• SkillCat Team

All about Thermostat

Updated: Jul 8


Basic Electrical Systems and Controls: Chapter 3


Thermostat


In this module, we will study the control system behind the thermostat. We will learn the flow of control in the system and study the heating and cooling modes of operation. Skip to quiz!


Temperature Control


Recall that the thermostat is a device that the user can manipulate to change the temperature of a room. The thermostat senses the room temperature and performs actions to keep the temperature near the desired point.


The thermostat reads the room temperature with the help of temperature sensors. Depending on the set temperature, the system will either turn on or off the compressor, outdoor fan, and blower fan.


Generally, the compressor, outdoor fan, and blower fan will start at the same time. Some HVAC systems start these components at different times. This order will then typically be:

  • Indoor fan motor,

  • Compressor, and then

  • Outdoor fan motor

Ambient temperature is the temperature measured by the temperature sensor. If the ambient temperature is below the set temperature, the heat pump will turn on. As soon as the ambient temperature reaches the set temperature, the heat pump will turn off.


If the ambient temperature is above the set temperature, the hair conditioning system will turn on. As soon as the ambient temperature reaches the set temperature, the air conditioning system will turn off. Next, we will take a closer look at the heating mode of operation.


Heating Mode

As we have learned, the thermostat controls the operations of heating and cooling. By selecting the heat option on the thermostat, the HVAC system will enter the heating mode.


When the heating mode is activated, the reversing valve is triggered, and the flow of refrigerant is reversed from the evaporator to the condenser. The condenser will act as an evaporator, and the evaporator will act as a condenser. The system will begin heating the inside of the home.


When the heating system is turned on, the compressor, condenser fan motor, and evaporator will all turn on simultaneously. In some systems, these components will start within a few seconds of one another.


The role of controls in the heating mode is to deliver the desired temperature. The controls in the heating mode are:

  • Switching from cooling mode to heating mode,

  • Increase or decrease the temperature of the room,

  • Control the speed of the fan motor, and

  • Set the timer

When the user sets the temperature, the thermostat sends a control signal to the furnace control board. Temperature sensors will also send control signals to the control board. The control board will compare these two signals. If heating is required, the control board will send a signal to the fuel valve.


The fuel valve will begin to supply gas to the system. The control board will then send a signal to the igniters. The igniters will start, and the gas that passes will be lit. The flame will provide heat to the system.


The system will maintain its temperature with the help of temperature sensors. To prevent equipment from constantly turning on and off, the thermostat allows the room to stay in a small range of temperatures around what is set. We will call this range of temperatures the wiggle room.


By incorporating wiggle room, the thermostat will allow the temperature to go slightly beyond what is set before turning the system back on. The system will be more efficient and save money. We will now take a look at the cooling mode of operation.


Cooling Mode

The thermostat features a button that can turn the air conditioning system into cooling mode. Like in the heating mode, the thermostat will use temperature sensors to get the system closer to the temperature set on the thermostat.


When the cooling system is turned on, the compressor, condenser fan motor, and evaporator will all turn on simultaneously. In some systems, these components will start within a few seconds of one another.


The role of controls in the cooling mode is to deliver the desired temperature. The controls in the cooling mode are:

  • Increase or decrease the temperature of the room,

  • Control the speed of the fan motor, and

  • Set the timer

When the user sets the temperature, the thermostat sends a control signal to turn on the compressor, outdoor fan motor, and indoor blower fan. The cooling system will now be in operation and will not stop until the temperature sensors read that the room temperature is the same as desired.


As in the heating mode, the thermostat allows the room to stay in a small range of temperatures around what is set. We call this range of temperatures the wiggle room.

Wiggle room is used so that the system does not have to turn on and off constantly. The system is efficient and saves money.


The thermostat is a device that the user can manipulate to change the temperature of a room. There are two modes of operation:

  • Heating mode, and

  • Cooling mode

Both the heating and cooling modes of operation make the use of controls to perform their function. The system allows for wiggle room between the actual temperature and the desired to prevent the system from constantly turning on and off.



Question #1: The thermostat senses the room temperature and performs actions to keep the temperature near the desired point.

  1. True

  2. False

Scroll down for the answer...











Answer: True

True, the thermostat senses the room temperature and performs actions to keep the temperature near the desired point.


Question #2: Which of the following is not one of the components activated by the thermostat.

  1. Evaporator coils

  2. Compressor

  3. Outdoor fan

  4. Blower fan

Scroll down for the answer...










Answer: Evaporator coils

Evaporator coils, the evaporator coils are not a component activated by the thermostat.


Question #3: Which of these components is not turned on when the heating mode is activated?

  1. Oven

  2. Compressor

  3. Condenser fan motor

  4. Evaporator

Scroll down for the answer...













Answer: Oven

Oven, the oven is not turned on when the heating mode is activated.


Question #4:The control board compares the signals from the thermostat and the temperature sensors and decides whether or not to begin heating.

  1. True

  2. False

Scroll down for the answer...













Answer: True

The control board compares the signals from the thermostat and the temperature sensors and decides whether or not to begin heating.


Question #5: Which of these components is not turned on when the cooling mode is activated?

  1. Refrigerator

  2. Compressor

  3. Condenser fan motor

  4. Evaporator

Scroll down for the answer...













Answer: Refrigerator

Refrigerator, the refrigerator is not turned on when the heating mode is activated.


Question #6: The system allows for wiggle room between the actual temperature and the set temperature to avoid constantly starting and stopping.

  1. True

  2. False

Scroll down for the answer...












Answer: True

Evaporator coils, the system, allows for wiggle room between the actual temperature and the set temperature to avoid constantly starting and stopping.

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