The Importance of Properly Sizing A/C Units

April 23rd, 2013 - Posted by Michael Bowers LEED GA under Technical

When installing a new air conditioning system, one of the most overlooked aspects is sizing. For decades, HVAC contractors have been neglecting the process and, instead, taking various shortcuts to simply “get the job done”.
Rather than taking the appropriate steps to calculate the correct size for an HVAC system, contractors often limit themselves to using “rules of thumb” that lead to incorrect sizing. They’ll use haphazard, blanket guides such as “600 square feet per ton” or “400 CFM per ton” to size units. The problem is that these statements are unjustifiable and will lead to incorrect system sizing.
Although these rules of thumb are quick and simple, it is extremely important that HVAC contractors avoid them and take the time to size HVAC systems properly. In order to do this, a fundamental understanding of ACCA’s Manual J is necessary. Manual J estimates heating and cooling loads for all types of residential structures. There are various computer programs that are available to help assist with the load calculations, as well. It is essential that every HVAC contractor take the time to carry out the Manual J load calculations to ensure the adequate sizing.
Properly sizing A/C units is extremely important, especially when you consider the consequences of incorrect sizing. Under-sizing units can lead to some obvious issues. If the unit is not sized to effectively remove the load in the space, the unit will continuously run, working fervently to attempt to eliminate the load. However, an inadequately sized unit will never be able to remove this load, leading to warmer indoor temperatures, longer run times, higher electricity costs, and potentially higher maintenance costs as well.
The more common issue among HVAC contractors is over-sizing the units. In an effort to try to overcompensate for any potential shortcomings and/or oversights, contractors often intentionally oversize. While this lackadaisical approach seems reckless and inefficient, it appears far too frequently in the field. To a layperson, it may seem like tacking on another ½ ton may not be a bad idea, as methodologies such as this are commonplace in other industries. Adding additional tonnage may seem like a redundancy or security, when, in fact, it can do a lot more harm than good.
Various issues arise when units are over-sized. Most of the issues stem from the short cycling of larger systems. Since the oversized units can sufficiently remove the load in a short time, the coil tends to run very short cycles. This short cycling does not allow the coil to effectively remove the moisture from the air, yielding elevated humidity levels. This increase in humidity can create discomfort and cause damage to items around the house; more importantly, humidity is a major contributor to indoor mold issues. The negative health effects of mold are well documented, and any kind of water intrusion can lead to mold problems. Although water intrusion can come from a number of sources within the building envelope, the HVAC system is often mistakenly overlooked as a source. Sizing the units correctly can eliminate a potential source of water intrusion, minimizing the risk of indoor mold.
Monetarily speaking, over-sizing an A/C unit can be as detrimental to your wallet it is to your health. Not only is a larger unit more expensive to buy, they also cost more to maintain and operate. Furthermore, the short cycling experienced with a larger unit accelerates wear and tear. The excessive cycling will increase the frequency of maintenance service and will ultimately lead to premature failure and replacement.
There are a myriad of issues that may arise when designing an A/C unit. From ductwork to fan power, and grille locations to air flow, designing an HVAC system is a complex process. One of the most overlooked aspects of HVAC design, however, is arguably the most important. Without correctly sizing a system, utilizing Manual J and incorporating every aspect of the conditioned space, the unit will be a source of severe future problems. In the past, it was commonplace to neglect the process and use arbitrary “rules of thumb” to size a unit. There is an information revolution occurring in the HVAC industry that is trying to eliminate these irresponsible practices. It is important that your HVAC contractor understands the importance of properly sizing the A/C units, as it will prevent issues from plaguing you in the future.
Michael Bowers LEED GA – Graduate from the University of Florida with a Bachelor’s Degree in Mechanical Engineering. LEED Green Associate with 2 years in the HVAC industry. Certified by the Air Conditioning Contractors of America in Residential System Design. Has worked closely with design and implementation of several HVAC projects, including multi-story residential buildings, universities, hospitals, and hotels. Mr. Bowers is currently working under the senior principal Chuck Wunder P.E. in preparation of securing his own PE license in the state of Florida, conducting inspection testing and analysis investigations with other principals in the firm and is currently the acting director of engineering and administration for U S Building consultants.

This post was submitted by Michael Bowers LEED GA.

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