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ENERGY SERIES: What about the Air Conditioning System?


2901-9001 (BSE-289NP)

Authors as Published

Robert "Bobby" Grisso, Ph. D., Extension Engineer, Biological Systems Engineering; Martha A. Walker, Ph.D, Community Viability Specialist, Central District; Philip Agee, Ph. D., Assistant Professor, Department of Building Construction, and John Ignosh, Extension Specialist, Biological Systems Engineering, Virginia Tech.

As you begin the process of selecting the most efficient air conditioning system for your home, investigate the critical issues of system size, placement, installation, and contractor experience. Your goal is to obtain an efficient system by: sizing the system for the specific cooling load of your home; selecting and properly installing the thermostats or controls; designing a ductwork system to deliver the correct amount of conditioned air to each space; and sealing and insulating all ductwork.

AC Size

Heating, ventilation and air conditioning load calculations are required for new homes in many municipalities. These design loads are usually performed for each zone in a dwelling in accordance with Air Conditioning Contractors of America (ACCA) Manual J, ACCA Manual N or the ASHRAE Cooling and Heating Load Calculation Manual. The load calculations should be based on the exact orientation, conditioned floor area (ft2) and type of construction for each component of the building envelope, as well as the heat given off by the lights, people, and equipment inside the building. If a zoned heating and cooling system is used, the loads for each zone should be calculated. An accurate load analysis will help to prevent problems that occur when a house is equipped with an oversized, or undersized, system.

Energy Efficiency of an AC System?

The cooling efficiency of a heat pump or an air conditioning system is rated by the Seasonal Energy Efficiency Ratio (SEER), a ratio of the total cooling output to the amount of electricity used. Federal Regulation mandates a minimum  SEER 14.0 for most residential air conditioners manufactured after January 1, 2015. Efficiencies of some units can be as high as SEER 26 or more. (Chances are that older homes have units with SEERs less than 10.) Note that it is important to understand that even though the SEER may be high, overall or system efficiency includes the air distribution system as well.

What about Removing Moisture?

The Sensible Heat Ratio, or SHR, describes the moisture removing capability of air conditioning systems. An SHR on HVAC equipment of 0.7 means that 70% of the air conditioning load is devoted to cooling and 30% to removing humidity. It is critical that the HVAC contractor accurately estimate the humidity, or latent load. It is important to note that many high SEER units have poor humidity removal capacity, so verify system performance before purchasing and ask the HVAC contractor to provide written confirmation.

Why are Size, SEER and SHR Important?

This is where it gets a bit complex. The SEER, the SHR, and the system tonnage must be in balance so difficulties do not occur with indoor air quality. Systems without an adequate SHR or with inaccurate tonnage, cool without removing moisture. An oversized air conditioner will “short cycle.” Short cycling will cool your home too quickly to remove moisture very effectively.

This results in a home that is cool and “clammy.” If units are not providing sufficient dehumidification, the typical owner response is to lower the thermostat setting. Since every degree the thermostat is lowered increases cooling bills 3% to 5%, systems that have technically high efficiencies, but inadequate dehumidification, may suffer from higher than expected cooling bills.

AC Component Locations

Central HVAC systems have a component called an air handling unit (AHU). The advantages of placing the AHU in conditioned space include: it is in a more benign environment; a central location can minimize duct lengths and optimize air flow; there is easier access for maintenance; and any leaks occur in conditioned space.

Another often-neglected area of installation concerns the placement of the outside unit (condenser). Manufacturer’s recommendations for proper clearance distances should be followed to ensure there is no blockage of air flow from the unit. Also, do not vent a clothes dryer within 10 feet of the outdoor unit as dryer lint will cling to the condensing coil, lowering both the system’s efficiency and service life.

