ACCA Manual J is a procedure. As with any procedure there are some essential or mandatory requirements. These 20 do’s should be followed as closely as possible for reliable and accurate results. Deviations may result in either oversizing or under sizing equipment; in both cases occupant comfort will be sacrificed.
Use the outdoor design conditions recommended by Table 1A of Manual J.
Outdoor design conditions are the expected extremes based on collected weather data for that location. We seem to remember the cold winter or hot summer. But these are very extreme. Equipment sized on these feelings results in oversized equipment. Local building codes may supersede these values.
Use indoor conditions within the comfort chart of Figure 3.2
Figure 3.2 provides the results of studies of what is a comfortable temperature for most people. The summer bull’s eye is 75 Fahrenheit and 50% relative humidity. The winter bull’s eye is 70 Fahrenheit and 30% relative humidity (no condensation). Local and state codes may supersede with actual values.
Consider orientation of structure.
Use actual orientation whenever possible. For a model home, which may have any orientation, use the worse case position. This is the orientation which has the highest heating or cooling load. Manual J calculations should be reran when orientation is known.
Verify all construction details prior to calculating loads.
If the structure has not been built, use the latest official set of plans. Document any information obtained in addition to the drawings. If the structure is existing, obtain plans drawn by a certified energy auditor.
Take full credit for documented window, glass door and skylight U-values and SHGC values.
Windows, skylights and glass doors are rated by the National Fenestration Rating Council (NFRC). Use these values if manufacturer and model are known. Otherwise use the Manual J tables for generic glass.
Take credit for overhangs.
Window and glass door loads in Manual J account for the sun’s radiant heating affect. Overhangs may block some or all of this load. Include the overhang geometry calculations for all glass.
Take credit for internal shade.
Window and glass door loads are based on sunlight penetrating into the structure. Internal shade, even if partial, can reduce that load. If internal shade details are not known, apply the Manual J default of medium color blinds with slats at 45 degrees.
Take credit for insect screens when specified.
Insect screens are an adjustment to the glass load.
Take full credit for rated or tested performance of construction materials.
Use the results of testing agencies like American Society for Testing and Materials (ASTM). For manufactured building systems, used independently tested data only. Verify with specified thickness and installation.
Take full credit for tightness of the building envelope.
Infiltration and exfiltration past the building envelope can be a major load factor. Building codes are requiring tighter buildings. Take this into consideration. If the building is being built to Energy Star or LEED standards.
Follow Manual J procedure for infiltration and ventilation.
As mentioned above, infiltration and also ventilation have a major impact. Using rule-of-thumb or guessing can lead to inaccuracies. The Manual J procedures allow the proper assessment of infiltration and ventilation, including their combined affect.
Take full credit for duct system sealing and duct insulation.
Air loss, heat gain and heat loss through the duct wall into unconditioned areas have a major impact on the heating and cooling loads. Older ductwork can be very leaky and have little to no insulation. Modern building codes are stricter on sealing and insulation for residential construction.
Match duct location as close as possible to the proper duct load table.
Determining heating and cooling loads resulting from ductwork can be quite tedious due to the many sizes and configurations. ACCA Manual J has simplified the procedure by providing multiple tables. Make sure you select the table which best fits your building.
Match duct system geometry.
Radial and spider duct layouts tend to have less duct surface area than extended plenum layouts.
Match return system geometry.
Account for systems with multiple returns or returns far from air handler.
Use duct wall insulation correction.
ACCA Manual J Duct Tables allow an adjustment for duct wall insulation. Older ductwork may need adjustment for less insulation, while duct meeting some building codes can be adjusted for more insulation.
Use duct surface adjustment factors.
Manual J Duct Tables are based on the average of an entire system for that particular configuration. If the ductwork surface is largely different than that average, use the adjustment factors.
Use occupancy and internal load that are compatible with structure.
ACCA Manual J has default values for typical internal load configurations. If your building is different, Manual J has table for calculating each individual internal load. Occupancy is based on building code occupancy.
Add blower heat to the sensible load.
The motor driving the blower (fan) is in the supply air stream. The heat from that motor is added to the sensible cooling load if not included in the equipment performance data.
Sit down with customer or client and explain the factors involved in an accurate load estimate. Spreadsheets and computer programs have evolved to properly perform the calculations. The tables are based on extensive study of the thermodynamic properties of buildings and building materials. Typically any inaccuracies will enter from faulty information and making inappropriate guesses when information is missing.
When ACCA Manual J calculations are performed from building drawings, it is recommended it is re-evaluated when the building is completely constructed. Windows will have NFRC labels and ceiling and in some cases floor insulation thickness can be verified. Check infiltration against blower door tests, if conducted.