How to Assure ‘Right Sizing’ HVAC Systems for Efficient Operation

“Right sizing” an HVAC system improves efficiency, reduces noise, offers greater cooling/heating comfort and saves money. An oversized system increases installation costs, wastes energy, and costs more in overall operating costs than correctly sized systems.

hvacfixIf you consider the worst-case scenarios when sizing your HVAC system, such as maximum occupancy, lighting needs and weather conditions, you’ll probably install an oversized system. And an oversized system can reduce the cooling capacity. That results in the compressors short cycling, which in turn affects the system’s ability to dehumidify the building effectively.

Most HVAC systems are sized to meet heating and cooling loads occurring around 1% to 2.5% of the time, but an efficient system works most effectively for the conditions occurring 97.5% to 99% of the time, according to the National Institute of Building Sciences.

Oversizing is common in commercial as well as residential applications because contractors use the rule-of-thumb sizing – a load calculation based on square footage, says Terry Townsend, president of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

Understanding how each system affects the others offers the best energy savings. The whole building design concept emphasizes designing a building’s HVAC system taking into account how one system affects another. It uses a systems approach rather than a component-based approach, considering the overall system configuration and the low load operation strategy. Whole building design, coupled with a right-sized, efficiently performing HVAC system results in as much as a 30% reduction in annual energy costs.

An efficient whole building design should plan for expansion without oversizing the equipment. The plan should not offer so much excess that it exceeds current required capacity and accounts for future loads that may never occur. Instead, the plan should build in physical space for adding equipment such as pumps, boilers and cooling towers. Use systems that can be modified, so additional equipment can be added later to expand the system.

‘Right Sizing’ HVAC System Considerations and Tips for Efficiency

According to the National Institute of Building Services and the Building Energy Codes Resource Center:

  • Do not rely on rules-of-thumb load calculations when sizing HVAC equipment. Obtain updated equipment load data matching the usage;
  • Calculate HVAC system design loads in accordance with accepted engineering standards and use handbooks such as the ASHRAE Handbook – Fundamentals;
  • Calculate an accurate load estimate, using established design data from ASHRAE;
  • The Air Conditioning Contractors of America (ACCA) publishes the Manual J, the standard for performing load calculations, for residential applications. For commercial facilities, the U.S. Small Business Administration recommends using the ACCA Manual N;
  • Use HVAC safety factors and pick-up load allowances in ASHRAE/IES 90.1 as the maximum limit;
  • Use safety factors sparingly. On the hottest or coldest day of the year, it’s unlikely the building would contain the maximum occupancy with every light and piece of machinery operating;
  • Choose cooling equipment meeting/exceeding the SEER (seasonal energy efficiency ratio) or EER (energy efficiency ratio) for the capacity requirement, advises the Building Energy Codes Resource Center. Additionally, choose cooling equipment meeting/exceeding the IPLV (integrated part-load value);
  • Use computer analysis tools to simulate how the building operation affects loads to prevent oversizing. Utilize tools provided by utility companies and the U.S. Department of Energy’s Building Energy Software Tools

Next Steps

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5 comments


  • I couldn’t agree more. Almost every homeowner or business owner wonders when it comes time to replace the air conditioner if the unit is big enough. As long as no major changes have occurred in the design of the space, quite often the unit appears to be too small because its performance, efficiency and capacity are all reduced due to its age. The question is easy to answer if you run a cooling/heating load on the space. I think too many contractors either don’t have the ability or the desire to go the extra step and ensure the unit that is being installed today will handle the needs of the space. I appreciate your nicely written article.

    April 4, 2012
  • Surprising statistics! I can see the logic behind designing the AC system for the worst case scenario, but it does make a lot more sense to design it for everyday situations. Great post!

    April 5, 2012
  • The biggest reason people are disappointed with their A/C system is the wrong sizing of equipment. Through forty plus years of experience I was always trying to fix problems cause by over and under sizing of the cooling system.

    April 8, 2012
  • […] divided by the electric energy input during a cooling season – and offers recommendations on right-sizing and installing equipment. Sizing and a proper installation slashes energy savings by as much as […]

    May 10, 2012
  • Ted

    More of a question. I currently have an old oil furnace and no central A/C. Next spring I want to change to a gas furnace and, eventually add central A/C. It is an 850 sf single family home. How do I determine what size of heating and cooling equipment I should have? Thanks.

    September 28, 2017

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