ACCA Manual J is a method. As with any method there are some practices to avoid. These 19 don’ts should be followed as closely as possible for reliable and accurate results. Deviations may result in the reduction of occupant comfort from oversizing or under sizing equipment.
Do not use ACCA Manual J on buildings for which it was not written or designed.
Do not use in these cases.
- Do not use for any type of commercial function even if it is in a residential building.
- Do not use for multi-family buildings or high-rise residential structures.
- Do not use if building has an indoor pool or hot tub.
- Do not use for earth berm or earth covered structure. Do not use for solar homes with passive solar features.
Do not use Manual J Abridged version outside of its purpose.
The ACCA Manual J Abridged version is for residences with basic and standard construction features. Comply with the Abridged Edition Check List on page 2 of the ACCA Manual J Manual.
Do not design for extreme record breaking weather conditions.
While they can be quite memorable, weather extremes are rare. Sizing heating and cooling equipment to meet these conditions results in oversizing of that equipment during all other times. Oversized equipment is less efficient because of frequent on and off. Indoor conditions will fluctuate and not be very comfortable.
Do not add a safety factor to weather data.
The ACCA weather data is based on actual weather observations and measurements over many years, often 20 to 30 years. It has been statistically calculated to be the expected extreme temperatures.
Do not design for abnormally high or low indoor temperature or relative humidity.
Performing load calculations for high winter indoor temperature and low summer indoor temperature has the same impact as using outdoor temperatures beyond the Manual J values.
Do not assume no internal shade on windows and glass doors.
The vast majority of buildings will have some type of internal shading on windows and glass doors. If they are not included in the initial construction, assume the home occupants will add them.
Do not fail to take credit for overhang.
The window calculations take into account energy from the sun. This is a large energy contributor, however an overhang can block some or all of that energy. This effect should be included in the window load.
Do not assume worse case orientation.
The heating and cooling loads can vary for the same building depending on which direction it faces. Make every attempt to find out the actual orientation of the building. Worse case can be used for model homes, then the loads updated when a home uses that model.
Do not reduce insulation rated R-value.
Use the tested R-value for the material. These values are easily obtained from the manufacturer. Do not reduce these values. Keep in mind Manual J is an estimate of the peak expected load. It is expected building materials like insulation will be installed correctly. Do not use R-value claims by manufacturers if not supported by ASTM testing. They may inflate the R-value due to reduced infiltration or radiant loads. But these effects are accounted for elsewhere in the load calculations.
Do not fail to take credit for a tight envelope.
More attention is being applied to constructing a tight envelope (the barrier between inside and outside). Each addition of the International Residential Code (IRC) has been tightening infiltration requirements. Know the applicable codes and use a matching value.
Do not assume infiltration will satisfy ventilation requirements.
As homes get tighter, infiltration may not meet ventilation requirements. For tight homes, the ventilation exceeds the infiltration. This needs to be evaluated in each time a Manual J analysis is performed.
Do not assume windows or doors will be open when making infiltration estimate.
ACCA Manual J estimated peak loads which is during peak winter and peak summer. Home occupants will not have windows open during these periods.
Do not assume all appliances and internal electrical devices are operating at full capacity at the same time.
Internal loads are difficult to assess. Take some time to estimate the time appliances are on and for how long. If these loads are not operating at or near peak periods, then they are not included.
Do not add extra occupancy for people.
Occupancy is based on long term presence in the home. This means they will need a place to sleep; this is why a bedroom count is used for determining occupancy. Extra occupancy, such as for a party or when company is present, is short term by Manual J standards. Estimating for this has same effect as estimating for rare peak temperatures.
Do not add internal load for special events.
The same reason here as explained above.
Do not arbitrarily assume ducts are unsealed.
More attention is being applied by code officials to duct sealing and insulation. Be aware of local trends; is it typical for local HVAC contractor to seal ductwork.
Do not fail to give credit for properly sealed and insulated ductwork.
Same reason here as indicated above. Use the appropriate adjustment.
Do not apply safety factor during any stage of the load calculation process.
The ACCA Manual J has been tested against actual building and their actual loads. Tests have shown Manual J to be accurate. Safety factors are not needed. Safety factors lead to oversizing equipment. This is an added expense to homeowner for less efficient operation and reduced comfort.
Do not apply a safety factor to the final answer.
The Manual J loads are peak estimates. Equipment is sized to meet these loads, so their capacity will be slightly larger. If equipment does not quite meet unusual high loads, occupants can make adjustments for their rare and short duration. For example, more blankets or portable heaters on especially cold nights. Completely pull shades, turn off unnecessary lights, and reduce or shift to later cooking during hot days.
ACCA Manual J procedure has been well tested for accuracy. Any inaccuracies leading to under or over sizing equipment is likely from improper data being used. For example, assumptions being made which turn out to be inaccurate.