During my 20 years of engineering experience, I have witnessed many buildings which have had their AC systems improperly sized. Unfortunately, by the time I get involved there has been significant damage that requires some sort of, typically very expensive, mitigation strategy. Some systems (although rare this days) were designed correctly, when originally commissioned, and later modified by the owners; but most systems are never sized correctly to begin with.

Here are the most common mistakes when sizing HVAC systems and the risks associated with them.

  1. Rule of thumb HVAC sizing : This is the idea that every HVAC system should be sized based on the square footage. The risks on this approach are to either over sizing the system resulting in energy waste and potential mold growth/damage, or under-sizing because all heat sources were not correctly identified.
  2. Bigger is better: Not on HVAC…..oversizing an HVAC system results in a term called “short cycling” which means that you will reach your set point room temperature too quickly, not allowing the cooling coil to properly dehumidify the indoor air. The risk on this approach is to develop indoor mold growth.
  3. Replacement in kind: This is the less damaging but still not a good idea and here is why. If you are replacing a unit that is 7+ years old, you are probably oversizing your system. More often than not you have: Replaced lights by either fluorescent or LED, changed appliances to high efficient type, replaced the roof and added insulation to save energy, or replaced windows and doors. All those factors affect HVAC sizing. The risk here is again potential mold growth.
  4. Sizing system using incorrect load assumptions: The best HVAC designer will provide an incorrect design if he/she is given the incorrect information to base the design. The most common items that are understated are 1) the number of building occupants and 2) A list of special equipment that will be added to the space. The risk associated to this approach is insufficient cooling during peak loads (summer time).

Replacing an HVAC unit, depending on the size of your system, is quite an investment. The potential risks associated with incorrectly sizing your unit can not only lead to energy waste but to other environmental risks such as uncontrolled mold growth. Therefore, hiring a mechanical professional engineer to right size and commission your new HVAC units is the best insurance policy you can buy. 

About the Author: Mr. Ivan R. Meneses, PE, CEM, EBCP is a professional engineer with a Master’s degree in Construction & Facilities Management and 20 years of experience in indoor humidity issues in government and commercial facilities. His Master Thesis from GA Tech involved a Case Study to solve Indoor Humidity issues at a federal government housing complex. His thesis can be viewed athttps://www.linkedin.com/redir/general-malware-page?url=http%3A%2F%2Fhdl%2ehandle%2enet%2F1853%2F4826. Mr. Meneses is also a certified energy manager and certified Existing Building Commissioning Professional. Mr. Meneses can be reached at ltmeneses@gmail.com .

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