The Evolution & Benefits of HVAC
HVAC: What is it?
The field incorporating heating, ventilation, and air conditioning (HVAC), is collectively known as the HVAC industry. All indoor spaces, both public and private, are equipped with varying HVAC methodologies to achieve a comfortable environment and a suitable level of breathable fresh air. This technology would allow you to maintain a comfortable temperature regardless of the ambient temperature in a particular location, which is subject to seasonal variations in temperature. An HVAC system controls the temperature of a given area by communicating with a thermostat, which is programmed to adjust the temperature based on a person's preferences, or by controlling it directly through a remote control. This field and the technology utilized to achieve daily comfort span nearly every facet of our life and can be found in cars, spaceships, and spacesuits, to name a few places.
Heating and cooling methods are commonly misunderstood or entirely foreign to most individuals since most are only interested in the desired outcome (hotter or colder air) instead of the inner workings of the systems achieving this outcome. Depending on the heating/cooling application, the technology available, and the environment to be conditioned, the equipment can dramatically vary in physical size and mode of operation. On top of this, brand variations make it so many different units can achieve similar outcomes, albeit looking entirely different in shape and design. Previously, HVAC technology was something meant to be felt but not seen. Older units were unsightly and bulky, and it was easy to tell if someone had air conditioning by just looking at their window. With the advent of smart home technology and the ever-improving HVAC equipment available, this is beginning to change.
HVAC: Evolution of Technology.
With the advancement of HVAC technology, everything has become smaller, which is better from an aesthetic standpoint as it allows our windows to remain functional, prevents the spread of viruses, and is more energy efficient. Window units only have a “cooling” option. Usually, we have to rely on building management to supply heating through gas/oil boilers, and oil boilers are known to be more terrible for the environment than gas. However, gas is still not an environmentally friendly option. Still, as HVAC technology evolves, we now have environmentally friendly options such as heat pumps which are a major component in the HVAC industry and don't rely on fossil fuels. With these options, our individual fossil fuel consumption is dramatically reduced, albeit the electricity being developed and supplied nationally still utilizes fossil fuels at the power stations.
HVAC: How does it work?
There is no physical cooling component in HVAC systems, as heat is transferred using physics. This process is facilitated by the refrigerant, a chemical that enables the operation of the heat pump's components. The heat pump's components will not be able to transfer heat without the presence of a refrigerant. To produce desired heat and cooling effects, the refrigerant in the system goes from a liquid state to a gas state. This process of heat transfer utilizes energy, and 50 % of our electricity bills come directly from this technology. Pairing the system with a smart thermostat that automatically regulates temperature ranges prevents the use of unnecessary electricity to get desired effects of the temperature within the space and only turns on the systems when needed, as opposed to keeping these systems operating all day. Smart thermostats will not be compatible with the older style window units, so upgrading your system is critical to achieving those savings.
As these systems operate, they purify the air by employing an air filtration process built into the indoor unit. All indoor units come with a standard filter, and these filters can be swapped out with different filtration ratings that would filter viruses relative to the ratings. The standard MERV-8 air filter that comes in your unit does filter the air but only to a certain extent. To achieve maximum virus-protecting benefits, you must acquire MERV-13 filters or higher, which is better for trapping potentially harmful nanoparticles. When considering switching out the filters for a better one, you will need to assess the compatibility of the upgrade to your current system. The best way to do this is to consult your HVAC professional; there are many things to consider when making the switch. A few considerations are the filter size and how powerful the system is to accommodate a more tightly packed filter. If you went from a MERV-8 to MERV-13 without much design consideration, you would experience a drastic decrease in airflow from your system unless the system has a sufficient amount of static pressure. However, most systems will be able to withstand the filtration differences. The higher the MERV rating, the higher the static pressure necessary to maintain the same volumetric flow rate. There exist MERV filters rated higher than the MERV-13 that will most likely require more drastic system changes.
HVAC: Sequence of Operations.
