One confusing thing about air conditioners is that when someone is talking about the tonnage of an air conditioner they are actually talking about the cooling capacity and not the actual weight of the air conditioner. This custom of air conditioner capacity being expressed in tonnage is a holdover from when cooling used to be accomplished by blocks of ice obtained over the winter from frozen rivers and lakes. Getting a better understanding of air conditioner capacity can help ensure you end up with the correct size of air conditioner for your home.
From Ice to Air Conditioning
The cooling capacity of ice can be easily equated to its weight due to the fact that almost all of its heat removal capacity lies in the latent heat it takes to melt it. Latent heat is simply the heat it takes for a substance to go to a liquid from a solid or a gas from a liquid. Since latent heat isn’t affected by temperature, the heat removal capacity of ice simply depends on how much ice there is.
Because the amount of latent heat it takes to change a given amount of ice into water is well known, it’s easy to calculate that it would take about 286,000 British thermal units (BTUs) to change one ton of ice from a solid to a liquid. However, we also want to know how fast the heat can be removed, so additional conversion is needed. With heat removal by blocks of ice, it made sense to determine the heat removal needed on a daily basis, but with air conditioners, it is standard practice to determine it per hour. This means one ton of air conditioner capacity is actually equal to 12,000 BTUs of heat removal per hour.
Measuring Air Conditioner Capacity
Determining the capacity of an air conditioner is markedly more difficult than determining the heat removal capacity of ice. This is because air conditioner capacity depends on the design and conditions they’re operated in. An air conditioner cools your home by using refrigerant to absorb heat from your home’s air. This refrigerant is then compressed to further raise its temperature so it can shed the heat into the exterior air. This means that its heat removal capacity is dependent upon numerous variables, including the temperature of both the interior and exterior air.
Because of the all the complexities involved, air conditioner capacity is determined through testing. The Air Conditioning, Heating, and Refrigeration Institute (AHRI) offers ratings that are more useful for comparing the capacities of different air conditioners than the manufacturer’s ratings because they use standardized operating conditions for their capacity tests.
However, even the AHRI ratings have limitations. If the conditions in your home are different from those used in the testing, your air conditioner’s capacity will be affected. Heat is more difficult to remove from cooler air, and AHRI uses an indoor temperature of 80, so if you keep your home cooler, AHRI will be overestimating capacity in this regard. The hot climate of northern Texas also means the AHRI rating generally overestimates air conditioner capacity in our area. AHRI bases their tests on an outdoor temperature of 95, but in our area design temperatures are usually in the upper 90s or higher.
Sizing an Air Conditioner for Your Home
Since the AHRI ratings probably won’t be accurate for an air conditioner operating in your home, you’ll want an HVAC professional to perform a Manual S sizing calculation. This calculation works in conjunction with the Manual J load calculation to ensure you have the most appropriately sized air conditioner for your home.
Performing both Manual J and S calculations also helps ensure your air conditioner will provide adequate humidity removal. High humidity levels in a home can be uncomfortable and cause problems with mold. The Manual J calculation provides both the sensible and latent heat loads of your home, or in other words, the amount of cooling and humidity removal it needs. The amount of latent heat removal as compared to sensible heat removal can vary between air conditioners, so it’s important to have a Manual S calculation performed to take into account conditions in your home and ensure your air conditioner will provide enough removal of each.
If you want more information on air conditioner capacity or want help finding a new air conditioner for your home, contact us at James Lane. We have supplied quality HVAC and plumbing services to Wichita Falls, Vernon, Burkburnett, Graham, Iowa Park, Olney and the surrounding areas since 1957.
