12 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. 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.

13 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. 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.