An Explanation of Building Heat Gain and Loss

The Basic Principles of how HVAC Load Calculations and Calculators Work

A Conceptual Introduction to Hot and Cold

According to a scientist by the name of Sir Isaac Newton, “all matter seeks equilibrium”. Cold air and warm air seek each other out and mix together. This is the same as if you were to combine two glasses of water, each with a different temperature. The water would “mix” together forming a consistent temperature.

All buildings lose heat to the outside or gain heat from the outside. That is the reason why structures need to make up for heat loss and gain. And we do that with heating and cooling systems.

Explaining Heat Loss

Temperature Differential is the difference between the outside temperature (ambient temperature) and the inside temperature (mean temperature). When it is 72° F inside your house, and 5 degrees outside your house: you have a temperature differential of 67° F. With a differential, the cold will suck the heat out of your house through the exposed walls and ceilings, and through the windows and floors. This is known as heat transfer.  

Cold air is also trying to infiltrate the building through any opening including the smallest cracks. This is known as infiltration.

Heat, on the other hand, is trying to escape through every nook and cranny. This is known as exhalation.

Cold air is always working to get into the structure and warm air is always. trying to leave the structure. The rate of this exchange depends on many factors including the temperature differential.

The total of all this leaking and loss at a specific low temperature for your region is known as heat loss. This total will be calculated in BTUs per hour, and the heating system will need to produce and distribute this same amount of BTU’s per hour to maintain your 68° F 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.

In the heat loss calculation, all windows are created equal, no matter which direction they face. Disallowing for wind factors, similar types of glazing 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, enter 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 on weighted toward solar radiation.

Heat Gain

Cold is the absence of heat. In other words, cold can be described as “empty”. The coldest it can ever get is about -460° F. This temperature is referred to as absolute zero. It is only theoretical because no one has ever recorded it.

Conversely, temperatures can reach into the billions. Earth maintains itself at the bottom of the thermometer with average temperatures in the neighborhood of 70° F.

The Function of the HVAC System

Heat gain and loss can be predicted using generally accepted engineering principles. Heat gain and loss is generally measured in BTU’s or British Thermal Units per hour.

It is the function of a cooling system to remove unwanted heat from a structure and relocate it to the outside. This heat exchange is accomplished using the refrigeration cycle as performed by your air-conditioning system. Since it is the job of the cooling system to remove heat, a cooling system’s capacity can be measured in BTUs per hour. In other words, how many British Thermal Units of heat can be moved from one place to another in one hour.

The function of the heating system is to replace heat lost through the heat transfer process. Since it is the job of the heating system to add heat, a heating system’s capacity can be measured in BTUs per hour. In other words, how many British Thermal Units of heat can be added to a structure in one hour.

There is More to This Than Just HVAC Equipment

Both systems combined should be referred to as a comfort system. However, a comfort system is made up of more than just properly sized equipment. It includes proper supply air ducting to deliver the proper amount of warm or cool air. It also includes a return air system that helps produce adequate circulation. There are other functions that this comfort system should serve. They include air circulation, air filtration, and humidity control.

Common Terms Used in Heat Gain Heat Calculations

Absolute Zero: The temperature at which all molecular motion ceases (-460° F and -273° C).

Ambient Temperature: The temperature of fluid (usually air) which surrounds objects on all sides.

British Thermal Units: The quantity of heat required to raise the temperature of one pound of water, one degree Fahrenheit.

Celsius: The temperature scale used in the metric system. The freezing point of water is 0° C, and the boiling point is 100° C.

Cold: The absence of heat.

Comfort System: The combination of a heating and/or cooling system as well as other necessary components capable of delivering comfortable living conditions regarding temperature, humidity, air filtration, and air circulation.

Exhalation: The passage of mean air (inside air) from inside a structure to the outside of a structure through openings such as doors, cracks, windows, etc.

Heat: A form of energy, which acts on substances to raise its temperature: energy associated with random motion of molecules.

Infiltration: The passage of ambient air into a structure through openings such as doors, cracks, windows, etc.

Mean Temperature: The temperature inside a structure.

Temperature Differential: The difference between the mean temperature and the ambient temperature.

Common Questions Around Heat Gain Calculations

Question: Why is this referred to as “heat gain”? I have heard “heat gain” and “heat loss” and also “heat gain/loss”. Which terms is most correct?

Answer: The terms “heat gain,” “heat loss,” and “heat gain/loss” are all used in the HVAC industry, and each has its specific context of use. The choice of term depends on the direction of heat transfer being discussed:

Heat Gain: This term is used when referring to the increase in heat within a space, typically due to external factors. In HVAC, heat gain is a critical consideration in warmer climates or during summer months. It includes the heat entering a building from the outside (through windows, walls, roof, etc.) and the heat generated internally by occupants, lights, and appliances. Understanding heat gain is essential for determining the cooling load necessary to maintain comfortable indoor temperatures.

Heat Loss: Conversely, heat loss refers to the transfer of heat from inside a building to the outside. This is a primary concern in colder climates or during winter months. Heat loss calculations help in sizing heating systems to ensure they can adequately warm a building despite the heat being lost through walls, windows, ceilings, floors, and due to air leakage.

