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.

 

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.

Educate customer.

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.

The optimal sized equipment is the smallest defensible size. This means using a method recognized as comprehensive and accurate. The ACCA Manual J does this, for more on Manual J. Here are the benefits of using this method.

Provide specified comfort and humidity control at design and part-load conditions.

The nature of residential hvac equipment is that they operate either on or off. Over-sized equipment provides too much capacity when it is turned on. For example, a furnace once on is providing much more heat than is needed to compensate current heat loss. This causes the indoor temperature to rise quickly often overshooting the desired temperature. Then the furnace turns off, allowing heat loss to cool the house and reduce the temperature. This oscillation of temperature will be perceived as discomfort. Some thermostats have tried to deal with this by turning the furnace on and off quicker. However, this leads to faster wear and tear on the furnace.

In the summertime, the air conditioner removes heat and humidity. That water dripping off of the cooling coil is humidity from the house. In most areas, during most of the time, the normal operating time of the air conditioner can keep the humidity at a comfortable level. But an over-sized air conditioner runs less frequently, so less humidity is removed from the indoor air. The thermostat senses temperature not humidity, so it is no help. High humidity throughout the house may be a indication of an over-sized air conditioner.

Provide reasonable comfort at outdoor design conditions.

The Manual J procedure uses the 99% and 1% weather data for your location. This means the actual temperature is the same or higher than the winter  design temperature 99% of the time based on statistical weather information. The actual temperature exceeds the summer design temperature only 1% of the time. Using this data, prevents equipment over-sizing due to infrequent temperature extremes. It may seem the equipment may not be able to handle rare extreme temperatures. In reality, this is not true. The homeowner can often take measures to help reduce heat gain or loss during extreme weather. For example, draw blinds or curtains on hot or cold days or adjust schedule so appliances are not used during peak summer afternoon. If existing equipment cannot maintain comfort yet is sized for these temperatures, it may be due to degradation of efficiency due to poor maintenance or increased infiltration due to unknown openings in walls or ductwork.

Optimize installed and operating costs.

Quite simply, larger equipment has a higher cost. The initial higher cost for larger equipment is obvious. The operating costs over the life of the equipment is also greater for larger equipment. Assume you are only meeting heat gain and heat loss, with the same running (steady-state) efficiency of equipment, then size doesn’t matter. But there is far more to the situation. Larger equipment will be starting and stopping more frequently. Efficiency and energy use is higher during the start sequence of the equipment. When it stops, residual energy in the equipment is lost. So even if run time efficiency is the same, energy use is higher for larger equipment due to more frequent starts and stops.

Improve reliability of equipment.

Most of the wear and tear on equipment is the turning on and off. As explained in the paragraph above, larger equipment cycles on and off more frequently. Modern HVAC equipment is designed to be reliable while it is running. Much of the wear and tear on equipment is during startup. For example, an air conditioner compressor needs oil to operate and the motor needs cool refrigerant. During startup most of the oil has dripped to the bottom of the compressor, so the initial few minutes the compressor has an insufficient amount of oil. Until the operating temperatures are reached, the motor is not being cooled as designed by the cool refrigerant. Yet during startup is when the motor pulls much more current resulting in heat.

Reduced size of ductwork.

The amount of air delivered tends to be lower in smaller systems. Ductwork sized to lower airflows is smaller which leads to lower material cost and in some case lower installation cost (smaller ducts are easier to fit into tight spaces).

Reduce load on utility system.There have also been discussions of the utility limiting your power

Simply, larger equipment puts a larger load on the utility system. This is especially critical for summertime cooling. Residential air conditioners running in the middle of the afternoon place a large load on the electric system. This leads to higher billing. Some utilities have gone to charging more for summer afternoon power.

Demonstrate “due diligence” in a court of law.

If a dispute erupts between you and the homeowner, you need to be able to show proof you conducted business in a professional manner consist with the trade practices.  Sizing equipment based on a rule of thumb or even your past experiences alone is not currently accepted trade practice. Providing an accurate Manual J provides the proof to support and justify your actions.

Bottom Line

These numerous benefits of completing a Manual J calculation are great. Go here to have your ACCA Manual J Analysis done for you.

Manual J is a procedure for estimating the heating and cooling load of a residence (single-family detached homes and mobile homes). This procedure is published and maintained by the Air Conditioning Contractors of America (ACCA). It is now in its eighth edition; older versions should not be used.

ACCA Manual J should be used anytime the heating and or cooling system is provided new or replaced in existing. It can also be used to evaluate an existing system. Manual J is required by code. “… building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies. “ IRC 2009 M1401.3

Each code jurisdiction will determine the scope and method of determining compliance. Also some energy rebate/incentive programs require Manual J calculations.

Manual J was written by experienced engineers within the Heating, Air Conditioning and Ventilating (HVAC) field. The method follows industry recognized procedures by the American Society of Heating, Air Conditioning and Refrigeration Engineers (ASHRAE) for all building types.

Manual J is a procedure; a set of rules, equations and many tables. The procedure may be performed in any manner. ACCA nor building codes specify how the procedure is to be completed. You can make all of the necessary calculations by hand using a calculator and forms. There are computer spreadsheets which increases the speed and accuracy of the results. This procedure can also be programmed into a computer. Software can ideally improve accuracy and time to complete. ACCA does approve software as following the Manual J procedure. Look for the “Powered by ACCA Manual J” logo. A list of these programs is at www.acca.org/industry/system-design/software.

Manual J can be performed in-house by HVAC Contractor using whichever method desired. This provides for the greatest control over completing the procedure. Changes can be made quickly and results can be checked when home is completed. The disadvantage is that at least one employee must take the time to learn the procedure. Even the software requires learning. The cost appears to be low, but can be quite high if the employee spends a huge amount of time completing.

The greatest variable in obtaining good accurate results is the accuracy and thoroughness of the input data. The old saying “garbage in is garbage out” applies here. Slight inaccuracies in some input data can yield large inaccuracies in the results. While other input data has minimal impact. The professional performing the calculations needs to know which data is critical and which data not to spend a lot of time on. For example, air infiltration is often incorrectly estimated. This can lead to large inaccuracy in the load estimate. On the other hand, spending much time on trying to get a detailed evaluation of internal loads is not necessary. Small electrical loads have little impact on equipment size. Usage is so variable and based on individual tastes; no estimate will be completely accurate.

For new homes, Manual J can be performed from a good set of drawings. For existing homes, the house needs to be surveyed and measured. Information on dimensions, building materials and the effects of age must be obtained. A blower door test for evaluating air infiltration is an absolute must. This information should be collected by an experienced energy auditor, preferably with Building Performance Institute (BPI) or Residential Energy Services Network (RESNET) certification.

Hire it done. While it may be good to have the expertise in-house, hiring an expert greatly shortens the time to complete the analysis. The expert may have years of experience and understands how all of the input parameters impact the results. Some energy rebate/incentive programs may require performance by independent party. Seeking a professional can be easy and low cost.

The old fashion method of sizing HVAC equipment based on house square footage is no longer valid or allowed by building code. Also keep in mind the litigious nature of modern society. This rule of thumb may be adequate some of the time, but what about the other times? Do you really have the time and money to fight off lawsuits?