Basics of heat loss, heat gain

May 10, 2000
Understanding the fundamentals of heat loss and heat gain is critical to sizing a new or replacement heating-cooling system.

This article offers an overview of the basics of these two important hvac concepts in residential system design. It’s an excellent description for newcomers to the hvac trade.

Heat loss

No matter what you do, when it is 68°F inside a house and 0° outside, the cold will suck the heat out of the house. It will pull at a certain rate through the exposed walls and ceilings, through the windows and floors. This is known as heat transfer.

The cold air is also trying to sneak into the building through every little crack in every nook and cranny. This is known as infiltration.

The home’s heat, on the other hand, is trying to escape through every nook and cranny. This is known as exhalation. It’s as if the house was breathing — breathing both air and temperature in and out.

The total of all this leaking and losing at a specific low temperature for your region, is known as the heat loss. This total will be calculated in Btu per hour (Btuh), and the heating system will need to produce and distribute this same amount of Btuh to maintain a 68° room temperature.

As most rooms differ from one another, each room’s heat loss must be determined. The total loss of all rooms added together will determine the size and design of the heating system.

In simple structures, the mere replacement of this lost heat is sufficient; but in complex houses with open floor plans and multiple levels, the flow of heat within the building becomes a factor. Heat rising from the first floor to the second increases the demand on the first floor while decreasing the demand on the second.

The formula used at the website, is a combination of three ingredients developed to reflect the internal conditions of a modern structure. It combines industry-accepted standards of heat transfer with the old-fashioned tin knocker “cfm method” of computation, blended together by 25 years experience designing and installing heating and cooling systems.

The result is an estimate of comfort.

In the heat loss calculation, all windows are created equal, no matter which direction they face. Disallowing for wind factors, similar types of glazings lose heat at the same rate.

On the other hand, when calculating heat gain, windows facing east and west gain more heat than those facing north and south. This results in larger quantities of air being distributed to rooms with east- and west-facing windows. This air is necessary for cooling but not for heating.

In the more northern climates, where heating is a priority, treat all window areas as east and west shaded, regardless of which direction they face. This will restore the emphasis on a balanced distribution system rather than one weighted toward solar radiation.

Heat gain

Cold is another word for empty. It isn’t really anything — it is, as a vacuum is, the lack of something. Cold is the lack of heat.

The earth we live on is almost empty. We live on the edge of a delicate temperature balance only some 500° above empty. The coldest it can ever get is about -460°, but “hot-wise,” temperatures can reach into the billions.

We live at the bottom of a thermometer that stretches to the moon. Our planet is a cool puddle in a desert of heat, but water works only at these narrow temperature ranges, and our life depends on this water.

It is the function of a cooling system to remove unwanted heat from a structure and relocate it to the out-of-doors. This heat exchange is accomplished by the use of the refrigeration cycle as performed by the air conditioning system.

The refrigeration cycle takes advantage of the relationships between pressure, temperature, and volume, in such a way that heat is collected inside and released outside. It uses a condenser, a compressor, and an evaporator to accomplish this task.

The condenser and compressor are located outside of the house, while the evaporator is located inside the air distribution system. The quantity of heat that needs to be removed to maintain indoor comfort, on a specific warm day for your region, is known as the heat gain for your structure.

A building gains heat from actual outdoor temperature and humidity levels. It gains heat from the people inside of it, from the lights, computers, copiers, dishwashers, ovens, etc. (Many contractors distribute an extra 1,500 Btu of cooling to the kitchen to offset the heat given off by the appliances, and an extra 400 Btu to various rooms for occupants.)

But mostly it gains heat from its exposure to sunlight, from solar radiation. The hot sun beats down on the walls and the roof, the sunlight pours through the windows and warming the floors it lands on.

The sum of all of this heat accumulation is known as the heat gain of the building.

Reprinted with permission from

Sidebar: American Standard intros e-business and expands Customer Care Program

American Standard Heating and Air Conditioning, Tyler, TX, unveiled an e-business program at its annual gathering of distributors in Florida that will allow American Standard dealers, other dealers, and consumers to use the Internet to do business.

Access would be controlled by distributors and would allow dealers to buy equipment and parts. Consumers would also be able to order service.

Distributors also learned about the expansion of the Customer Care Program, which was designed to make sure customers are 100% satisfied with their purchase.

Participating dealers have customers respond to a survey to rate their satisfaction with the product, the installation, and the professionalism of the dealers. If customers are not satisfied, dealers are notified so they can respond immediately.

Dealers who sign up for the Customer Care Program also have the opportunity to take advantage of several new programs from the company, including truck leasing, insurance, consumer financing, an Internet Web site, and a consumer referral program.

Dale Green, spokesman for American Standard, also reported record sales for the company in 1999.