Heat Gain/Loss: This combined term is often used because, in many situations, both processes need to be considered. For instance, in temperate climates or in buildings with varying occupancy and use patterns, both heat gain and loss can be significant at different times of the year or even within a single day. HVAC systems in such cases need to be designed to handle both cooling and heating demands efficiently.

In terms of which term is “most correct,” it really depends on the specific aspect of thermal dynamics being addressed. In general, “heat gain/loss” is a more comprehensive term that encompasses the entirety of thermal exchange considerations in HVAC system design and operation. However, when discussing specific scenarios or challenges, using “heat gain” or “heat loss” individually provides more precise communication about the issue at hand.

Question: What scientific principles are used in calculating HVAC heat gain?

Answer: HVAC heat gain calculations primarily rely on principles of thermodynamics, particularly the law of conservation of energy. This law states that energy cannot be created or destroyed, only transferred or converted from one form to another. In the context of HVAC, this involves understanding how heat energy moves from warmer areas to cooler ones. The calculations also use principles of heat transfer, including conduction, convection, and radiation, as well as material properties like thermal conductivity and specific heat capacity.

How is the heat load of a building determined?

Answer: The heat load of a building is determined through a process called a “load calculation.” This involves considering various factors such as the size of the building, the number and type of windows, insulation levels, occupancy, and the types of appliances and equipment in use. Tools like the Manual J load calculation method from the Air Conditioning Contractors of America (ACCA) are often used, which account for these variables to estimate the building’s heating and cooling requirements.

What role does insulation play in HVAC heat gain calculations?

Answer: Insulation is crucial in HVAC heat gain calculations as it significantly impacts the rate of heat transfer. The effectiveness of insulation is measured in terms of R-value, which indicates its resistance to heat flow. Higher R-values mean better insulation, leading to slower heat gain in summer and reduced heat loss in winter. Properly accounting for insulation levels in different parts of a building allows for more accurate sizing of HVAC systems.

How do environmental factors affect HVAC heat gain calculations?

Answer: Environmental factors such as external temperature, humidity, and solar radiation greatly influence HVAC heat gain calculations. For instance, higher external temperatures or direct sunlight exposure can increase heat gain, requiring more cooling capacity. Likewise, local climate data, including average temperatures and humidity levels, are factored into these calculations to ensure the HVAC system can handle peak load conditions.

How do we verify the accuracy and reliability of heat gain calculations?

Answer: The accuracy and reliability of heat gain calculations are verified through empirical data, adherence to established standards (like ACCA’s Manual J), and the use of simulation software. Field testing and historical performance data of HVAC systems also provide real-world validation. Continuous updates to calculation methodologies based on new research and technology advancements ensure that these calculations remain accurate and relevant.

Question: Who is ACCA and what is Manual J?

Answer: ACCA® stands for (Air Conditioning Contractors of America®).

ACCA is a non-profit association in the United States that represents heating, ventilation, air conditioning, and refrigeration (HVACR) contractors. Established to promote professional contracting, energy efficiency, and healthy, comfortable indoor environments, ACCA has a significant role in setting industry standards. The association develops and publishes standards for the design, maintenance, installation, testing, and performance of indoor environment systems. It also provides training, certification, and advocacy for HVACR contractors and professionals.

Manual J:
Manual J®, developed by ACCA, is the industry-standard methodology for performing HVAC load calculations. Known officially as “Manual J: Residential Load Calculation,” it provides a comprehensive approach for determining the heating and cooling loads (i.e., the capacity required by an HVAC system) for residential buildings. This methodology takes into account a variety of factors affecting the thermal characteristics of a building, including:

  • Size and layout of the structure
  • Insulation levels
  • Window types and locations
  • Orientation of the house
  • Local climate data
  • Occupancy and usage patterns
  • Internal heat sources (like appliances and lighting)

The purpose of Manual J is to ensure that HVAC systems are properly sized. Oversized systems can lead to increased energy costs, poor humidity control, and uneven temperature distribution, while undersized systems may not adequately heat or cool a space. Accurate load calculations using Manual J are crucial for optimizing system performance, enhancing comfort, and promoting energy efficiency in residential settings. HVAC professionals widely use Manual J for designing and installing efficient and effective residential HVAC systems.

Trademark Acknowledgments

Aptora Corporation® is an independent entity and is not affiliated with the Air Conditioning Contractors of America (ACCA)®. References to ACCA or Manual J in this article are for informational purposes only and do not imply endorsement or partnership. Manual J® is a registered trademark of the Air Conditioning Contractors of America®. The use of this trademark and the information derived from Manual J in this article is purely for educational and illustrative purposes to discuss industry practices and standards. All rights and acknowledgments for Manual J and other ACCA materials are reserved to ACCA.

According to a scientist by the name of Sir Isaac Newton, “all matter seeks equilibrium”. Cold air and warm air seek each other out and mix together. This is the same as if you were to combine two glasses of water, each with a different temperature. The water would “mix” together forming a consistent temperature. All buildings lose heat to the outside or gain heat from the outside. That is the reason why structures need to make up for heat loss and gain. And we do that with heating and cooling systems.