For many years some professional HVAC businesses have believed that residential HVAC equipment selection was performed following guidance in ACCA Manual J. For example, a home with a 30,000 Btu/h cooling load would use a 2.5 ton air conditioner, right?

WRONG!!

Manual J is only used to calculate the heating and cooling loads. The current, nationally-recognized Manual J8 guides HVAC system designers to use ACCA Manual S Residential Equipment Selection to select equipment that is the right size (see section 10-4).

Why use Manual S
There are three good reasons to use Manual S: It is the nationally-recognized standard, it prevents problems associated with equipment over and under sizing, and it's a requirement in the 2009 International Residential Code (IRC).

Manual S, Residential Equipment Selection, is the ANSI-recognized, national standard providing clear instruction for interpreting and applying original equipment manufacturers' (OEM) expanded performance data. Manual S instructs designers how to select equipment which meets the application requirements (heating, sensible cooling, and latent cooling) at the design conditions that were used for calculating the loads.

Manual S also sets equipment sizing limits, as summarized in Table 1. These sizing limits ensure equipment capacities will keep customers comfortable while preventing the problems associated with equipment over and under sizing which include: health issues associated with excessive humidity, callbacks from comfort complaints, higher building costs (larger equipment is more expensive, more materials are needed, and more labor is used to install it), larger energy consumption, and greater wear and tear on the equipment¹.

Finally, Manual S is a code requirement. A modification adopted in the 2009 IRC clarifies an existing requirement for sizing HVAC equipment (change is underlined).

"Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCAManual J or other approved heating and cooling calculation methodologies."

How to Apply Manual S: Heating Example (Part I)
Manual J heating load calculations produce values, in Btu/h, for selecting the heating equipment. This heating example will select a furnace for a home that has a heating requirement of 56,000 Btu/h. The furnace needed must have the capacity to deliver 56,000 Btu/h in order to maintain 70°F in the home when the outdoor temperature dips to the outdoor design temperature (refer to Section 3-6 and Table 1A of Manual J8).

Figure 1: Manual S: Equipment Selection Sizing Limitations

Based on the home's load and the sizing limitations from Table 1, the furnace must produce more than 56,000 Btu/h, but less than 78,400 Btu/h. The home's heating capacity requirement is 56,000 Btu/h and the furnace can have a capacity no larger than 140% of the heating load (140% x 56,000 = 78,400 Btu/h).

Using the manufacturer's product data (as typified in Figure 1), the XYZ model FR 80-036 or FR80-024 has the output capacity of 64,000 Btu/h, which is enough to meet the requirement. The FR 60-036 has too little capacity (48,000 Btu/h, 86% of the load), and the FR 125-036 has too much output capacity (100,000 Btu/h, 178% of the load), selecting either the FR 60-036 or the 125-036 would cause comfort complaints or lead to a more expensive system.