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ACCA Manual J Do’s & Don’ts

Improper Practices –

Never use “rules-of-thumb”. The idea that the required equipment capacity equals the floor area multiplied by some magic number has resulted in many customer complaints and legal actions. Heating and cooling loads depend on individual circumstances.

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        • Unless superseded by local code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        • Unless superseded by local code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).

ACCA Manual J Don’ts –

  • Do not use Manual J (any version) for:
    • Any type of commercial application (even if located in a residential structure).
    • Large multi-family buildings or residential high rise structures.
    • A room or space containing an indoor swimming pool or hot tub.
    • Earth-berm or earth covered dwellings.
    • Solar homes that have passive features.
  • Do not use MJ8ae to estimate loads that are not compatible with Abridge Edition Checklist.
  • Do not design for record breaking (or news making) weather conditions.
  • Do not add a “safety factor” to Table 1A design conditions.
  • Do not design for abnormally low or high indoor temperature or humidity conditions.
  • Do not assume that there will be no internal shade on ordinary windows and glass doors (bare glass is an acceptable assumption for glass specifically installed for “day-lighting”).
  • Do not fail to take full credit for overhangs.
  • Do not assume that the load for the worst case site orientation can be used for other orientations. (Rotating the dwelling on a site can change the cooling load by a half ton or more. Room air flow requirements change as the orientation changes. If the same design is used for any orientation, some rooms may have too much supply air and other rooms will not have enough supply air for temperature control and comfort.)
  • Do not reduce known ceiling, wall or floor R-values “just to be safe”.
  • Do not fail to give full credit for the builder’s effort to produce a tight envelope.
  • If a local code specifies a fresh air requirement (typically an air change per hour value), do not assume the infiltration rate will satisfy this requirement and do not use the code ventilation requirement as the input value for the infiltration rate.
  • Do not assume that windows and doors will be open when making the infiltration estimate.
  • Do not make worst case “everything is going full blast” assumptions about internal loads.
  • Do not add extra occupancy loads for “entertaining groups of people.”
  • Do not add internal loads for special events.
  • Do not arbitrarily assume that ducts are unsealed (i.e., do not assume that they are leaky).
  • Do not fail to give full credit for efforts to provide tight, properly insulated ducts.
  • Do not apply “safety factors” during any stage of the load calculation process.
  • Do not apply a “safety factor” to the final answer or to the equipment selection procedure.

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.

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