ACCA Manual J Do’s (Mandatory Requirements) –

Per Manual J:
“Be honest and aggressive. Manual J is an engineering tool that has an inherent and appropriate factor of safety. Any attempt to add other safety factors or manipulate the procedure may result in unacceptable performance (especially at part load).”

• Use outdoor design conditions recommended by Manual J Table 1A.
  • Unless superseded by local code
• Use the default indoor design conditions recommended by Manual J Comfort Chart.
  • Unless superseded by local code

In most cases local codes are amended in order to provide “safety factors”, e.g. an “up to” higher outdoor design temperature or “down to” lower indoor design temperature, which in either case, is not best practice.

• Consider orientation of the structure on the site.
  • Use actual orientation, whenever possible
  • Use “worst case-best case” for plans that may be built using various orientations.
• Verify all construction details prior to calculating loads.
• Take full credit for:
  • Documented window, glass door and skylight U-values and SHGC values,
  • External overhangs (should be applied to all windows/glass doors), and
  • All internal shading devices (default is medium colored blind, w/slats at 45ºangle).
• Take credit for insect screens when installed or specified.
• In general, take full credit for the rated (or tested) performance of construction materials, insulation materials and construction features.
  • As specified for new construction.
  • As installed (verify the installation conforms to methods and materials protocols).
  • As tested (see quality control programs for new construction, investigate existing construction)
• Take full credit for tightness of the envelope construction.
  • As specified by builder or code.
  • As installed (verify the installation conforms to methods and materials protocols).
  • As tested (see quality control programs for new construction, investigate existing construction).
• Follow the manual J procedures for infiltration and ventilation procedure to evaluate the fresh air requirement.
  • Use the Table 8A procedure to evaluate the fresh air requirements.
  • Use Table 5A to estimate infiltration rates for heating and cooling (ignore intermittent exhaust fans).
  • Decide on the installation of an engineered ventilation system (mandatory if the code fresh air requirement is larger than an honest estimate of the Manual J infiltration rate).
  • Intermittent bath room and kitchen exhaust fans are not “ventilation devices” or “ventilation systems.”
• Take full credit for duct system sealing and duct insulation when such efforts are confidently anticipated or certifiable.
  • Use the default (0.12/0.24) scenario for (untested) ducts that are reasonably sealed.
  • Take full credit for sealing efforts that are certifiably tighter than the default scenario for sealed ducts.
  • If the duct sealing work is deficient (existing homes only) – seal the ducts and take credit for sealed ducts (use unsealed option only to show why the sealing work is required).
• Match location as close as possible when selecting a duct load table.
  • For attic locations, match roof material, roof color, use of radiant barrier and attic ventilation.
  • For closed crawl space locations, match crawl space tightness, crawl space wall insulation and crawl space ceiling insulation.
• Match duct system geometry (radial and spider systems tend to have less surface area than extended plenum and trunk and branch systems).
• Match return system geometry (use advanced Manual J procedures when the system has more than one or two large returns or when the returns are not located close to the air handler).
• Be sure and use the wall insulation correction if the R-value of the insulation is not R-6.
  • It’s best to use the correct R-values for duct wall insulation.
• Be sure to use the surface adjustment factor for the exposed duct surface area when the surface area of the actual duct system is significantly different than the defaults listed in Manual J.
• Add blower heat to the sensible load if equipment performance data is not adjusted for blower heat. (if equipment manufacturer or blower power is unknown, assume 1,707 Btuh for indoor blower motor heat).

At Enerlogic we would be happy to answer all of your HVAC System design questions and provide you with design services in a timely manner.