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When getting a new unit the contractor needs to complete a heat load calculation in order to know the right size of air conditioner to install. The calculation is also necessary in the Phoenix Metro in order to receive a rebate from SRP or APS. We perform heat loads for all of our installs and were going to walk you through how this calculation is made. Basically, how much heat enters the home is calculated. There are 7 components to this.


  1. Design Temperatures
  2. Heat gains from exterior walls, ceilings, and floors
  3. Heat gains from windows, and doors
  4. Heat gain from infiltration (conditioned air leaking out of the home)
  5. Internal heat gain from appliances, and occupants
  6. Duct Loss Gain Factor
  7. Moisture Removal

Each Of These Steps Are Very Important To Know What Size Of Air Conditioner Is Needed


1. Design Temperatures

There are two design temperatures, the outdoor design temperature, and the indoor design temperature. The outdoor design temperature is sort of the average maximum high temperature and is referenced from an outdoor design temperature chart (ex. Phoenix is 108, Tucson is 105, and Prescott is 96). The indoor design temperature is what the homeowner likes to set their thermostat to (ex. 72 F). The design temperature determines the delta T value and is one of the cooling factors.

2. Heat Gains from Walls, Ceilings, and Floors

To calculate the heat gain from the walls you need to do two things. First, calculate the area of the walls, and then determine the R Value of the wall. The R Value of the wall determines the cooling factor and can be determined by how thick the wall is (about 3 values per inch). Area x Cooling Factor = heat gain from a wall.

3. Heat Gains from Windows and Doors

To calculate the heat gain from windows and doors you need to know the area and their cooling factor. The cooling factor of a window is determined by the number of panes it has in the direction that it is facing (north, south, east, and west), and delta T. The cooling factor of a door is determined by its material (wood/metal), and delta T. Area x Cooling Factor = heat gain from windows or doors.

4. Heat Gain from Infiltration

To calculate the heat gain from infiltration, you need the volume of the structure, the Air Changes Per Hour, and delta T. The Air Changes per hour is the percentage of air that leaks out of the house in one hour. This is based off the area of the house, and its age value which can be looked up (20+ years old poor, 10-20 years old Fair, 0-10 years old Good). Air Changes per hour can also be measured with a blower door.

5. Internal Heat Gain from Appliances and Occupants.

The heat gain from appliances and occupants can be measured at about 1,200 BTU an hour for a kitchen and 300 BTU an hour for a person. Base the number of people on the number of bedrooms (2 per bedroom, and how many kitchens (usually 1)).

6. Duct Loss Gain Factor

The Duct Loss Gain Factor is determined by the R Value of the insulation that the ducts are contained in, and the duct location. The R Value per inch for Pink Fiberglass is about 2.5 values per inch. Duct Loss Gain Factor is usually somewhere between 1.0 and 1.2, you multiply the sum of all the Heat gains above by it.

7. Moisture Removal

Multiply the Heat gain with Duct Gain accounted for by 1.3 to account for moisture removal. This doesn’t change from a Humid Climate, because Phoenix still has high humidity during monsoon season.


What you will end up with is a number measured in BTU/hr. 1 Ton of cooling equals 12,000 BTU/hr. so divide your heat gain by 12,000 to get your tonnage, then you will know what size air conditioner to install!

That is the process in a nutshell that contractors go through to find out what size of air conditioner your home needs.

Contact SMW Refrigeration if your thinking about replacing your air conditioner or are building a new home and need HVAC equipment. We will make sure the right size air conditioner is installed.