Air Conditioning Load
Have you ever heard the terms load and capacity when describing air conditioning? Let us first distinguish the two terms, and then we will discuss the three types of air conditioning load.
Load is the amount of cooling that a building requires. Capacity is the amount of cooling an air conditioner can provide.
First, there is the design load, or the expected cooling needs for a specific building. This is calculated based on the building’s specifications, including the layout and orientation. Other factors, such as outdoor temperature at any given time of day and time of year cause variations in this calculation. The design load varies further due to indoor temperatures set by the building occupants. Manual J by the Air Conditioning Contractors of America lists load calculations based on average summertime temperatures for specific geographical locations and corresponding recommended indoor temperatures.
Another form of load calculation is extreme load. This takes into account the worst case scenario, such as the highest outdoor temperature occurring at the geographical location in question. The extreme load calculation is not ideal for determining what capacity air conditioning to install for a building because such conditions occur only a very small percentage of time. Besides, Manual J pads design load calculations to account for hotter than average conditions. Using extreme load figures to select air conditioning equipment will only result in installing a system with far greater capacity than what would be required.
Finally, there is part load, which takes into account the fact that conditions usually do not match or exceed design load conditions. Remember, outdoor temperature considered in design load calculations is based on the seasonal high, which is only met or exceeded not very many days out of any given year. Add to that the situation where insulation if the building is very effective, and therefore not so easily affected by seasonal high outdoor temperatures.
Determining air conditioning load and capacity is a very complex undertaking. Consulting your air conditioning professional is the best course of action.
Bailes, Allison. “The 3 Types of Heating and Cooling Loads” Energy Vanguard. January 14, 2015.