Questions the HVAC Contractor Should Ask

The HVAC contractor should ask you the following questions to properly conduct a comfort analysis and system design for your family and home:

  • Would you change anything about yourcurrent air conditioning and/or heatingsystem?
  • What do you like most about your presentsystem?
  • What benefits do you expect from your newsystem?
  • Does your existing system heat and cool yourhome to your satisfaction?
  • Are there rooms that are too hot or too cold?
  • What temperature is your thermostat set onduring the summer? Winter?
  • Do you have a scheduled lifestyle that encourages adjusting the thermostat frequently, or while you’re not at home?
  • Do you set the thermostat at differenttemperatures for the hours that you’reawake and the hours you’re asleep?
  • What types of heating or coolingproblems have you experienced?
  • Have you had any problems withcondensate drainage?
  • What is your average summer electric bill?
  • Who performs your regular energy-savings check-ups?
  • How long do you plan on residing inthis home?
  • Do you plan to remodel and/or expandyour floor plan in the future?
  • Have you made any changes to yourhome since the existing air conditioningand/or heating system was originallyinstalled?
  • How many people reside in your home?
  • Does anyone residing in your homehave allergies?
  • Do you understand ratings like SEERand SHR?
  • Do you understand how HVAC systems workor, more specifically, do you understand howthe system I’m recommending placing in yourhome, works?

Many of the same questions should be asked when determining what HVAC system should go in a new home. In addition…

  • Be sure your contractor is licensed (ifrequired in your municipality), welltrained, and experienced. Ask to see a validlicense, proof of coverage for workers’compensation, and certificate of insurancecoverage for liability and property damage.Inquire about references and membership incontractor associations. Ask for proof thatyour contractor is certified to handlerefrigerant in cooling systems.
  • Request a calculation of your savings.Heating and cooling equipment comes with two price tags: the cost to buy the equipment and the cost to operate it. Your contractor should be able to calculate your utility bill savings and total lifetime costs.
  • Request a load calculation. Ask yourcontractor to calculate equipment size usingcomputer software or professional guidelines.This will require taking measurements in yourhouse and asking questions. Don’t use acontractor who wants to size your unit solely onthe square footage of your house.
  • Inspect ducts. Ask your contractor to inspectyour ducts for leaks, incomplete connections,and compatibility with the rest of your system.Evaluate your system’s performance. Ideally,your contractor should use diagnosticequipment, and, if necessary, fix leaks using aquality duct sealant. In some cases, proper ductrepairs may include actual duct modifications toensure proper supply and return airflow.
  • Consider a house pressurization test. Testyour house and appliances for “backdrafting,”which occurs when the fumes from thecombustion process are pulled back into the home, threatening the health and safety of occupants.
  • Replace both indoor and outdoor coils.If you’re replacing an air conditioner orheat pump, be sure to replace both indoorand outdoor coils on a matched system formaximum efficiency and reliability.
  • Obtain a written contract. Always obtain awritten contract or proposal before allowingyour contractor to install a new system. Besure to ask about warranties for labor andparts.
  • Weigh the costs. Remember that the lowestprice may not always be the best price.Carefully evaluate a contractor’s proposal toensure you get the equipment and service thatbest meets your needs. Paying slightly morenow may get you better equipment, serviceand save you money in the years to come dueto lower costs of ownership.
  • Install for easy maintenance. Make sure theinside coil can be reached for its annualcleaning. The air filter(s) should also be easilyremoved, cleaned or changed when dirty.Check it monthly during peak season.

References and Resources

U.S. Department of Energy (n.d.) , BuildingEnergy Codes Program, Glossary

Developed as part of the NASULGC/DOE Building Science Community of Practice. The factsheet editors are: Robert "Bobby" Grisso, Ph. D., Extension Engineer, Biological Systems Engineering; Martha A. Walker, Ph.D, Community Viability Specialist, Central District; Philip Agee, Ph. D., Assistant Professor, Department of Building Construction, and John Ignosh, Extension Specialist, Biological Systems Engineering, Virginia Tech. 

DISCLAIMER – This piece is intended to give the reader only general factual information current at the time of publication. This piece is not a substitute for professional advice and should not be used for guidance or decisions related to a specific design or construction project. This piece is not intended to reflect the opinion of any of the entities, agencies or organizations identified in the materials and, if any opinions appear, are those of the individual author and should not be relied upon in any event. 

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Publication Date

February 3, 2020