By Rich Lindemon, PE, LEED AP
It is very easy to fall into the trap of thinking that bigger is better when you consider something that you like or enjoy or that benefits you and those around you. That super-sized value meal with the extra-large fries and drink must be better than the regular sized portions. An extra 100 horsepower out of your car’s engine can make you go faster whenever you want. Who wouldn’t want a 70-inch TV on the wall of their family room, right? But that line of thinking doesn’t always apply favorably to all things, and it’s probably easy to come up with several of them rather quickly. One that is often overlooked is thinking that a bigger air conditioner must be better because it would have the ability to condition a space quicker and easier, and never fall short on capacity. There are several flaws with this way of thinking, which demonstrate that in the case of oversizing packaged direct expansion (DX) HVAC equipment – bigger is NOT better.
Before getting into the reasons why oversizing DX HVAC equipment is a bad practice, we should start by noting that all HVAC equipment should be sized using proper heating and cooling load calculations. There are several methods for doing this, but bypassing this critical design step in lieu of a “Rule of Thumb” method or a generic load-per-square-foot will usually get you off to a bad start. An accurate load calculation will allow you to select the correct size HVAC unit. Once you know your load demand, you need to focus on why you are designing an air conditioning system in the first place – usually simply for comfort. Comfort results from properly controlling the temperature, relative humidity, and air circulation uniformly and consistently through the space. In most cases, we concentrate primarily on regulating the temperature because we want it cooler when we feel uncomfortably warm. But we also feel uncomfortable when the humidity levels are too high. This heightened sensation of heat causes the desire to try to compensate by lowering the set point on the thermostat. This in turn causes the unit to overcool the space, leading to a more uncomfortable environment. A right-sized HVAC system that is properly matched to the calculated load will operate far more efficiently than an oversized unit. An air conditioning system that doesn’t run efficiently can unnecessarily drive up energy costs, going against today’s focus on creating a sustainable environment.
When an air conditioner runs, it does two jobs. It lowers the temperature of the space while also removing moisture from the air. To do the second task, it must run for a long enough time to allow the water to condense out of the airstream. Oversized air conditioners typically do not run long enough to completely dehumidify because they satisfy the space temperature and then shut off. As the space temperature rises again the unit will start running, but then quickly shut off when the space temperature setpoint is reached. This is called short-cycling and leads to another problem with an oversized air conditioner. As it short-cycles frequently – “bang on, bang off” – it takes far more initial energy to start the unit as it does to maintain continuous operation, leading to increased wear and tear on the components, particularly the compressor. An oversized short-cycling packaged DX air conditioning unit will not only fail to satisfy all the required comfort conditions, but it will ultimately fail completely long before reaching its life expectancy.
While sacrificing human comfort with an oversized packaged DX HVAC system, you may also be causing inadvertent damage in other areas as well. These include the building structure, personal health, and lifecycle costs. Limiting the ability to properly condition the space and control moisture can lead to environmental issues like the development of destructive organic growth that can damage your building structure. Indoor air quality can suffer greatly, leading to all sorts of potential health risks. Ultimately, you will pay more to purchase, install, operate, and repair an oversized HVAC unit that will also fail prematurely.
Now, about that supersized value meal…