A typical modern system (heat pump) operates in the following order: the user selects the desired temperature and mode on the thermostat or remote control. The controller will determine whether there is a difference between the set and measured temperature in the room. The reading will signal the compressor (located in the outdoor unit) to turn on; once the compressor has built enough pressure, the electronic expansion valve will begin to modulate, allowing refrigerant flow to occur. Once the specified temperature has been reached, the expansion valve closes, and the compressor is turned off. The cooling cycle can be seen above in the image on page 2, and the heating cycle is similar, but what effectively happens is the condenser and evaporator “trade” places. The direction of the refrigerant flow (to the condenser first or the evaporator first) is modulated by the 4-way valve in the condenser and effectively allows heat pumps to do both heating and cooling.
During heat mode, the indoor unit fan will not begin cycling until the indoor coil temperature reaches a specific temperature, allowing heat to accumulate. During cooling mode, the fan turns on immediately to remove heat from the air. As the refrigerant passes through the coils, they either absorb heat from the environment (changing from liquid to gas) or release heat into the atmosphere (changing from gas to liquid). When the compressor is turned on, the outdoor fan also turns on, and the refrigerant flows in only one direction, depending on the mode set. A reversing valve (AKA 4-way valve) alternates the flow inside the heat pump system. The condenser will dump cold or hot air into the atmosphere, depending on the mode of operation in which the system is acting.
One undesired outcome of this comfort is the production of condensation (water) in the condenser and evaporator, depending on the time of year. This consideration isn’t as serious for the condenser since the water it produces simply drains below wherever it’s installed, but this isn’t the same for indoor units. HVAC technicians often collaborate with plumbers to drain the system’s condensate properly. If this isn’t possible, they usually have to find the best way to route the water to the exterior of the premises as safely as possible. In some cases, undesired water buildup can damage property or temporarily lock a system out of operation. Although it is not common, if the units are kept in good condition (coils kept clean), this phenomenon can be beneficial, as the water produced will be distilled and reusable, extracted directly from thin air.
HVAC: Related Physics & Chemistry Concepts.
To understand HVAC systems operate, it is critical to understand a few fundamental concepts.
Heat transfer- This is the process by which heat is transferred from one object to another by several possible means. The three transfer modes are radiation, conduction, and convection, which all accomplish the same thing but differently. Heat transfer by conduction requires the bodies to be in physical contact with one another, whereas convection does this through fluid traversing a body. Both modes require a temperature gradient between the two or more bodies, and the transfer always goes from the hotter object to the colder object if no outside forces are involved. Radiation, however, doesn’t require a temperature difference since this mode of energy transfer utilizes the electromagnetic wave spectrum to carry the energy from one source to another and is usually the source of energy we wish to eliminate in the summer.
Thermodynamics - The study of heat and its relationship with other forms of energy (mechanical, electrical, chemical). Utilizing several concepts from this branch of science, engineers created heat pumps, which effectively remove or add heat from one location to another. They do this by cycling a refrigerant (fluid contained in a closed-loop copper system) between indoor and outdoor units, providing heating/cooling where necessary. Traditional air conditioners only utilize the refrigeration cycle in “one direction” to provide cooling, but by running the cycle in reverse, they provide the opposite effect, heating. Engineers design components to make these systems more efficient every year. They are now the prime means of heating and cooling for residential and commercial locations.
Fluid Mechanics - The study of fluids in various states and their reaction to forces acting upon them. When a fluid goes from a liquid to a gas (evaporation), it absorbs a tremendous amount of heat to undergo this phase change, and this heat is rejected at the condenser (outdoor unit) during cooling. As the name suggests, the refrigerant condenses at the condenser when the system is cooling, turning it back into a liquid. This phase change from a liquid to a vapor mixture and back again to a liquid is what modern HVAC systems monitor while they are operating. Two mechanical components control these changes: the compressor (heart of the system) and the EEV, which meters the rate at which the phase is allowed to change its state. As counterintuitive as it sounds, during heating, refrigerant colder than the ambient temperature picks up all the heat in the cold outside air and carries it inside. This is done by utilizing the same heat transfer/Thermo concepts; refrigerant can reach such low temperatures that outside air would still be “hot” in comparison. Other notable phase changes in HVAC systems are the condensate (water in the air in contact with cold coils condenses the water into liquid droplets) in the indoor and outdoor units and along the piping if contractors don’t take care to insulate the lining.
