How to Size a Heating and Air Conditioning (HVAC) System

Introduction

According to a scientist by the name of Sir Isaac Newton (he’s they guy who had the apple fall on his head), “all matter seeks equilibrium”. Cold air and warm air seek each other out and mix together. This is why your home or office often feels drafty.

This is the same situation as if you were to combine two glasses of water, each with a different temperature. The water would “mix” together forming a consistent temperature.

Hot and cold air are always trying to get together and equal things out. 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. That’ why we make a living installing and maintaining HVAC (heating and air conditioning) systems.

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. Your 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 losing at a specific low temperature for your region is known as the heat loss. This total will be calculated in BTU’s 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 typical heat loss calculation (like more heat gain software), 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 actually the absence of heat. In other words, cold can be described as “empty”. There is no such thing as cold; only the absence of heat.

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; but scientists have gotten extremely close in a laboratory setting.

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

Conclusion

Heat gain and loss can be predicted using generally accepted engineering principles. Heat gain and loss is generally measured in BTU’ 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 by the use of 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’ capacity can be measured in BTU’ 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’ capacity can be measured in BTU’ per hour. In other words, how many British Thermal Units of heat can be added to a structure in one hour.

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.

Heat Gain and Loss Related Definitions

Absolute Zero: The theoretical temperature at which all mater stops moving (-460° F and -273° C). This temperature has never actually been achieved.

Ambient Temperature: Outside air temperature. More specifically, 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. There is no such thing as “cold”.

Comfort System: The combination of a heating and/or cooling system as well as other necessary components capable of delivering comfortable living conditions with regard to 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 windows, doors, cracks, vents, etc.

Mean Temperature: The temperature inside a structure.

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

Heat Gain and Loss Software

There are several software programs on the market designed to allow heating and air conditioning technicians to quickly and accurately calculate the correct size of heating and air conditioning (HVAC) equipment. Software such as Easy Load Calc, uses the simple and quick “whole house” concept. This method is appropriate for most normal residential HVAC sizing scenarios.

Other heat gain and loss calculation software programs, such as WrightSoft’s Right-J® software, use a more complicated method known as “room-to-room. Their calculations are based on the Manual J calculation concepts developed and owned by ACCA ® (Air Conditioning Contractors of America®). Please check their website for more information.

Heat gain or loss comments?