What Does ACCA Manual S Mean for HVAC Equipment Selection?
The residential HVAC system should have its size determined on the basis of more than just the square footage of the house or the nominal tonnage given on the unit, with ACCA Manual S being the standard used when selecting and sizing residential heating and cooling equipment after the building's heating and cooling loads have been calculated.
A Manual J load calculation is used to establish the amount of heating and cooling that a house requires. Using those load figures together with the local design conditions and the manufacturer's performance data, Manual S decides if a particular furnace, air conditioner, heat pump, or other HVAC system is an appropriate match.
For instance, a house could have a cooling load of 32,000 BTU/h, but this does not necessarily imply that a nominal 2.5-ton or 3-ton system is the appropriate choice; the equipment has to be assessed in light of its actual capacity under the operating conditions that the project anticipates.
This is the stage at which Manual S is incorporated into the residential HVAC design process.
What Is Manual S in HVAC?
Manual S is the ACCA standard concerned with the selection of residential equipment, and it sets out the procedures for selecting and sizing heating, cooling, dehumidification, and humidification equipment on the basis of the home's calculated loads, the local design conditions, and the manufacturer's performance information.
ACCA presents Manual S as a guide to selecting and sizing residential heating and cooling equipment taking into account the local climate and construction conditions; the current entry on the ACCA page is for the third edition, ANSI/ACCA 3 Manual S – 2023, together with Addenda A and B from 2024.
Manual S goes beyond being merely a chart for sizing equipment; it involves matching the building's calculated requirements with the real-world performance of the equipment that is available.
For instance, a contractor must not choose an air conditioner simply because the nameplate states it is a "3-ton" system, since the real capacity of that system can differ according to the operating conditions and the combination of equipment.
An accurate calculation of the Manual S value takes into account the relationship between the load and the equipment's stated performance.
What Is the Purpose of Manual S for HVAC Equipment Selection?
The main purpose of Manual S for HVAC equipment selection is to ensure that the chosen equipment can effectively meet the calculated heating and cooling loads without being either too large or too small.
A good equipment selection should consider several factors, including:
- Heating load
- Cooling load
- Sensible cooling capacity
- Latent cooling capacity
- Outdoor design conditions
- Indoor design conditions
- Equipment operating conditions
- Manufacturer performance data
- Equipment type
- Available equipment combinations
- Heating capacity at relevant outdoor temperatures
- Cooling capacity at relevant entering-air conditions
- Equipment sizing limits
This is why HVAC system sizing should not rely solely on the square footage of a house.
A 2,000-square-foot home in Phoenix has different cooling needs compared to a 2,000-square-foot home in Seattle. Factors like insulation, windows, orientation, air leakage, ceiling construction, internal gains, ventilation, and outdoor design conditions all influence the load calculation.
ACCA clearly states that when choosing equipment, you must consider local climate and construction conditions instead of just using the same equipment size for homes in different areas.
Manual S vs. Manual J vs. Manual D: What Is the Difference?
The easiest way to understand the three standards is to think of them as different stages of residential HVAC design.
Standard | Main purpose | Primary question |
Manual J | Load calculation | How much heating and cooling does the home need? |
Manual S | Equipment selection | Which HVAC equipment can properly meet those loads? |
Manual D | Duct design | How should air be distributed through the home? |
Manual J: Calculates the Load
Manual J determines the home's heating and cooling loads.
It considers factors such as:
Building size and geometry
Insulation
Windows and doors
Solar exposure
Air infiltration
Internal heat gains
Ventilation
Outdoor design conditions
Indoor design conditions
The standard for calculating the heating and cooling loads required for houses, such as single-family detached dwellings, townhouses, condominiums, and some small multifamily buildings, is Manual J as identified by ACCA.
The outcome isn't just that 'this house needs an AC unit of 3 tons'. Instead, it gives detailed heating and cooling requirements which can then be used when selecting the equipment.
Manual S: Selects the Equipment
Manual S uses the load information and manufacturer performance data to decide if specific equipment is suitable. Here, the designer checks the actual equipment capacity instead of just relying on nominal tonnage.
Manual D: Designs the Duct System
Manual D follows the equipment selection and is used to design the residential duct system. The duct design considers factors like:
- Required airflow
- Available static pressure
- Duct length
- Fittings
- Duct material
- Supply and return paths
- Air distribution requirements
ACCA describes Manual D as the ANSI-approved standard for residential duct design.
In practical terms:
Manual J → Manual S → Manual D
Calculate the load → Select the equipment → Design the ducts.
These standards work together instead of replacing each other. ACCA's materials for reviewing residential system design show the same relationship. Manual J determines the heating and cooling needs. Manual S assesses equipment choices using load calculations and OEM data. Manual D sets duct sizes based on the chosen equipment and the features of the duct system.
