Selecting HVAC Manual J Designer – Base YOUR Decision on ACCA Manual J8 Appendix A Criteria

It is critical a proper Manual J load calculation be performed prior to installing an HVAC system.  It is also important the person performing your load calculations is knowledgeable in Manual J, S & D theory and ACCA Certified Manual J Software.

Appendix A of the ACCA Manual J book (Enhancements to Scope for Mechanical Contractor/HVAC Manual J Designer) recommends the following:

  • Design Analysis to be completed by Engineer or Contractor, also known as the “Designer”
  • Calculations to be performed using ACCA Manual J Version 8 Prerequisites:
    • All inputs/defaults/building characteristics used for the ACCA Manual J Version 8 calculations must be verified by the HVAC contractor/Mechanical designer in the field and/or must be verified and signed off by the builder, that what is being built is consistent across all of the builder’s houses in that community or division.
    • Design conditions as listed in Table 1A of ACCA Manual J version 8 for the following city shall be used <insert city>. Indoor conditions shall be 75°F Temp 50% RH Cooling  and 70°F Temp 30% RH Heating
    • The “Designer” shall provide the following information to the builder and HVAC installer:
      • Provide room-by-room and building heat gain/loss demand loads based on installed lights and equipment, envelope heat transfer, occupants, required ventilation and all other factors normally considered in these calculations.
        • Loads are to be determined by analysis of actual load calculations performed using ACCA Manual J Version 8 and not by “rules of thumb”.
  • Determine room supply, return or exhaust air requirements based on the heat gain/loss calculations, total and outside air change requirements, temperature, humidity, ventilation, and pressurization criteria for each room or space.

Combining several adjustments only compounds the inaccuracy of the calculation results. The results of the combined manipulations to outdoor/indoor design conditions, building components, ductwork

ACCA Air Conditioning Contractors of America

ACCA MJ8 says of compounding safety factors: The compounding of arbitrary safety factors must be avoided in the load calculation process

conditions, and ventilation/infiltration conditions produce significantly oversized calculated loads. Not only does oversizing impact the heating and cooling equipment costs, but also duct sizes and numbers of runs in order to account for the significantly increased system airflow.

Again, ACCA MJ8 says of compounding safety factors:

“Research studies and the experience of multiple system designers indicate that aggressive use of Manual J procedures provides adequate accuracy. No additional safety factors are required when load estimates are based on accurate information pertaining to the envelope construction and duct system efficiency. Large errors are possible if there is uncertainty about insulation levels, fenestration performance, envelope tightness or the efficiency of the duct runs installed in the unconditioned space.”

The compounding of arbitrary safety factors must be avoided in the load calculation process. The HVAC design industry’s practice of using extreme outdoor design conditions, de-rating the insulation value, and over-estimating the infiltration rate render the calculated load values meaningless.   Savoy Engineering Group only uses ACCA & ASHRAE approved design criteria to make sure your comfort system is sized Just-Right!

ASHRAE References:

Be sure to check out Savoy Engineering Group’s post on the ASHRAE recommended Manual J. It will help you understand residential HVAC design.