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In a nutshell, a central HVAC system heats or cools your home by feeding heated or cooled air through your ductwork. While the process seems simple enough, getting your home to the perfect temperature requires a lot of moving parts with a lot of different components at play.

Understanding the central heating and cooling processes will help you better maintain your HVAC system. Once you know the basics, you’ll know exactly what’s going on in your home the next time you hear your HVAC device kick on.

How does central cooling work?

Air conditioners function by taking heat and humidity from your home and releasing it outside, to ultimately drop the temperature in your home. In order to complete this process, all parts of the central cooling system must work together.

Central cooling system options

In most cases, central air conditioning refers to either a split-system air conditioner or a heat pump, which both have an outdoor and indoor unit. The indoor and outdoor units work together to distribute cool air through a system of ducts in your home. Together, they are composed of five main parts: a thermostat, an outdoor unit (which holds a fan, condenser, and condenser coil), an indoor unit (which holds a fan and evaporator coil), copper tubes that connect the outdoor and indoor units, and ductwork throughout the home. While a split-system AC only cools, a heat pump can reverse the flow of refrigerant to heat the home, so the process works in reverse (more on that later!).

A packaged air conditioner is the final type of central cooling. A packaged unit works the same way as a split system AC or heat pump, except the whole unit sits outside the house; there is no equipment inside the home. The packaged unit takes air from your house through the return ductwork, cools it, and then supplies it back to the house through a second set of ductworks, called the supply ductwork.

The central cooling processes

Central cooling kicks off when a thermostat senses that the temperature in your home needs to be changed. In the case of central cooling, the thermostat will alert your cooling unit that the temperature needs to drop, and all parts will work simultaneously to get your home to your desired temperature. Once the systems are alerted, they take these steps to cool your home:

  1. A fan from the indoor portion of your AC pulls hot air out of your home.
  2. This air is filtered, so dust, lint and debris are taken out
  3. The filtered, hot air is blown over the evaporator coil, a portion of your air conditioner that gets filled with liquid refrigerant (a chemical cooling compound). The evaporator coil absorbs the heat from the air, which turns the refrigerant into a gas.
  4. The newly cooled air is pumped back into your home via ducts.
  5. While that cool air is being put into your home, the heat that was recently absorbed needs a place to go, too. So, the heated, gaseous refrigerant passes through a copper tube into the outdoor portion of your air conditioner.
  6. The refrigerant is put into the compressor, which pressurizes the gas and sends it to the condenser coil.
  7. The condenser coil releases the heat that used to be in your home from the refrigerant, turning it back into a liquid.
  8. The process starts again.

How does central heating work?

Central heating systems can work in the same general way as central cooling systems by moving air around to change the temperature in your home. Instead of moving hot air out, central heating systems move hot air in to get your home to your desired temperature. But, keep in mind, this isn’t always the case. Depending on which central heating system you own, your HVAC device might actually be producing its own heat.

Central heating system options

There are two main options for central heating systems: heat pumps and furnaces.

  • Furnaces are the most traditional form of central heating. They work by burning a natural fuel source to make heat from scratch, then distribute that heat through ducts. Furnaces work the best when the temperature outside is really low (below 40°F), because they produce really hot heat at a rapid pace.
  • Heat pumps are the most common type of central heating, because they work throughout the year in all temperatures. We mentioned heat pumps earlier in the central cooling section because they can switch between heating and cooling functions depending on the temperature. Heat pumps are ideal for mild climates, because they work best to heat and cool in moderate temperatures.

The central heating processes

All central heating processes start the same way central cooling processes do: a thermostat registers the temperature and kicks off the HVAC system.

As we mentioned before, heat pumps have the same parts as a split-system air conditioner and work in the same way when they’re needed for cooling. When the temperature drops and a heat pump needs to provide heat, the process essentially reverses itself. Once a heat pump registers the function shift, a reversing valve in the outdoor unit kicks on. This allows the heat pump to absorb heat energy from the outside air and transfer it into the home to warm it up, instead of taking heat from the home and putting it outside to cool it down. The reversing valve is never something that the homeowner needs to touch; the heat pump knows to switch functions on its own!