What Does Manual S Include?
The standard includes the procedures needed for selecting and determining the correct size for residential heating and cooling equipment; according to ACCA's present description, Manual S covers heating, cooling, dehumidification, and humidification equipment and specifies size limits which are meant to help make sure that properly installed equipment will be able to give suitable performance.
The second edition, for example, included sections covering:
Equipment size limits
Cooling-only equipment
Heat pump equipment
Furnaces
Boilers
Humidity control
Central ducted systems
Ductless systems
Single-package equipment
Manufacturer performance data
Matching evaporators and condensing units
Altitude effects
Dehumidifier performance
Equipment cycling
Performance models
Equipment selection examples
The current third edition includes more detailed guidance on variable-capacity heat pumps, dehumidification, and code compliance. The specific requirements depend on the edition in use, the type of equipment, and the project needs. Designers should use the applicable current standard instead of depending on an outdated worksheet or rule of thumb.
Manual S Equipment Selection Criteria
The Manual S equipment selection criteria rely on more than just the nominal capacity shown on an equipment label. A designer typically needs to assess the equipment based on the calculated building loads and the conditions expected during operation. Important factors to consider include:
1. Calculated heating load
The chosen heating equipment must offer sufficient heating capacity for the home's design heating condition. For heat pumps, this is particularly crucial since heating capacity can vary with outdoor temperatures.
2. Calculated cooling load
The cooling equipment must meet the calculated cooling needs without going over the size limits. A bigger system is not necessarily better.
3. Sensible cooling capacity
Sensible capacity involves lowering the actual air temperature. For example, if a home has a high sensible cooling demand, the chosen equipment must deliver enough sensible capacity under the expected operating conditions.
4. Latent cooling capacity
There is a connection between latent capacity and the removal of moisture.
This is important since a fan can reduce the temperature without necessarily offering proper humidity control.
A system which is excessively large might be able to meet the temperature load rapidly and then switch off before it has had a chance to remove enough moisture.
5. Manufacturer performance data
One of the most important aspects of choosing equipment is this.
Manufacturers issue performance data which shows how equipment performs under various conditions, and a designer uses this data to find out the real capacity of a specific equipment combination.
The ACCA makes it clear in Manual S how manufacturer performance data should be used when choosing equipment.
6. Outdoor design conditions
Equipment performance depends on outdoor conditions. This is especially important for heat pumps because their heating capacity and efficiency can vary greatly with changes in outdoor temperature.
7. Equipment type and operating characteristics
Single-stage, two-stage, and variable-capacity equipment can perform differently. For example, a variable-capacity heat pump may operate effectively across a wider temperature range than a conventional single-stage system. It's important to evaluate the actual performance data of the equipment instead of assuming that all equipment with the same nominal tonnage works the same way.
How to Use Manual S for Sizing HVAC Equipment
The procedure for using Manual S to size HVAC equipment may be outlined in a number of steps.
Step 1: Complete the Manual J load calculation
Begin with a Manual J calculation that has been properly prepared.
The calculation must determine the heating and cooling requirements of the house, including those on a room-by-room basis where appropriate.
The equipment selection will be based on a weak foundation if the load is not accurately known.
Step 2: Establish the project design conditions
Choose the suitable outdoor and indoor design conditions for the project.
The place where something is located is important; a system developed for Phoenix isn't guaranteed to be tested under the same outdoor conditions as one that was designed for a considerably colder climate.
Step 3: Determine the required equipment type
The project may call for:
- Central air conditioning
- Heat pump
- Gas furnace
- Electric heating
- Dual-fuel equipment
- Ductless equipment
- Packaged equipment
- Other applicable residential HVAC equipment
The system type affects how the equipment is evaluated.
Step 4: Identify candidate equipment
The designer then reviews available equipment from manufacturers.
This involves looking beyond the nominal product size.
For example, two systems advertised as "3-ton" systems may have different actual capacities under the same operating conditions.
Step 5: Review manufacturer performance data
The equipment's published performance data shows how much heating and cooling capacity it can really provide.
This may involve checking:
- Total cooling capacity
- Sensible cooling capacity
- Latent cooling capacity
- Heating capacity
- Outdoor temperature
- Entering air conditions
- Equipment combinations
- Blower settings
- Other applicable performance conditions
Step 6: Compare equipment performance with the Manual J loads
The available capacity of the equipment is compared with the calculated load.
The designer is not simply asking:
Is it a unit of three tons?
The more useful question is:
Does the particular combination of equipment have the right capacity under the design conditions of the project?
Step 7: Check Manual S sizing limits
The equipment that has been selected must also meet the sizing limits specified in the relevant Manual S.
ACCA's residential equipment-selection materials clearly list equipment size limits as a key part of Manual S.
