Technical Papers

The Ins and Outs of HVAC

Monday, June 10th, 2019 by Starcom Intern



The Ins and Outs of HVAC

Have you ever wondered what exactly your heating, ventilation, and air conditioning (HVAC) system, is doing? We know it keeps us comfortable by keeping our homes at the temperature we want, but how? Most of us have an HVAC system; in fact over 85% of American homes have an HVAC system that keeps them satisfied (Hadhazy, 2013). Every one of those people knows how important the HVAC system is. The minute it is not working right, people notice. And yet, the majority of people have no clue how an HVAC system works or how they can get the most out of it. Here are the ins and outs and everything you need to know to get familiar with your HVAC system.


An HVAC system is comprised of three main components. Two components are installed  - Image 1inside your home; the ducts that are in the walls, and the inside unit (figure 1). The last is the unit installed outside your home (figure 2).


Let’s start with the simplest of the system’s parts, ventilation. When a house is built, air ducts typically made of steel are installed in the walls and ceilings. There are two different kinds of air ducts in a home; return, and supply ducts. Return ducts bring air from inside your home to your HVAC system. Supply ducts disperse the treated air throughout your home. These ducts should be strategically placed as to supply the HVAC system with enough air to heat and cool, as well as supply the house with enough treated air.

A ventilation system that does not supply enough air to its units will “choke” the system. This is like asking you to only breathe through a straw every day. This would certainly lead to some complications that will hinder your performance. The same will happen to your HVAC system. Your system will have to work longer and harder to bring your home to a pleasant temperature, leading to higher energy bills, more frequent breakdowns, and a shorter lifespan for your system. According to the National Comfort Institute, around 25% of typical American homes have less return air than required and their ventilation system works at 57% efficiency. Make sure you work with your HVAC technician to figure out the most efficient way to set up your ducts.

Air Conditioning

Now that you know how air is distributed through your home, let’s find out how the air gets cooled. An air conditioning system works on the basic premise of removing heat and moisture from the air inside a home and expelling it outside. This leaves only cool air to be circulated through the home by the ventilation. Refrigerants are used to cool the air while it is passing through the system. The two most commonly used refrigerants are R-22 and R-410a. These particular chemicals are used because of their ability to change phases extremely easily. R-22 is an older refrigerant that is being phased out for a more environmentally friendly option. R-22 and R-410a are hydrochlorofluorocarbons, or HCFCs, which are very harmful to our ozone layer (Hadhazy, 2013). It is vital that your HVAC technician be skilled and professional as there are mistakes that can have deadly consequences. Fumes like carbon monoxide are produced in heating mode and need to be brought outside the home. If refrigerant is accidentally ignited, phosgene gas is produced and if inhaled, can cause death. Refrigerants must go through four main components that allow the air in your home to get cooled. Below is a diagram (figure 3) that lays out the general path of refrigerants. As you get read further, you will understand the diagram better.

  - Image 2

The Evaporator

The evaporator, or the evaporator coil (figure 4), is a piece of machinery that is installed in your home on top of your furnace or inside an air handler.  - Image 3When refrigerant flows through the evaporator coil, it starts as a cold liquid. As it travels through the coils, it is absorbing heat from the passing air, thus evaporating into a gas. This phase change from liquid to gas causes heat to be pulled from the air brought into the unit by return ducts. This leaves only cooled air to be distributed throughout the home through supply ducts. The key in this mechanism is that the coil is colder than the air passing through it, not to pass on its coldness, but so that it has the ability to absorb heat from the passing air (Silberstein, 2012). If you have pets, be sure to change out the air filter located in your return duct every month as it will get clogged fast. If the filter gets clogged, air will be forced around the filter if possible and not get filtered at all. This will lead to a clogged coil full of pet hair and dust. A clogged coil has many risks. For instance problems such as reduced efficiency, higher energy costs, higher chance of mold growth, and unpleasant odors are all problems associated with a clogged evaporator coil (Fink, 2018). If you are having trouble with mold and bacteria growth, you may want to look into having a UVC germicidal lamp installed into your plenum. These are special lamps that use UV light to kill nearly all bacteria.

