AC cooling equipment should be sized at 95-115% of Design Total Heat Gain, or the next nominal size that is available to satisfy the latent and sensible requirements.
Heat pump cooling equipment should be sized at 100-125% of Design Total Heat Gain, or the next nominal size that is available to satisfy the latent and sensible requirements unless a larger size is dictated by the heating equipment selection.
Listed Output Heat Capacity must be sized at 100-140% of Design Total Heat Loss, or next nominal size unless a larger size is dictated by the cooling equipment selection.
Sizing HVAC systems using ACCA Manual S for energy efficient air conditioning and heating:
AHRI vs OEM Expanded Performance Data:
Manufacturers of heating and cooling equipment are responsible for testing and certifying the performance of their products. The Air Conditioning, Heating and Refrigeration Institute (AHRI) produces standards for rating such equipment, but data published in AHRI product directories should not be used. The test conditions simulate 80 F indoor and 95 F outdoor design.
AHRI certification data are based on a different set of conditions than the ACCA Manual J indoor design conditions: ACCA Manual J uses 75 F indoor design temperature NOT 80 F. Additionally, 95 F outdoor is a very small geographical location. Minnesota can have an 87 F summer outdoor design temperature while Arizona uses 108 F. Sizing your equipment using an AHRI certificate will NOT work.
Cooling equipment must be selected using the OEM data which offers a range of performance at different outdoor and indoor conditions. AHRI data also does not list sensible or latent capacity.
What is the recommended CFM and static pressure based on OEM performance data? OEM (Original Equipment Manufacturer) performance data must be supplied to the HVAC designer.
Several items from the Manual S are used in the Manual D duct design. The heating mode looks at the temperature rise requirement (Manual S section 2-6) while the cooling load looks at the air flow (CFM) associated with the selected equipment's capacity (Manual S section 3-11).
Why do most permit offices insist on Manual S sizing for your heating and cooling equipment? Because Manual S equipment selection procedures have a proven track record over many decades just like ACCA Manual J. By using proper residential HVAC selection techniques you prevent an oversized HAVC that cycles on and off continuously resulting in increased utility bills and wear and tear. Heating and cooling equipment based on properly sized equipment will run better which will save both money and energy!
Why select Savoy Engineering Group for your ACCA Manual S calculations?
Example of an AHRI Certifcate - this CANNOT be used for proper ACCA Manual S HVAC selection
VIDEOS: What is a Manual S HVAC sizing calculation?
The 6 things to consider when selecting HVAC comfort system. Most residential HVAC systems are mediocre. They are either on or off. They can't control where flow goes or provide fresh air. They can't dehumidify on demand to keep your home comfy and healthy. Know what can? Your car. Let's compare, and when you buy a new HVAC system for your house, it will actually have a chance of making your house truly comfortable - no hot or cold spot
Watch Nate the House Whisperer’s videos:
Manual S VIDEOS: Please watch Nate the House Whisperer define "What factors go into HVAC selection?" further down the page
What is ACCA Manual S References:
Everything you ever wanted to know about residential Manual J load calculations, Manual S equipment verification and Manual D duct design - videos range from simple to technical
What is Manual S? ACCA Manual S is HVAC sizing based on the manufacturer's OEM data for your outdoor and indoor design conditions so your A/C is 100-115% of the cooling load and the furnace is 100-140% of the heating requirement. Manual S shows how to select and size heating and cooling equipment to meet loads for a particular home, which are based on local climate and home construction specifics. Contractors in Miami, Phoenix, and Seattle - all extremely different climates - would never use the same equipment and expect the same results. Manual S provides sizing an in-depth explanations of how to use manufacturer's performance data.
2012 International Residential Code (IRC) clarifies the existing requirements for sizing HVAC equipment: "Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J."
Air Conditioning Contractors of America (ACCA) Manual S, Residential Heating and Cooling Equipment Selection, provides clear instructions for interpreting and applying original equipment manufacturer (OEM) expanded performance data to select equipment that meets application requirements (heating, sensible or latent cooling) for the applied design conditions used to calculate loads with ACCA Manual J, Residential Load Calculation.
In addition, Manual S sets equipment sizing limits. These sizing limits ensure that equipment capacities meet the minimum needs of occupants while preventing the problems associated with oversizing. The size of the cooling equipment must be based on the same temperature and humidity conditions that were used to calculate the HVAC load calculation.
Example of Manufacturer OEM data needed for proper ACCA Manual S HVAC selection. OEM data shows different outside summer temperatures, wet bulb temperatures (how humid) and CFM settings
Developed by Hank Rutkowski
So you are getting ready to replace an old HVAC or perhaps you are building your new dream home. Before making that HVAC purchase or even selecting the installing contractor, there are three important factors to investigate to ensure optimum comfort in your home for many years to come. This webpage addresses the 2nd step - ACCA Manual S HVAC equipment sizing developed by Hank Rutkowski.
Energy Star, LEED and many permit offices require that after the residential HVAC designer has completed the ACCA Manual J load calculation to determine the heating and cooling needs of the home, then the designer needs to size the physical HVAC equipment based on the manufacturer OEM data for the home's specific design conditions. DO NOT use the AHRI certificate (A/C capacity results at 80 F indoor & 95 F outdoor) to size HVAC equipment. ACCA Manual S is the industry standard for selecting HVAC equipment for comfort.
Most permit offices require all new residential and multifamily homes must demonstrate compliance with ACCA Manual J, S & D. Additions and alterations may also require code compliance if the mechanical contractor is installing new heating and cooling equipment.
Manual S ensures YOUR heating and cooling equipment is properly sized and will maintain the required design comfort conditions calculated by ACCA Manual J. Also ACCA Manual S verifies the HVAC has enough blower power to move the required amount of air (CFM) through the duct system. An understanding of how to interpret the manufacturers equipment (OEM) data is essential to correct HVAC sizing.
Most HVAC equipment is NOT selected following Manual S calculations. Manufacturer OEM expanded performance data MUST be used to select properly sized HVAC equipment.
If your HVAC contractor does not perform a Manual S calculation, there is a good chance that your unit will fail to provide the comfort that a properly sized HVAC will deliver. We often see HVAC inventory clearance advertisements online. What are the odds that the HVAC on clearance matches your home's exact cooling sensible and latent needs much less the tonnage? You may save a little money initially, but you pay for it in comfort and efficiency for the life of the unit.
Residential HVAC sizing has a substantial impact on the efficiency and operating costs of the HVAC. During the selection process, make sure your heating and air conditioning can meet the following criteria:
The total cooling load is determined by the ACCA Manual J heat gain calculation. The summer heat gain is made up of the sensible load (outdoor temperature vs indoor temperature) and the latent load (humidity). Cooling equipment capacity shouldn't exceed the total load by more than 15% according to ACCA Manual S.
The sensible load is the amount of actual cooling required to maintain a 75 F indoor temperature. The latent load (dehumidification) is the amount of moisture the A/C must remove. The better the A/C is sized to the cooling load, the longer the unit runs on the hottest days. The longer the unit operates the more moisture it removes from the air. Less moisture typically makes the home more comfortable.
The heating load (heat loss) is determined by the Manual J heat loss calculation. PER ACCA Manual S, furnace size should stay within 140% of the peak heating load requirement.
THE BLOWER CAPACITY:
CFM (cubic feet per minute) needed to meet the volume of air needed after all the sources of pressure loss, such as duct fittings, filters and other accessories.