We reviewed the importance of the home load calculation in the last post. Provided the cooling and heating calculations have been done as accurately as possible, the load results provide both a target Cfm for reference during the air distribution design process and a sensible heat ratio (SHR) to assist with coil selection. Referencing the SHR during the equipment selection process helps ensure we have the evaporator coil capacity required to cool the air stream, and also to dehumidify the air stream to comfort conditions. Armed with those two pieces of information we can begin the process of identifying the air conditioning equipment necessary to meet the cooling load. And to do that, we need to get our hands on the expanded (or sometimes referred to as extended) performance data for that particular piece of equipment.
Stop and think about it. When any OEM produces a piece of air conditioning equipment, they have no idea in what part of the country, or world, that piece of equipment will be installed. An air conditioning unit may perform at more than, or less than it’s nominal Btuh rating, depending on the geographic location in which it is installed. As an example, a 3 ton air conditioner installed in Seattle Washington is going to have a greater output capacity on a design cooling day, than the same unit installed in Houston Texas on a design cooling day. It will also have a different (lower) dehumidification load than that same unit in Houston .
We need to know those details up front, in order to make the right equipment selection decisions. That type of information comes from the OEM expanded performance data. There is generally a great deal of additional technical data within expanded performance material, i.e., amperage values for the compressor under different load conditions and in some cases, high and low refrigerant pressures for various outdoor temperatures, etc., that make access to this material valuable to any installing or service technician as well as the person making the equipment selection decisions.
The details associated with proper equipment selection processes are defined and illustrated within ACCA Manual S. The newest version of this manual was just released early this year. And it is loaded with updates to the old content as well as new material to address technology changes that have occurred over the last decade. You’ll find it worth your time to review.