The Compressor

After the refrigerants pass through the evaporator coil, the next step is the compressor. The compressor can be thought of as the heart of the system. It is responsible for pumping refrigerants through a system and is located outside of the house in the outside unit. The refrigerant gets brought to the compressor by copper tubing; specifically the suction line. This is the larger of the two copper tubes and should be cold and moist to the touch. It receives refrigerants as a cold gas, and then compresses the gas into a hotter and higher pressure version of itself. This creates a pressure difference which is vital for the flow of refrigerants. The pressure difference is constantly trying to equalize, therefore the refrigerants will flow (Silberstein, 2012).

The Condensor

Next, after the refrigerants pass through the compressor, they go into the condenser coils (figure 5) as a hot gas. The condenser coil has a fan fixed to the top of the unit. The fan will suck air into the  - Image 4unit through its sides, around the hot condenser coils, then out the top of the unit to release heat from the hot refrigerant into the outside air. This heat transfer causes the refrigerant to condense back into a warm liquid, then eventually to a cold liquid. As you can see in figure 5, it is vital that you keep the fins that direct air around the coils clean. Otherwise they may get clogged and impede the process of transferring the refrigerant’s latent heat to the outside air. 

The Thermostatic Expansion Valve (TXV)

After refrigerant goes through the condenser coil, it goes back into the house through the smaller copper pipe, called the liquid line. This is because by the time the refrigerant gets back to the smaller copper pipe, it has cooled off enough in the condenser to become liquid once again. But before it goes to the evaporator coil, it must go through one small device. This device goes by many names. The thermostatic expansion valve, the thermal expansion valve, TXV, TEV, or TX valve (figure 6), is an extremely important yet very small device. The device itself is located in the evaporator coil box on top of your furnace. It regulates how much liquid refrigerant is passing through the evaporator coil at a time. This assures that the heat transfer occurring in the evaporator is enough to evaporate all refrigerant present. Since liquid cannot be compressed, if any liquid refrigerant were to make it back to the compressor, it would cause serious problems. After the TXV releases refrigerant into the evaporator, the cycle starts anew.

 - Image 5

Moisture Removal

Air conditioners not only cool air, they condense water out of that air as a byproduct. When you remove heat from air, you take away the air’s ability to hold water. This is because warm air has a higher energy level with very fast moving molecules suspended in it. When the air is cooled, the molecules slow down; so much so that they eventually have such little energy that they condense into water droplets (2015). This works out very well for home comfort as high humidity levels can be uncomfortable. In 1902, the first modern styled air conditioner was made by Willis Carrier. He was attempting to reduce the humidity in the printing plant that he worked in. As a byproduct, he got cold air from his machine. He realized the possible benefits and turned his dehumidifier into a marketable air conditioner (Oremus, 2013)

Condensation happens inside your home at the evaporator coil. When the evaporator is removing heat and making cold air, it wrings out moisture from the air. This moisture condenses into water and collects in a pan under the evaporator. This pan drains to an opening designed to have PVC pipes installed to drain the water to a pump, a sink, or straight out of the house. As discussed in the evaporator segment, evaporators can get clogged with dust, grime, and pet hair. Sometimes clumps of these contaminants will fall into the pan under the evaporator and eventually block water flow into the PVC pipe. When this happens, water will accumulate wherever it can. Whether it be into the furnace, possibly frying electrical boards, or down into your home. This can cause extensive damage to your home through flooding and mold growth. Most AC systems have water switches installed as a safety precaution. If a water switch senses water is where it should not be, it will send a signal to the furnace or air handler to turn off the system so no more damage can be done.


 - Image 6

Heating is a simpler mechanism. There are a few ways to heat your home; gas or oil furnace, boilers, and heat pumps. Machines that heat your home tend to go through fewer steps than those that cool your home. Figure 7 shows a basic diagram of the different heating systems.

Gas/Oil Furnace

Your furnace will turn on if the thermostat connected to it recognizes the temperature in your home is lower than requested. The thermostat will send a low voltage signal to your furnace to open the gas valve. This starts releasing natural gas or oil into the burners and also turns on the blower. The burners are where fuel and air will be combined. Then the furnace’s pilot light will ignite the fuel/air mixture and blow fire into the heat exchanger. The blower then forces air around the heat exchanger to absorb heat and be dispersed throughout the house through its ventilation system. The heat exchanger can be thought of as the opposite to the evaporator coil in the air conditioner. Through the combustion of fuel, the furnace creates noxious fumes like carbon monoxide. These fumes are expelled outside the house by the flue, piping made of PVC or steel usually coming out the top of your furnace. The fumes are pumped through the flue by a draft inducer motor. The draft inducer motor turns on before the gas can burn; creating a small vacuum that pushes the fumes out the flue.

