The Impact of Heat Loads on Energy Use

Space conditioning accounts for approximately 50 percent of the total energy requirements of a building. The proper design, sizing, and operation of heating and cooling systems can translate into a typical savings of 20 percent as well as reduced noise, optimized equipment operation, and lower initial costs for equipment. It is therefore important to understand the basics of how heat flows through a building and how to identify the largest sources, or loads, of heat.

Understanding Heat Flow

The fundamental law of heat flow is that heat always flows from a warm object or substance to one that is cooler. The amount and rate of heat flowing through the building envelope depend on three factors: total surface area, temperature differences, and material properties.

Total Surface Area

The larger the area of a material, the more heat it will conduct. A 400-square-foot wall, for example, will conduct twice as much heat as a 200-square-foot-wall made of the same material.

Temperature Differences

The greater the difference between indoor and outdoor temperatures, the greater the heat flow between the two. For example, if the difference between indoor and outdoor temperatures increases from 30°F to 60°F, the heat flow through the building envelope will double.

Material Properties

The materials that form the building envelope are evaluated according to four factors: thermal conductivity, thermal conductance, thermal resistance, and overall coefficient of heat transfer.

Thermal Conductivity: Thermal conductivity measures a material’s ability to conduct heat. It is defined as the amount of heat (measured in Btu or watts) that passes through a homogeneous material 1 inch thick and 1 square foot in area, in an hour, with a temperature difference of 1°F between the outer and inner surfaces. The symbol for thermal conductivity is K. The lower the material’s K-factor, the higher its thermal insulation value. Units of K are expressed as:

(Btu x in.) / (hr x ft2 x °F)

Thermal conductivity varies considerably among different materials. Metals are the best conductors, whereas wood, asbestos, cork, felt, and plastic—commonly known as insulators—are poor conductors.

Thermal Conductance: Thermal conductance is similar to thermal conductivity except that it measures the rate of heat flow for the actual thickness of a substance 1 square foot in area at a temperature difference of 1°F. It is the number of Btu per hour conducted through one square foot of a material or materials for a 1°F temperature difference between the faces. Units of thermal conductance are measured using the following formula:

Btu / (hr x ft2 x °F)

The symbol for thermal conductance is C. The lower a material’s C-factor, the higher its insulating value.

Thermal Resistance: Thermal resistance is the measure of a material’s ability to retard heat flow rather than promote it. The symbol for thermal resistance is R. The higher a material’s R-value, the higher its insulating value. Thermal resistance is the reciprocal, or inverse, of thermal conductance. Thermal resistance units are measured as:

(Hr x ft2 x °F) / Btu

Overall Coefficient of Heat Transfer: The overall coefficient of heat transfer, also known as thermal transmittance, measures the resistance value of all building materials, air spaces (including walls and ceilings), and surface air films. The overall coefficient of heat transfer is generally referred to as the U-factor.

Recognizing Heat Loads

Heat sources in a building are also called heat loads. Each heat load in a building should be closely scrutinized to identify if any changes may be made to improve efficiency and comfort. Many space planners often call for features—such as kitchens or computer rooms—with little regard for heat gain. The following common heat loads should be reviewed for potential efficiency upgrades.

Solar Loads

The heat gained from the sun in winter months can have a positive effect in the office environment, whereas heat in the summer months can have a negative impact.

Human Heat

The normal heat generated by the average person is approximately 400 Btu. When sizing a space, special attention needs to be given to the expected number of occupants. For example, a shopping mall has to be precooled to the point that it feels chilly to the first shoppers to arrive so that it will become comfortable as the crowds grow.

Computers and Peripherals

The presence of many computers in a concentrated space can have a cumulative heat gain.


Lighting contributes to heat load, especially with the presence of incandescent light fixtures, and is the largest source of energy use in a building. In a typical office building, lighting accounts for approximately 29 percent of energy consumed. Upgraded, energy-efficient lighting systems can reduce excess heat and energy and improve lighting quality, increasing occupant comfort and productivity.

Fans and Motor Systems

Fans that move the heated and cooled air through a building constitute approximately 11 percent of energy consumed by a building. There are several ways to improve the efficiency of fans and motors, including rightsizing fans to fit the building appropriately, installing variable-speed drives on motors that allow speed reduction, and installing energy-efficient motors.

By analyzing where your heat gain is coming from, you will be able to avoid using more energy than you need to. Reduced energy usage is the quickest, easiest way to reduce your energy costs.

This article is excerpted from the BOMI International course Energy Management and Controls, part of the SMT® and SMA® designation programs. More information regarding this course and BOMI International’s education programs is available by calling 1-800-235-2664. Visit BOMI International’s website,