by Stan Johnson
When it comes to performing heat gain/loss calculations for residential buildings, you have several choices.
- The best choice is to invest some time in understanding ACCA’s new load calculation procedure, the Eighth Edition of Manual J® (MJ8). MJ8 has more information on new construction materials, building technologies, and technical developments than the Seventh Edition of Manual J (MJ7) it replaces.
- Another choice is to continue using the old MJ7 load calculation procedure with its defaults, construction listings, and formulas from 1986.
- A non-choice, but an option many contractors still appear to prefer, is the use of seat-of-the-pants, rules-of-thumb, and other estimating procedures that were not appropriate even two decades ago.
It should be recognized that ACCA released MJ7 well before computers were prevalent in the home and workplace. To enable MJ7 to be a procedure for field estimating (i.e., hand calculation) of home heat loss/heat gain, a great number of assumptions and simplifications were included in it. During the past 15+ years, new research and investigation has led to a better understanding of residential heat gain/heat loss mechanisms, and the interactions that envelope, equipment sizing, comfort requirements (temperature and humidity), weather effects, construction practices, etc. have on the residential load determination process. During this same period, the marketplace developed and introduced new residential HVAC equipment, new building products, and varied construction processes/methods for cost-saving and energy efficiency reasons. As a result, MJ7 simply lacks the capability and sensitivities needed for today’s real world applications.
Which MJ is Appropriate?
If the residence you’re preparing to run a load analysis on was constructed prior to 1986 you’re probably okay with the construction materials and building technologies embodied in MJ7. However, you are still omitting technical advances and improved relationship knowledge between structure, use, and load. If you and your customer are okay with those omissions, then maybe MJ7 is good enough for you to continue to use-for a while. On the other hand, if you are involved with new residential construction, or major remodeling of existing homes, and you have not upgraded to MJ8, you are likely to be using incorrect information for various construction materials and building approaches. With the liability issues surrounding proper HVAC equipment sizing, perhaps you should check with your homeowner (or builder), attorney, and insurance company to see if using obsolete, 1986 information and procedures is okay with them.
Ease of Use
Early in the MJ8 development stages it was recognized that contractors would not be performing “hand calculations.” Rather, it was envisioned that contractors would employ computerized MJ8 applications (as offered by third-party software developers such as Elite and Wrightsoft). Since very little new information is sought from MJ software users, the effort and time to execute an MJ8 load calculation is very similar to that of performing an MJ7. Unfortunately, it appears that MJ8’s creators may have lost sight that without a simplified hand approach, it becomes more difficult for contractors to learn key concepts and for technicians to implement load calculations in the field. ACCA has recognized this shortcoming and is currently developing a “simplified hand approach” that can provide good load approximations for a number of selected applications.
It has been purported in some sectors of our industry that MJ8 is not ready for primetime, and that the loads resultant from MJ8 are considerably higher than those from MJ7. At a July 2003 meeting of industry stakeholders (OEMs, educators, researchers, contractors) this issue was reviewed in considerable detail. The upshot was that MJ8 may erroneously result in higher loads. However, as a consensus, the group noted that the largest contributor to such higher loads was user error! It was observed that MJ8 offers many more permutations than MJ7, and that users are simply selecting construction descriptions that appear to be close, but are not exact replacements for those provided by MJ7. Additionally, users tend to use “worst case” values/assumptions that are much too conservative. All of this can result in higher loads when comparing the results of MJ8 vs. MJ7. Contractors are strongly encouraged to follow these MJ8 guidelines:
Window shading: Always consider internal shading (curtains, blinds, etc.). Always consider internal shading (curtains, blinds, etc.). At a minimum, recognize that privacy areas such as bedrooms and bathrooms nearly 100% of the time have internal shading.
- Overhangs: Always take into consideration roof overhangs and their shading effect on windows.
- Conservative assumptions: Select the attributes that best match the home’s construction (some testing or additional verification might be required.) For example, determine if duct runs in unconditioned spaces (such as hot attics and open crawlspaces) are, or will be, sealed tight and well insulated.
- Do not merely assume ducts are "leaky, unsealed" or un-insulated. Special consideration is warranted here because there are serious system sizing and performance issues associated with incorrect duct assumptions. Do not assume "leaky" or "average" air infiltration through the envelope when it is "tight." MJ8, with many more default options than MJ7,
requires diligence by the user. Be careful to select the most appropriate construction elements for walls, ceilings, windows, etc.
- Compass orientation: Compass orientation must be observed and can result in substantially different equipment as a building is pointed in varying directions.
- Default outdoor design conditions: Use those provided in Table 1 of Manual J.
- Default indoor design conditions: 75F cooling, 70F heating.
- Ventilation fans: Ignore kitchen and bath fans as these are rarely in operation.
- Safety factors: Do not add safety factors to the resultant load.
- Equipment selection: Select HVAC equipment that is within 15% of the determined load, and is able to satisfy both the sensible and latent requirements (refer to Manual S for full guidance).
Properly observing the above recommendations will produce accurate MJ8 loads that are consistent with those produced by MJ7. Ignoring the above recommendations can easily add an unneccessary 12-to-1 ton or more to the job.
In most cases, a contractor who insists on performing Manual J calculations does not receive support from the builder, the power utility, or the local engineer. The contractor is put in what he considers an indefensible position. The very existence of his business is on the line and, if not that, then the loss of the builder’s business could also be debilitating. Consequently, contractors tend to use conservative assumptions and to oversize equipment to gain an “extra margin of safety” for those very hot days. Yet, this is not required and should not be done.
As an industry, we collectively have our work cut out for us to educate homeowners, builders, architects, professional engineers, utilities, etc. on the value of correct load analysis. This education process starts with the HVAC contractor. If a homeowner complains about inadequate cooling, and the summer weather is above the outdoor design condition, the contractor should provide an MJ8 load showing proper equipment selection and explain why sizing is important. We need to continue to educate all stakeholders on the benefits of proper load determinations and proper equipment sizing/selection.
Each edition of Manual J has been a vast improvement over the previous edition. Similarly, MJ8 will be revised as the body of air conditioning and construction knowledge grows.
While contractors in the replacement market might continue to use MJ7 for a while, they need to be learning MJ8. Many older homes have already been upgraded to current efficiency standards, and today’s new homes are not too far away from tomorrow’s replacement systems. For new construction, any analysis on something other than MJ8 is already invalid. n
Stan Johnson is pesident of Stan’s Heating and Air Conditioning, Inc. in Austin, TX. He is a vice chairman of the Board
of Directors of the Air Conditioning Contractors of America. His e-mail address is: [email protected].