Step 8: Verify the equipment combination
With split systems, it is necessary to assess the indoor and outdoor units as a complete and matched system.
The choice of evaporator coil, condenser, furnace or air handler, and any other relevant components may have an effect on system performance.
Step 9: Use the selected equipment for Manual D
After the equipment has been chosen, the airflow it requires and all the other characteristics of the system become inputs for the design of the ducts.
Manual D can be used to work out the suitable duct sizes and the way in which the air should be distributed.
Manual S Equipment Selection Example
A Manual S example calculation is easier to understand with a simplified scenario.
Suppose a Manual J calculation produces:
Cooling load: 30,000 BTU/h
Heating load: 38,000 BTU/h
A designer is considering a nominal 2.5-ton air-conditioning system.
Because:
2.5 tons × 12,000 BTU/h per ton = 30,000 BTU/h
it may appear that the equipment is an exact match.
But this does not complete the Manual S selection.
The designer still needs to determine the equipment's actual performance at the project's design conditions.
For example, assume the manufacturer's data shows that the proposed system provides:
Total cooling capacity: 29,500 BTU/h
Sensible cooling capacity: 22,000 BTU/h
The designer would then compare those values with the applicable Manual J cooling load and sensible load.
If the equipment does not provide the required performance, another equipment combination may be needed.
The same concept applies to heating.
Let us assume that a heat pump has a nominal heating capacity of 36,000 BTU/h under a particular test condition; that figure does not mean that it will actually deliver 36,000 BTU/h when the outdoor temperature reaches the home's design temperature.
The designer should look at the manufacturer's performance data under the relevant conditions.
The example given here is intended as a simplified illustration for educational use. In reality, making a selection for the Manual S requires reference to the relevant standard, the design conditions of the project, and the manufacturer-specific performance data.
Why Nominal Tons Are Not Enough
A frequent error in HVAC sizing is to confuse nominal tonnage with actual delivered capacity.
It’s common to see a “3-ton” air conditioner described in terms of its nominal capacity, but the actual capacity may differ depending on the operating conditions and the equipment it’s matched up with.
The same goes for heat pumps.
This is one reason why a rule of thumb like "one tonne per 500 square feet" should not substitute for a load calculation and equipment-selection procedure.
Consider two homes:
Home A
2,000 square feet
High-quality insulation
Low solar exposure
Efficient windows
Low air leakage
Home B
2,000 square feet
Older windows
Greater solar exposure
Higher infiltration
Less insulation
Although they have the same floor area, their heating and cooling requirements can be very different.
The loads are determined by Manual J, and then Manual S decides which of the available units can adequately meet them.
Why HVAC Systems Should Not Be Oversized or Undersized
Choosing the right HVAC system size is about more than selecting the largest unit for extra capacity or the smallest unit to reduce upfront costs. Equipment that is either oversized or undersized can create comfort and performance problems.
An oversized air conditioner may reach the thermostat setting too quickly and shut off before running long enough to remove sufficient moisture from the air. This can lead to:
Short cycling
Uneven temperatures
Poor humidity control
More frequent starts and stops
Reduced comfort
Increased equipment wear
An undersized system can create the opposite problem. During periods of peak heating or cooling demand, the equipment may struggle to maintain the desired indoor temperature. Possible signs include:
The air conditioner runs for long periods
The home does not reach the thermostat setting
Certain rooms remain uncomfortable
Heating equipment cannot maintain the desired indoor temperature
The system operates near maximum capacity for extended periods
The goal is not to choose the largest or smallest HVAC system available. The goal is to select equipment that properly matches the home's calculated heating and cooling loads and performs appropriately under the project's design conditions.
For a more detailed comparison, see our guide on oversized vs. undersized AC units
Why Manual S Matters for Heat Pumps
Manual S has become particularly relevant with the increasing use of heat pumps in residential HVAC systems.
The performance of a heat pump is not the same at all outdoor temperatures; since the heating capacity varies with changes in the outdoor conditions, choosing the right equipment involves more than just referring to the nominal heating capacity.
The expanded sizing tolerances for variable-capacity heat pumps and the extra guidance concerning heat pump heating sizing are both specifically stated in ACCA's present Manual S information.
For a heat pump project, a designer may need to consider:
Heating load
Cooling load
Outdoor design temperature
Heat pump capacity at relevant temperatures
Supplemental heat
Defrost operation where applicable
Equipment controls
Variable-capacity operation
Manufacturer performance data
This is particularly relevant in climates where heating and cooling loads differ substantially.
Manual S and Building Code Compliance
Manual J, Manual S, and Manual D are not merely optional worksheets used for internal HVAC planning.
Building codes and various jurisdictions throughout the United States refer to ACCA standards. The ACCA says that Manual J is required by the national building codes as well as by many state and local jurisdictions, and its materials for code officials describe Manual J, S, and D as being referenced and required in America.
