Everything you need to know about your HVAC

by: Anthony Kinslow II, Ph.D., CEM

Learn tips and tricks on how to save money, by maximizing the energy efficiency of your HVAC.​

For the majority of Americans, the largest energy consuming equipment in their home is their Heating, Ventilation, and Air-Conditioning (HVAC) system. Whether you are in the north and your HVAC unit is primarily used for heating or the South and your HVAC unit is primarily used for cooling, it is more than likely your largest energy hog.

The following is organized into three areas

• Five (5) little known facts about your Air-Conditioning
• How does a HVAC system work?
• How Your Air-Conditioning impacts the environment

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Five (5) little known facts about your Air-Conditioning

1. Whenever your A/C comes on it comes on full blast. Units purchased after 2018 may have a Variable Frequency Drive (VFD) that adjust the power based on the load, saving you energy.
2. There is no mechanical way to cool your space faster because no matter how low you turn the thermostat, the air coming out of your vents will always be the same temperature. The A/C will keep blowing that air until it reaches the temperature you set it to. Therefore, set your temperature to your comfort level and wait or risk it is becoming too cold.
3. Your A/C determines when to come on and turn off based on the temperature sensor located in the thermostat. To avoid your A/C coming on to often, make sure your thermostat is not being hit by direct sunlight.
4. If your A/C unit comes on and then goes off within 10 minutes than it is most likely over-sized (i.e., too big). When a A/C unit is over-sized, you may experience hot and cold spots throughout the space.
5. A/C unit efficiency is determined by the Seasonal Energy Efficiency Ratio (SEER) value. The higher the SEER value the more efficient the A/C unit, but if your SEER value is under 13 then you should be thinking about replacing it. Your utility company may have an incentive or rebate program for replacing an old unit.

How does a HVAC system work?
An air-conditioning (A/C) unit controls the interior moisture (relative humidity) and offsets heat that is gained from the external environment and heat generated from internal loads (e.g., lights and appliances). The purpose of A/C is to provide a comfortable environment for occupants through temperature, humidity, and air movement. The four major components of an A/C unit are:

• Thermostat. The thermostat enables the occupant to set the internal temperature they would like the house to not exceed. This value is called the setpoint. When the thermostat setpoint is exceeded, the thermostat sends signals to the A/C unit to turn on. A residential A/C unit has only one speed; therefore, the A/C unit is running at full power whenever on.
• Refrigerant Cycle. The refrigerant cycle process is based on the ideal gas law (PV = nRT) and contains four components: the compressor, the compressor coil, the expansion valve, and the evaporator coil. The compressor takes cold, low-pressure gas and makes hot, high-pressure gas. This gas moves through the condenser coil becoming a liquid by releasing the heat from that gas to the outside. As the fluid reaches the expansion valve, it is a warm, high-pressure liquid. After passing through the expansion valve, the fluid becomes a cold, low-pressure liquid. The evaporator coil absorbs heat from the air, making it a cold, low-pressure gas again. The work done by the compressor is proportional to the difference in entering and leaving pressures. The higher the temperature in the evaporator coil entering the condenser, the lower the pressure that needs to be applied. Therefore, the delta in temperatures from the evaporator coil and the condenser coil indicates the work for the motor in the compressor. The smaller the delta, the less work and the less energy consumed.
• Fans. The fans blow air over the coils. The refrigerant in the coils absorbs the heat from the air, providing cool air into the home to displace the heat gains.​​
• Size. The air conditioner (A/C) removes heat (i.e., cools a space) at a rate (BTU/hr). The size of the A/C unit determines the rate of heat removal. In America, the sizes are typically measured by tons, and a 1-ton A/C unit means that 12,000 BTUs can be removed from the space in an hour. The increase in tonnage is proportional to the heat removal rate, so a 2-ton A/C unit removes 24,000 BTUs/hr and a 3-ton A/C unit will remove 36,000 BTUs/hr, etc. The size of the unit also determines the power output. In standard conditions, which are specified by the manufacturer, the power output of a 2-ton A/C unit is approximately 0.586 kW while a 3-ton A/C unit is 0.879 kW, assuming an energy efficiency ratio of 12. 

How Your Air-Conditioning impacts the environment
A/C is often the largest energy consumer during utility peak day and utility summer peak hours. During peak moments, the utility company must bring additional energy sources to the grid to avoid power outage. These supplemental energy sources often produce the most greenhouse gases. For examples, the grid could be running on hydro and natural gas normally, but during peak hours, coal plants are brought online to supplement the extra load. As such, reducing A/C energy consumption can often have greater reductions in greenhouse gas emissions than other energy efficiency measures. Minimizing A/C energy consumption can occur in two ways: reduce the power required to provide the necessary cooling through efficiency or reduce the runtime that A/C is cooling through behavior and/or controls. 

About the author: Anthony D. Kinslow II, Ph.D.

Anthony Kinslow II, Ph.D., CEM, is the Founder and CEO of Gemini Energy Solutions. Gemini democratizes the small commercial energy audit sector, creating affordable investment-grade energy audits and supporting efforts to increase workforce diversity in the energy efficiency sector drastically. Dr. Kinslow is also a Commercialization and Equity consultant for Clean Energy Works. This nonprofit organization seeks to accelerate private capital utility investments in inclusive clean energy solutions at the grid edge.  
Anthony also lectures at Stanford University, where he co-teaches two courses Racial Equity in Energy and Quest for an Inclusive Clean Energy Economy. 
 
His efforts center around accelerating the U.S. transition to a clean energy economy. And he argues that without prioritizing inclusive and affordable solutions for marginalized communities, the transition will be slow, expensive, and inequitable.  Dr. Kinslow II's work to drive equity and inclusion extends past his professional responsibilities. For example, the City of San Jose recognized his active involvement in understanding wage theft in the construction industry. Moreover, Dr. Kinslow II's role in designing and implementing Stanford's workforce program has introduced the concept of environmental justice to hundreds of high school students.
 
In addition to being a Historic Black Church and University (HBCU) alum (North Carolina A&T), Dr. Kinslow earned his Ph.D. from Stanford University in Civil and Environmental Engineering, where he incorporated Bayesian statistics in building science analysis to improve the cost-effectiveness of energy efficiency programs. His M.S. (Sustainable Design & Construction) was also from Stanford. In his free time, Dr. Kinslow II enjoys walking trails with his wife, Maria, and sons, Marcus and Samuel.