Evolving Industry, Revised Standard
In 2009, the ICC’s International Residential Code and International Energy Conservation Code first referenced the ANSI/ACCA 2 Manual S – 2004 (Residential Equipment Selection) as the methodology for properly sizing residential HVAC equipment. But the industry has not stood still since then, and neither has Manual S.
Think about how much our industry has changed in the decade since 2004. There are new state and federal codes, regulations, and programs that simply did not exist then, but these continue to affect everything from equipment efficiency to refrigerant production. The available equipment and controls have become more advanced and complex, not to mention the rising popularity of “newer” types of equipment (i.e., multi- and variable-speed equipment, mini- and multi-splits, etc.).
Building codes have also changed drastically, with more emphasis placed on a home’s energy performance. This is evidenced in stricter requirements for air sealing and insulation, as well as basic mechanical equipment efficiency.
Not surprisingly, the changes on multiple fronts (regulation, available HVAC equipment, codes) have exacerbated a designer’s job. Increasingly, we are seeing new homes that are tighter than ever, with much smaller cooling and heating loads. Right-sizing the equipment is as important as ever given that (1) grossly oversized equipment may lead to moisture issues like mold and mildew and (2) tighter homes retain more moisture. However, standard equipment offerings generally start at 1.5 or 2.0 tons with one-ton jumps between 4.0 and 5.0 tons, and one-ton increments in multi-speed equipment. Hence, these smaller loads, coupled with available equipment sizes have resulted in many projects that cannot easily comply with code given the oversize limits in the 2004 Manual S.
Revision and Consensus
In 2011, ACCA started the process of completely revising Manual S in order to reflect the current state of equipment selection. Based on comments received from practitioners since 2004, as well as a review by staff, a list of over 50 discreet revision issues was drafted for consideration by a committee of industry experts.
The next step was for the advisory committee leading the revision effort to meet and discuss each of these issues to develop the standard draft. Once the draft was completed, it was time for the ANSI public review. In total, Manual S underwent three successive public reviews, which ended in April 2014.
So what does the Manual S look like now? The major changes from the 2004 version are as follows:
Manual split into two parts:
- The first part is normative and written in code enforceable language (for contractors and code officials),
- The second part contains examples and the details/nuances (for contractors and educators);
Expanded to include equipment not previously covered:
- AHRI-certified cooling-only equipment, AHRI-certified heat pump equipment, electric heating coils (new), fossil fuel furnaces, water boilers (new), water heaters used for space heat (new), dual fuel systems, ancillary dehumidification equipment (new), AHAM appliances (new), direct evaporative cooling equipment (new);
- Revised oversize limits.
Probably of most interest to readers are the revised oversize limits for cooling-only and heat pump equipment. These sizing limits are derived by comparing the equipment capability (at specific design conditions) to the home’s heating and cooling load requirements. This table summarizes the sizing limits.
You will note two things: there are different sizing limits for single-, two-, and variable-speed equipment and the lower limit has been dropped from 0.95 to 0.90 x cooling load across the board.
These two items, coupled with the option of selecting a different indoor unit for a given condenser (+/– a quarter- to a half-ton impact) and varying indoor CFM delivery, which has a large impact on the latent and sensible capabilities, will greatly ease equipment selection.
Manual S also provides an alternative option for sizing heat pumps, which allows for a maximum capacity of 15,000 Btuh over the Manual J cooling load. In order to use this option, the home’s sensible heat ratio must be ≥ 0.95 (i.e., very dry), and the ratio of the location’s heating degree days (base 65°F) to cooling degree days (base 50°F) must be ≥ 2.0 (i.e., a heating dominated location).
This is just a summary excerpt. Manual S goes into detail as to what everything in the table means, and must be consulted by a system designer in order to identify the correct oversize limits for a specific application. We encourage everyone to get a copy of the fully updated 2014 Manual S and check out the changes in detail.
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