Furnaces, on the other hand, function in a totally different way than other HVAC products. While the thermostat still kicks off the heating process, all furnaces produce heat from a gas source, like propane. Depending on the type of unit you own, the gas source could be located in an outdoor unit or underground, but both will be connected to your home via a pipe. Once your gas source is connected, the furnace follows these steps to raise the temperature:

  1. Gas comes into the furnace through the pipe and lights the furnace burner.
  2. Cold air from your home meets the burning gas. The cold air is warmed via the gas within the furnace heat exchanger.
  3. Exhaust from this interaction between the cold air and hot gas is piped out of the furnace through a vent and then put outside your home via an exhaust pipe.
  4. A blower fan inside the furnace directs the newly warmed air through the various ducts in your home.
  5. More cold air is directed from your home into the furnace via the return ducts.
  6. The process repeats itself until your home is warmed, at which point the gas valve switches off and the furnace stops producing heat.

Controlling your central heating and cooling systems

As we’ve already mentioned, thermostats are essential components of central heating and cooling systems. Without thermostats, your HVAC system wouldn’t be able to tell when your home needed a temperature change.

Today, most homes are equipped with an electronic thermostat. These are electronic devices attached to your wall with sensors that determine if your home is at the correct temperature, then communicate that message through wires to your HVAC device. Electronic thermostats are also referred to as smart thermostats, because they can allow you to program different functions and temperatures.

Homes with older HVAC systems might have non-programmable thermostats. These systems function with either a bi-metal or metal strip. When the temperature in your home drops or rises, the strip senses the change and moves to one side, causing mercury inside the system to flow to that side. When the mercury flows to one specific side, that signals to either the heating or cooling device that it’s time to go to work.

How do smart thermostats work?

Smart thermostats make controlling your HVAC easier and more efficient. Depending on which kind of electronic thermostat you own, they can come with scheduling features, voice activation, app integrations, and more.

Scheduling features allow you to pre-program your desired temperatures based on your routine, depending on time or day of the week. This means, you can program your home to be 72 degrees during the week while you’re at work, but 68 degrees on the weekends when you’re home. After pre-programming, the thermostat will know to send signals to your HVAC device to make your home the ideal temperature for that day.

Some smart thermostats allow for voice control, so you can change the temperature without even getting up from the couch. Others can be linked to an app, so you can raise the temperature in your home while your family is out running errands but lower it before you get back from a long, hot day at the beach.

Heating and cooling installation: What to expect

Understanding heating and cooling product installation is important before you buy an HVAC system. HVAC systems should always be installed by a licensed HVAC professional and homeowners should never attempt any kind of installation or maintenance on their system themselves. But there are steps you can take before installation to ensure everything runs smoothly and your HVAC device is the perfect fit for your home.

What steps should homeowners take before installation?

  • Determine your heating and cooling needs. Do you live in a place where the temperature outside gets close to freezing? If yes, consider investing in a furnace to provide hot, dry heat quickly. Does the temperature where you live stay consistent and mild most of the year? If so, a heat pump could provide the heating and cooling you need all year long. Assess your HVAC needs based on your geographic region, yearly temperature patterns and home size to ensure you’re installing the right device.
  • Clear your schedule for installation. Most new HVAC devices take a few hours to a day to install, and it’s important that you’re home for the process in case your technician has any questions for you. In some cases, like if you’re having a furnace installed, installation could take multiple days because furnaces need gas sources and if you don’t have one on your property, your technician will need to arrange for one.
  • Hire a licensed HVAC professional. Installation should be left to a state and local licensed professional. Bonus points for your technician if they have additional credentials, like a NATE certification. Remember, whenever you’re looking for a certified technician, Trane can help you find the perfect one for you.

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