Chemistry (Phase transition of Refrigerant) - R22, R410A, now R-454b with a lower global warming potential starting in 2023. Refrigerant is a compound that boils at -15 degrees Fahrenheit (-26 degrees Celsius) and cools at -154 degrees Fahrenheit (-103 degrees Celsius). This is different from water which boils at 212 degrees Fahrenheit (100 degrees Celsius) and cools at 32 degrees Fahrenheit ( 0 degrees Celsius). This is why it is utilized, but it is toxic, so it needs to be handled by professionals. Some are flammable, others are not, so depending on what system is being used, there may be additional considerations to maintain safety.
HVAC: Components of the System.
The following components of the system execute fundamental concepts.
Heat Exchanger - Heat pumps primarily utilize copper coils, piping, and fittings as the conduits for the refrigerant in the system. The coils are made of one continuous pipe and are bent sinusoidally to increase the surface contact area between the coil and the air passing over it. Fins are added to these coils to increase the surface area further, but these are typically made of aluminum which has more desirable heat transfer properties.
Blower Motor - Both indoor and outdoor units have motors that pass the air over the coils.
Combustion Chamber - Needs the perfect amount of fuel, heat, and oxygen for combustion. A flow of air must be provided to filter out the carbon monoxide that the combustion creates to ensure a safe breathing environment, such as a chimney or gas vent. (only applicable if a furnace is installed in tandem with an old conventional air conditioner).
Compressor - The heart of the system; as the name suggests, it compresses refrigerant and produces the pressure gradient necessary for the refrigerant to flow. Whether in heating or cooling, the compressor’s job doesn’t change. In cooling, it behaves exactly as it does in heating. In heat mode, it doesn’t produce heat by compressing the refrigerant. Still, instead, the refrigerant absorbs heat from the ambient air moving over the condenser coil and transfers it to the indoor coils, which then transfers this heat to the indoor air. The cycle is reversed in cooling, and it’s worth noting that the naming convention (condenser and evaporator) is only accurate when the system is in heating, as the coils “trade” places when the mode switches. (i.e., in heating, the indoor coils are condensers, whereas they are evaporators in cooling).
Condenser Coil or Condenser and Evaporator Coil - Evaporator and condenser coils are the inverses of each other as they function in opposite ways. In a split system, refrigerant cycles through the system, reaching the evaporator and condenser coils in a continuous loop. In cooling, the indoor unit is an evaporator; in heating, it’s a condenser because it refers to the substance inside the piping. Condensing happens in the outdoor unit when cooling, and evaporating occurs in the indoor unit when it is cooling. Both are always on, and the refrigerant chemical goes from the indoor unit to the outdoor unit and will be conserved as long as there is no leak present. Systems will not operate properly with low refrigerant levels, but this can be corrected by a technician and verified by reading dead pressures and operating pressures. When the pressure is “dead,” that usually refers to the system being off. As air passes through one of the coils, heat is transferred to heat or cool the surrounding environment; as mentioned earlier, the refrigerant held within the coils determines the air's temperature depending on the phase of refrigerant within the coil.
Filters - The filter serves as a filtration for the air; filters start at MERV 8 and will increase as they become more efficient in filtration; the more efficient (higher rating) the MERV filter, the likelihood of being exposed to potentially harmful nanoparticles is decreased. The higher the filtration level, the more air resistance, the more air resistance, and the more the flow motor in the indoor unit will need to operate to produce an equal amount of airflow. The type of filtration a system will require may require more design consideration at the beginning of an installation because of the flow motor capabilities and compatibility. Most systems will come with a MERV 8 filter, which can be removed from the indoor unit and washed. Not cleaning these filters can overwork your unit and blow the motor on the indoor unit because there is an increased air resistance similar to a more advanced MERV filter which is why this more advanced filter cannot be installed on any unit and is expected to work at full capacity. This airflow calculation is measured in cubic feet-per-minute CFM and helps determine the specifications required to operate efficiently in a given environment.