Nevertheless, the requirements at the local level may differ.
Cities or counties may have particular requirements as to the documentation needed when submitting a permit. The designer should therefore check which edition of the documents, the forms, the calculations, the equipment documentation, and the supporting information are required by the authority having jurisdiction for the project.
When carrying out permit work, it is not sufficient just to choose equipment that appears reasonable; the documentation must support the design and show how the equipment selected relates to the calculated loads.
Common Mistakes in Manual S Equipment Selection
Choosing equipment based only on square footage
Square footage is not a substitute for a Manual J load calculation.
Selecting equipment by nominal tonnage
Nominal capacity does not tell the entire performance story.
Ignoring sensible and latent capacity
Cooling capacity includes both temperature reduction and moisture removal. Both can matter for comfort.
Using generic equipment data
Manufacturer-specific performance data should be used when evaluating specific equipment.
Ignoring local climate conditions
Equipment performance depends on operating conditions, so the project's location and design conditions matter.
Selecting equipment before calculating the load
Starting with an existing equipment size and working backward can produce an inappropriate design.
Treating Manual J and Manual S as the same thing
Manual J calculates the load. Manual S selects equipment based on those loads and equipment performance.
Forgetting the duct system
The equipment cannot be considered in isolation from the air distribution system. After equipment selection, Manual D provides the duct-design procedure needed to deliver the required airflow.
Who Should Perform a Manual S Calculation?
Manual S equipment selection requires you to know HVAC load calculations, equipment performance data, design conditions, and residential system design.
A qualified HVAC designer or engineer should be able to:
Have a look at the Manual J results.
Set up the design conditions that apply.
Identify suitable equipment.
Review manufacturer performance data.
Take the real equipment capacity and compare it with the calculated loads.
Check applicable sizing limits.
Document the equipment selection.
Arrange the chosen equipment in accordance with the duct design.
Although software can simplify the calculations, it does not take the place of engineering judgment or compensate for incorrect input data.
ACCA also warns that its Speed-Sheets are aids to learning and for carrying out calculations and not substitutes for the full technical manuals or for ACCA-approved software.
Frequently Asked Questions About Manual S
What is Manual S in HVAC?
Manual S is the ACCA standard for residential HVAC equipment selection. It provides procedures for selecting and sizing heating, cooling, dehumidification, and humidification equipment based on calculated loads, design conditions, and manufacturer performance data.
What does Manual S include?
Manual S includes procedures for evaluating residential heating and cooling equipment, equipment size limits, manufacturer performance information, and equipment-specific selection guidance. The current edition also includes updated guidance related to heat pumps, dehumidification, and code compliance.
Is Manual S the same as Manual J?
No. Manual J calculates the heating and cooling loads of the home. Manual S uses those loads and manufacturer performance data to select appropriate HVAC equipment.
Is Manual S the same as Manual D?
No. Manual D is used to design the residential duct system. It follows equipment selection and uses information about the selected equipment and required airflow.
How does Manual S help with HVAC system sizing?
Manual S helps determine whether specific HVAC equipment can provide appropriate heating and cooling capacity under the project's design conditions. It moves equipment selection beyond a simple nominal-tonnage calculation.
Can I use an air conditioner size calculator instead of Manual S?
An online air conditioner size calculator can provide a rough estimate, but it should not be treated as a substitute for a properly prepared Manual J load calculation and Manual S equipment-selection process for a permit or engineered HVAC design.
Does Manual S apply to heat pumps?
Yes. Manual S includes procedures for residential heat pump equipment selection. The current edition includes expanded guidance concerning variable-capacity heat pumps and heating sizing.
Why does manufacturer data matter in Manual S?
Manufacturer performance data provides the information needed to determine how a particular equipment combination performs under specified operating conditions. Two systems with the same nominal capacity can have different performance characteristics.
Does Manual S determine duct sizes?
No. Manual S selects the equipment. Manual D is used to design and size the duct system after the equipment selection is established.
Need a Manual S Calculation for Your HVAC Project?
Picking out HVAC equipment involves more than just matching a house to a nominal tonnage; a correct Manual J calculation links the building's calculated heating and cooling loads with the actual performance of the equipment in question.
We offer residential HVAC design services which include carrying out Manual J load calculations, Manual S equipment sizing, and Manual D duct design; these services can be provided as a coordinated Manual JSD package when a project involves the need for load calculations, equipment selection, and duct design all being carried out together.
If you are putting together an HVAC permit package or require assistance in deciding on the right equipment for a residential project, then get in touch with Heatload to talk about your project requirements, plans, the time you need for completion, and the prices.
Get a Quote -
Phone: 602-245-5273Email: support@heatload.net
Comments