Manual S Overview
In a nutshell, the ACCA Manual S procedure is a method to obtain the adjusted capacity of HVAC equipment for the conditions in which it will be operating. Of course, there is much more to Manual S, and for complete details you can order the ACCA Manual S book from here: https://www.acca.org/store?webproductid=18b6715f-bc20-e511-80f5-c4346bac9a78
The nominal AHRI rated cooling capacity is measured using a 95 F outdoor temperature. There are also other factors considered in the nominal rating of a unit, and a summary of these additional factors can be seen here: http://dnr.louisiana.gov/assets/TAD/education/ECEP/hvac/a/tbl-a345.htm Documentation on the complete rating procedure can be seen here: http://www.ahrinet.org/App_Content/ahri/files/standards%20pdfs/ANSI%20standards%20pdfs/ANSI.AHRI%20Standard%20210.240%20with%20Addenda%201%20and%202.pdf
For most locations in the U.S. where the summer outdoor design air temperature is 95 F or less, the nominal rated capacity of a unit will be very close to its actual adjusted capacity. However, adjusted capacity calculations become extremely important when the outdoor design temperature is significantly above 95 F and/or the indoor/outdoor conditions are very humid (high wet bulb temperatures). For example, in Phoenix, AZ with an outdoor design temp of 108 F, a nominal rated 4 ton unit might only have an actual net capacity of just 3 tons. High temperatures and humidity decrease the capacity of a cooling system.
HVAC designers and installers that live and work in extreme climates are already well aware of the need to check the adjusted capacity of the units they install. But many other HVAC professionals who don't live and work in extreme climates should also be checking the adjusted capacity of the units they install. Sometimes the combination of an outdoor temperature slightly higher than 95 F along with a higher than normal wet bulb temperature can cause a significant capacity decrease from the nominal rating. Every HVAC professional in every climate situation should be aware of how to obtain the adjusted capacities of heating and cooling equipment.
The easiest way to find out the adjusted capacity of a unit is when the manufacturer provides expanded ratings software that allows you to select a model and then enter its operating conditions. In years past, this type of software would come from the manufacturer as a PC based program with a name like Perf or ESP. In current times, this type of software is most often supplied as an application on a web page.
Unfortunately, at the time of this writing, only about 40% of the manufacturers offer such web applications. Furthermore, these applications are not open to the general public. Typically, you have to be a dealer for the manufacturer in question in order to be able to create an account, login, and then make model selections. Sometimes, others who have some professional relationship with the manufacturer are allowed to create an account for use of the ratings software. Examples of those who might qualify would be specifiers, trainers, and consultants.
If you think you might qualify to use such software, contact the territory or regional sales manager of the manufacturer in question. Use your favorite search engine to find that contact. But again, be aware that typically it is only the largest manufacturers that offer such web applications.
Even if a manufacturer does not have ratings software that you can access, sometimes you can get the expanded rating data for a model in .PDF or printed form. This is usually a single page of tabular data showing capacity values under various conditions. Unfortunately, every manufacturer uses their own custom format for arranging these data. The differences can be so great from manufacturer to manufacturer that Elite Software believed it worthwhile to include in the Rhvac Online Help pages five different examples on how you might encounter these data and how you can best work with them.
Unfortunately, just as with the ratings software itself, expanded ratings data tables are usually not available for public download. See if you can get a web account made with a manufacturer such that you can download the data. If that is not possible, contact your closest local distributor of the manufacturer in question. Sometimes they can email you data sheets, and sometimes you have to go by their place and pick up data sheets. Be aware also that some manufacturers don't offer expanded ratings data at all, let alone special ratings software.
You can find data for several manufacturers here: https://www.elitesoft.com/web/equipmentperformancedata/equipmentperformancedata.html
If your manufacturer provides a web application for adjusted model ratings, you will not need to fill out the cooling performance data screens shown in the Rhvac Online Manual S examples as you will already have the adjusted net capacity that you can enter directly into the Adjusted Total Capacity input on the Manual S Performance Data page. More often though, you will have expanded ratings data in PDF or printed form. If so, review the examples provided in these help pages and enter the necessary items from your expanded ratings data so that Rhvac Online can "interpolate" the data provided and arrive at an accurate total net capacity.