Air Conditioning: What’s it about?

It’s all about “AIR!”

First, your home’s air conditioning system must be sized correctly to provide the proper CFM of air and BTUs of cooling to each room of your home.

What is CFM?
CFM (Cubic Feet per Minute) is a measure of volume of airflow rate. Visualize a basketball. A basketball is approximately 1 cubic foot in volume.

How many CFM’s does your system require?
For example, a typical 3 ton system (36,000 BTUs) requires a minimum of 1,050 CFM to a maximum of 1,200 CFM to perform to manufacturer’s specifications.

What are BTUs?
(British thermal unit) – 1 BTU is equal to the heat produced by burning a single wooden match or can be defined as the amount of heat required to raise the temperature of 1 pound of water by 1°F.

Once you have the proper airflow, you can cool it, heat it, dehumidify it and clean it, to provide you and your family with total personal comfort.

Bedroom airflow issues
How to know if it is a lack of supply air or lack of return air:

FIRST: Does the room cool fine during the day with the entrance door open?

SECOND: Is your problem only when the door is closed for privacy issues?

THIRD: If the room cools satisfactorily and only has issues when the door is closed, add a properly sized return air grill with duct work ran back to the air handler plenum.

FOURTH: If cooling is not adequate with door open, increase airflow supply, enlarge supply duct or add additional supply register with accompanying duct work coming from supply air plenum, plus add a return grill with accompanying duct work ran back to return plenum for good performance.

NOTE: The new building code requires all major rooms to have properly sized return air grills to allow free flow of return air back to air handler.

Who and What is ACCA?

WHO – is a non-profit association serving more then 60,000 professionals and 4,000 businesses in the HVAC community, working together to promote professional contracting, energy efficiency and healthy, comfortable indoor environments.

WHAT – they are the go-to organization for all residential home indoor energy conservation, design, and installation procedures. All NATIONAL, STATE, and LOCAL building codes utilize ACCA’s expertise in the writing of the Building and Energy Conservation Codes. ACCA’s technical manuals are the bedrock of air conditioning in America. ACCA manuals will be referred to many times throughout this website.

  • Manual J, Residential load calculations
  • Manual D, Residential duct design
  • Manual B, Air balancing and testing
  • Manual S, Residential equipment selection

Air Conditioning Sized for Your Home

There are three specific pieces of engineering data that are required when sizing the air conditioning system for your home:

1) ACCA Manual J8- “Load calculation”
2) Manufacturer’s Expanded Performance Data
3) ACCA Manual S- Heating and Cooling Equipment Sizing

Summarized Detail:

  • 1. An ACCA Manual J8 load calculation must be performed. This calculates the amount of BTU’s that enters your home in the summer and exits in the winter. Your load calculation’s “Design Conditions” are based on your LOCAL weather station data. Design conditions for Southwest Florida are 93 degrees outdoor temperature with an indoor temperature of 75 degrees and 50% relative humidity ratio (63% wet bulb–moisture content) in your home.
  • 2. The Manufacturer’s Expanded Engineering Data- manufacturers publish engineering data for all “Design Conditions”. Your Consultant must retrieve from the manufacturer this engineering data, in order to choose the proper size of equipment for your home. Your Consultant must never use the AHRI rated BTU output; this is against the Building Code. The AHRI data is a nominal, rating-test based on 95 degree outdoor- 80 degree indoor with a 67 degree wet bulb (moisture content). These parameters are only used to compare all brands of equipment. This is similar to the mileage sticker on the window of new car–not accurate and good for comparison only.
  • 3. An ACCA Manual S computerized report must be generated while your Consultant is in your home. The Consultant will use the above collected data to generate this report. Only now will the Consultant know if this equipment is an approved match for your home. This completed report will be submitted to your Building Department in order to pull your replacement permit.


Greater Detail:


#1: ACCA Manual J8 – load calculation

Why is this so important? Your system must NEVER be over sized. If it is, you will never be able to control the humidity and comfort level in your home. Click on personal comfort for the comfort table.

FACT: Today’s high efficient air conditioning systems have LESS ability to control your home’s humidity. The truth is they have lost approximately 30% of their dehumidification ability, compared to your existing system, due to Government regulations. Over-sized systems only compound the problem. Visit this site for more information. (Proper design information from ACCA)

There are only two methods to obtain the information that is necessary to perform an ACCURATE Load Calculation:

Method 1: A full set of blueprints for YOUR HOME, with all necessary details, including the following:

  • Square footage of the home being air conditioned
  • Ceiling insulation value R-? value
  • Wall insulation R-? value
  • Linear square footage of exterior walls
  • All glass windows, orientation, size, shading, eave protrusion and drop and patio over hang
  • All exterior doors and construction material
  • Skylights, if any
  • Number of occupants
  • Infiltration – how “tight” is the construction?
  • Quantity of appliances
  • Air distribution loss factor, if duct work is in attic
  • Ventilation, fresh air, or code requirements

Method 2:
The air conditioning company representative must do thorough measurements at your home in order to gather all the information that is required and listed in Method 1.

NOTE: The Building Code does not allow designer safety factors, provisions for future expansion or other factors which effect equipment sizing.

BEWARE: Garbage Information IN Equals Garbage Information OUT!

ACCA WARNING: Homeowners beware of inappropriate load calculation software! Only software that is based on ACCA Manual J8 (J8 is the latest version available) should be used in the load calculation for your home. The following exclusive ACCA-approved system design software programs have earned the right to be recognized by the following term–Powered by ACCA Manual J®:

  • RHVC Residential Load Calculation from Elite Software
  • Right-J from Wrightsoft
  • AccuLoads from Adtek Software Company
  • EnergyGuage from the Florida Solar Energy Center
  • Carmelsoft Residential Load Calculation

Special Note: There are Load Calculation Software Programs that are being utilized every day in Southwest Florida that constantly oversize your home BTU’s requirements.    


#3: ACCA Manual S – Equipment Sizing            

Information Detailed on the ACCA Manual S form:

Box 1-  Customer name and address

Box 2-  Design Information for Southwest Florida

            OUTDOOR Design Temperature – 93 degrees     summer

            INDOOR Design Temperature     –  75 degrees     summer

            INDOOR Design %RH                     –  50 %         summer

            OUTDOOR Design Temperature –  47 degrees     winter

Box 3-  Proposed Equipment to be Installed.


            This information comes from Manufacturers Expanded Performance Data

            Manufacturer            Brand Name

            Outdoor Unit             Model Number

            Indoor Unit                Model Number

            System Cooling          SEER Rating

            System Heating          HSPF Rating (Heat Pump Information)

Box 4-  Manual J – Load Calculation Information

            This information is from the ACCA Manual J8 load calculation form

            Heating Load Calculation – BTU’s

            Cooling Load Calculation   – BTU’s sensible (ability to lower temperature)

-BTU’s latent (ability to remove moisture) 

Box 7-   Manufacturers Cooling Performance      Expanded Engineering Data

This information is from the Manufacturers Expanded Performance Data.

             Equipment Capacity at design conditions – BTU’s sensible

             Equipment Capacity at design conditions – BTU’s latent

            Equipment Capacity as a % of Design – (maximum over sizing allowed is 15%)

WARNING:        If an air conditioning company representative is at your home and fails to provide you with a replacement quote following Method #1 or Method #2 above and doesn’t utilize an approved Heat Load Calculation software program, save yourself from making a FIFTEEN YEAR mistake. SHOW the REPRESENTATIVE the door; this person lacks knowledge to be designing your replacement air conditioning system.