The well-being of your HVAC system should be your priority among the appliances and machinery you use in your home.

Why?

Because your heating and air conditioning system ensures the quality of air in your home and if the air in your home is compromised, the possibilities of you and your family being sick increases. That’s why taking your air conditioning units or heating systems for granted is not an option.

You need to constantly monitor, troubleshoot your HVAC system to minimize the health hazards and maximize efficiency. Another way to do so is by learning how these air conditioners and Heat Pumps that make residential HVAC systems and commercial HVAC systems work, especially how they are operated or controlled.

Understanding how heating and cooling systems are controlled in your home can help you tackle the AC repair situations before they become serious and today, I decided to shed some light on this topic.

Thermostat is the key

By any chance, if you don’t know what a thermostat is (which is highly unlikely if you own an HVAC system) – let me remind you.

According to the Know-It-All Wikipedia, – “A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.”

This is what a thermostat is according to Wikipedia. But in layman’s terms, a thermostat is a component that takes the thermal reading of your room. If the temperature is not what you have set for your rooms, the thermostat will ensure necessary steps are taken to reach your targeted temperature.

Let’s say you have preset your temperature to be 21°C or 70°F. That means you need the temperature in your home to be around that point which is a comfortable temperature for your current weather. Now, let’s say, the weather in your area has gone below that point and your current thermal reading in your thermostat states 17°C. Now, it’s up to your thermostat to maintain and control the preferred temperature in your room. So, in this case, your thermostat will manage to increase the temperature of your home by igniting the Gas Furnaces, heat pumps or whatever heating system you have installed in your home.

Basically, the thermostat in your home combines both the sensing or reading of temperature fluctuation and control action elements that ensures the temperature of the controlled area is consistent or set to the owner’s preference. You can say – a thermostat is a heat-sensitive switch, that monitors, controls and regulates the temperature of your home.  

How a thermostat monitors, controls and regulates the temperature using your HVAC system

Till now, I was talking about the function of a thermostat in layman’s terms. If you want to know about its function in details, this part is for you.

Let’s get going with the terminology.

Set Point: The set point is the exact point on the Celsius or Fahrenheit scale that you are comfortable with. If you set your preferred temperature to 21°C or 70°F, this 21°C on the Celsius scale and 70°F in the Fahrenheit scale is the set point for the thermostat.

Controlled system/environment: Controlled system or environment is the area where you want the temperature to be consistent or to be set at your preferred set points using an HVAC system.

Now let’s get into the functions of a thermostat –

The thermostat responds to the changes or fluctuations in the temperature of the surrounding air it is placed in. If the temperature is above the set point, it turns on the air conditioner. But if the temperature is below the set point, the air conditioner will stop and the heating system of the controlled environment will turn on.

The main working component of a thermostat is the bimetallic element it has. This element contracts or expands depending on the temperature. If the temperature of the controlled system stats to rise, the bimetallic element of the thermostat expands and lets it know that it is time to turn on the central air conditioner. On the other hand, when the temperature decreases, the bimetallic indicator element contracts and tell your HVAC system that it’s time to turn off the central ac unit turn on the heat pump.

In the older thermostats, you have 2 exposed contacts. When the temperature decreases, the bimetallic element bends and creates the first one electric contact. After that, it creates the second one which triggers the system to be fully activated. Closing this second contact turns the heating system and the anticipator of your thermostat on. This anticipator then heats the bimetallic strip element and causes it to break or bend, thus creating the second electrical contact. Till then, the first contact has not broken and the heater keeps running as the temperature rises past the preset thermal points on the thermostat.

But in recent HVAC systems, like Trane system, GoodMan system, Carrier system and other modern heating and cooling systems from HVAC companies, you have thermostats with coiled bimetallic strip element. In these thermostats, the contacts are sealed at one side of the glass which protects it from the dust, dirt and debris.

In this case, when the temperature drops, this bimetallic element starts to uncoil. This force from the exertion separates the stationary steel bar from the magnet which is attached to end of the coil. This magnet then comes close to the contact and pulls up the inner-tube contact arm. It causes the contact to close thus completing the electrical circuit. Basically, this is how the heater and the anticipator are turned on. On the other hand, when the temperature drops, the circuit breaks as the arm drops. This keeps the contacts open allowing the heater to turn off.

In a nutshell, this is how a simple thermostat in your home affects the entire HVAC system and runs it.

See the summary in this infographic – Tips On Controlling Heating and Cooling Systems For Longevity & Maximum Efficiency [Infographic]