Thermostat - This device monitors and controls the temperature of an environment where a system may be installed. The device serves as a brain to condenser and evaporator coils. The settings on the thermostat will have a range; these devices must be compatible with your system and should meet the same specifications so it operates properly. For example, single-stage systems require a single-stage thermostat, and dual-stage systems require a dual-stage thermostat.
Single v. Double-stage system.
Single-stage is either on or off and will run at 100 % capacity; this stage is considered less efficient but will cost less. Parts will be readily available for a system like this and won't require reaching out to a manufacturer.
Dual-stage will run at different speeds; this stage is considered more efficient but will cost more initially, but in the long run, it will be better for your pocket. They will run at 70% capacity in the first stage and 100% capacity in the second stage; more often than not, the systems operate on the first stage, and you will see the savings from the non-utilization of the spare 30% in your capacity. The dual-stage system is better for homes with multiple thermostats and zoning for different temperature variations. The stages allow you to break up your environment into sections, so the system is not running as a whole when it only needs to operate for one section. Parts for the dual-stage may need to be purchased from the manufacturer, but there will still be some basic parts you can buy at your local HVAC store, or an HVAC technician will likely have them. Since the system takes less energy, it is also quieter.
HVAC: Why is HVAC on the rise?
HVAC technology offers many benefits, including energy efficiency, preventing the spread of viruses, and becoming less bulky as time goes by. Global warming has led to the Department of Energy implementing minimum efficiency standards in 2023, which promote upgrading old technology that consumes enormous amounts of energy; as mentioned earlier, HVAC accounts for 50% of total energy consumption. Owners may also consider upgrading systems because older systems take R22 refrigerant, which is also not being made available anymore because of the known negative effects the refrigerant has on the ozone layer. Manufacturers are also innovating many existing models to become more efficient, which will raise equipment costs in 2023; the increase in price and the new requirements imposed by the DOE will drive the HVAC market next year. You can make use of rebate programs specifically designed for upgrading older systems. Most rebate programs will pair you with a few qualified contractors within your area.
Several prevention measures were also explored during COVID-19 to prevent the spread of the disease. Frequent use of HVAC was associated with reduced positive results in neighborhoods. A significant part of the reason for this disparity in HVAC is the price tag associated with the technology; however, something as essential as HVAC must be provided in all neighborhoods irrespective of its cost. HVAC technology will eventually become a basic necessity, and local governments will develop programs to provide it to low-income households as a result of further research.
Works Cited
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U.S. Department of Health & Human Services. “Improving Ventilation in Your Home.” CDC, https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/improving-ventilation-home.html. Accessed 10 December 2022.
U.S. Department of Health & Human Services. “Ventilation in Buildings - Ventilation in Buildings.” CDC, https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html. Accessed 10 December 2022.
U.S. Environmental Protection Agency. “Can running the HVAC system in my home help protect me from COVID-19? | US EPA.” EPA, 3 March 2022, https://www.epa.gov/coronavirus/can-running-hvac-system-my-home-help-protect-me-covid-19. Accessed 10 December 2022.
U.S. Environmental Protection Agency. “What kind of filter should I use in my home HVAC system to help protect my family from COVID-19? | US EPA.” EPA, 21 March 2022, https://www.epa.gov/coronavirus/what-kind-filter-should-i-use-my-home-hvac-system-help-protect-my-family-covid-19. Accessed 10 December 2022.
Weinberger, Dawn, and Lexie Pelchen. “All About HVAC: What It Is, How It Works, And What Does HVAC Stand For? – Forbes Home.” Forbes, 25 July 2022, https://www.forbes.com/home-improvement/hvac/how-do-hvac-systems-work/. Accessed 10 December 2022.