How is HVAC Sizing Determined?

B. Turner

Heating, ventilation, and air conditioning (HVAC) systems are used to heat or cool the air in the home while replacing stale air with fresh supplies from outdoors. One of the primary factors to consider when choosing an HVAC system is the size of the system. HVAC sizing refers to the performance capabilities of the system, rather than the size of equipment or air handling units. The size of an HVAC system is rated in British Thermal Units (BTUs), which measure the volume of air the system can produce.

HVAC system size is rated in British Thermal Units, measuring the volume of air the system can produce.
HVAC system size is rated in British Thermal Units, measuring the volume of air the system can produce.

The majority of residential HVAC systems are sized incorrectly, which can produce a number of drawbacks to homeowners. An under-sized system will not heat or cool sufficient quantities of air to keep the home comfortable, while an over-sized system will often cause the home to be too hot or cold. Systems that are too small will not be capable of controlling moisture and humidity within a home, and will not be able to provide effective ventilation. Over-sized systems are often noisy and expensive to operate. They also cost more up front because of the larger heating and cooling units that must be purchased.

Traditional HVAC sizing calls for 12,000 BTUs for each 500 square feet (46 square meters) of living space. Some contractors use tons of air per hour rather than BTUs, with one ton equivalent to 12,000 BTUs. This rule of thumb is widely used by many HVAC contractors, yet it often results in homeowners receiving a system that is not sized properly. More and more contractors are turning to more effective methods of calculating loads in order to size HVAC systems more effectively.

The Air Conditioning Contractors of America (ACCA) produces a publication known as Manual J, which is aimed at helping contractors with HVAC sizing. