What is a Heat Load Calculation?

Presidential Heating & Air Conditioning

Gaithersburg HVAC Contractors


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human hand rotating thermostatHVAC designers and consultants must be able to perform heat load calculations accurately and reliably. It is a fundamental skill that is absolutely necessary for designing an HVAC system. You might be wondering, what is heat load calculation? Essentially, it is a procedure that determines how much heat has to be removed from a given space in order to properly cool it.

Heat Load/Heat Gain

There are a lot of potential heat sources that can add heat to the system. Computers, copy machines, and lighting generate heat. People generate heat, too, and the number of occupants in a building has to be factored in as a heat source. Leakage or warm air entering through open doors and windows, is another heat source.

The biggest source of heat, though, is the sun. Sunlight entering through windows can directly heat internal surfaces but the sunlight hitting the walls and roof can heat the whole building at once.

All of these sources combined are known as the heat load or heat gain, and that is expressed in either British Thermal Units (BTU) or Kilowatts (kW). For an air conditioner to effectively cool a room or building, it must have an output larger than the heat load.

Main Heat Sources

Solar Heat Gain

Sun peaking through blindsSolar heat affects the inside of buildings in three ways: convection, conduction, and radiation.

Conduction occurs primarily in walls and roofs. While these areas are in direct contact with sunlight and warm air on one side, the other side of the walls is in contact with the cooler air inside. Heat is conducted through the wall from the warm outside air to the cooler inside air. The heat of conduction is transferred slowly, and your building’s walls will store it and release it all night long.

Convection is the result of air movement. This can be either the movement of hot outdoor air leaking into the building through open doors or windows, or the air nearest the walls warming up and moving throughout the interior space.

Radiation is the direct transfer of heat, usually occurring when sunlight enters through windows. This effect is immediate- you will feel the warmth as soon as the light hits you, and feel cool as soon as you pull down the shades.

The alignment of a room will affect it is exposure to solar heat. A room with walls and windows facing east will warm up in the morning and cool off in the afternoon, while a room with walls and windows facing west will be cool in the morning and warm in the afternoon. In the northern hemisphere, north-facing windows and falls have the lowest solar heat load.

Heat from Occupants

Human experiencing heat discomfortWe generate our own body heat by burning calories. That means we radiate our own heat, and that heat can easily be felt in a closed space. Essentially, every human body is its own furnace.

In buildings with low occupancy, this is less of an issue. We do not generate that much heat individually, and with only a few people in a building that heat dissipates quickly. In spaces with dense occupancy, it can be a big part of the heat load.

Outdoor Air Heat

As people open and close doors, air from outside leaks into the building. While this can be minimized by not leaving doors open any longer than they have to be, it is unavoidable.
When outside air is hotter than the indoor air, it, naturally heats up the inside air, adding to the heat load. In some buildings, this may be negligible, but in a busier building that has lots of people coming and going, it can be more significant than most people realize.

Heat from Electronics

Computers generate enough heat that they come with built-in cooling fans to prevent overheating. Most modern buildings have computers in nearly every room. Electrical lighting creates heat, as do appliances like coffee makers and water heaters. Even refrigerators throw off heat.

The heat generated by all of the appliances and electronics in a building can add up fast. Understanding the heat load from these sources is crucial to calculating the total heat load for a building.

Heat Load Calculation Procedure

hvac technicians smiling The first step in any heat load calculation is surveying all of the rooms in the building to identify all of the heat sources. Once that data has been gathered, the calculation can be done. Gathering all of the necessary data and performing the calculations is a complex and time-consuming process that should only be attempted by an HVAC technician.

Manual calculation involves the use of several equations and tables. The specific equations and table values that are used will depend on the construction materials, building geometry, and appliances. Based on these equations and the data gathered, the HVAC technician will recommend a suitable HVAC system.

Most HVAC technicians currently use software for their Heat Load Calculations However, it is not recommended that you purchase software to do this yourself. Using the software correctly still requires a lot of technical knowledge and experience.

No matter which method your HVAC technician employs, they will likely ask you for other documentation, including building plans. They will also calculate the heat load for each room or area of the building, as well as the total building heat load.


If professional HVAC design services seem too expensive, remember that you are paying for more than installation. You are paying for their expertise. Without a proper understanding of the heat load in your building, it is impossible to buy and install an HVAC system that will adequately cool your building.

If that happens, you are likely to find yourself paying for expensive repairs on equipment that is being overworked as it is trying to cool down a building it cannot effectively cool. The cost of not having your heat load calculated is, in the long run, much higher.
To have your building’s heat load calculated and receive a recommendation for an HVAC system that will be able to cool your building properly, contact Presidential Heating and Air today.

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