Since furnaces can have potentially deadly consequences if a mistake is made during installation, theyThe Ins and Outs of HVAC - Image 7 have multiple safety devices preinstalled. There is a pressure switch that assures the draft inducer motor has turned on. There is a small diaphragm that gets pulled in when the inducer motor is on. If the pressure switch recognizes the diaphragm has been sucked in, it will allow fuel to flow (Benetti, 2015). There are also sensors to assure a fire has been lit. Otherwise fuel is being pumped into the furnace and accumulating. If a fire eventually did start, an explosion could happen. A thermocouple is a sensor that recognizes if the pilot light is lit. If a hot surface igniter is being used instead of a pilot light, a flame sensor will be used to assure the fire in the burners is burning. If not, the system will turn off. If a furnace gets too hot, a limit switch will turn off the system. For example, if the blower motor broke and did not circulate air through the system, there would be no heat exchange and the furnace will overheat. Furnaces also have a slot usually located just outside the bottom on the side of the return duct meant for air filters. These filters should be changed every few months. To give you a good idea of what these parts look like, figure 8 shows many of them.

Hot Water and Steam Boiler

Instead of using air to heat a home, boilers use water and steam. These systems are often called hydronic systems. The mechanism in the actual boiler is very similar to a furnace. Fuel is lit and shot into a combustion chamber which heats a heat exchanger. The heat exchanger passes on heat to water passing through pipes to be sent throughout the house. Instead of a fan, boilers use pumps to circulate hot water or steam. Some systems circulate hot water in the floor. This is called radiant floor heating.

Heat Pump

A heat pump is basically an air conditioner but running backwards. As we established, air conditioners take the heat away from the air inside a home and sends it outside through the outside unit. A heat pump has this capability, but it can also run in reverse. Meaning, it will reverse the flow of refrigerants and start taking in heat from the outside air. Even when it is very cold outside, there is still heat energy present. This heat is then brought into the house carried by the refrigerants. Once in the evaporator coil the refrigerants can transfer its heat into the air being blown through by the blower. When a heat pump is installed, a furnace is not needed. Instead, an air handler is installed in the house. This has the evaporator coil, the blower, and a backup electric heater installed. The electric heater is used if there is not enough heat energy outside to be extracted and brought into the house. It is a set of coils located above the evaporator that gets red hot through electricity.


Now that you know all that you do, it will be much easier for you to keep your HVAC system running smoothly. Though, it is important to remember that you contact your local HVAC technician if you believe there is a problem with your system. If you open a unit and attempt to repair it yourself, you could get shocked or accidentally let out noxious fumes into your home. So remember to leave it to the professionals so you and your HVAC system are happy.



Works Cited

Benetti, A. (2015, November 16). Gas Furnace Components and Parts Explained. Retrieved June 4, 2019, from

Fink, R. G. (2018, November 27). Mold/Bacteria Protection of A/C Coil. Retrieved June 3, 2019, from

Hadhazy, A. (2013, August 06). Science of Summer: How Does Air Conditioning Work? Retrieved June 3, 2019, from

How is it that warm air is able to hold more water vapour than cold air? (2015, October 23). Retrieved June 4, 2019, from

Oremus, W. (2013, July 15). A history of air conditioning. Retrieved June 4, 2019, from

Pippin Brothers. (2013, March 11). Is Your HVAC System Wasting Money by Breathing Through a Straw? Retrieved June 3, 2019, from

Silberstein, E. (2012). “Section 1 Evaporator.” Residential Construction Academy: HVAC, Second Edition. Clifton Park, NY: Thomson Delmar Learning, 27-28. Print.

Silberstein, E. (2012). “Compressors.” Residential Construction Academy: HVAC, Second Edition. Clifton Park, NY: Delmar,. 29-33. Print.

Vandervort, D. (2018, March 8). How a Gas Furnace Works. Retrieved June 3, 2019, from

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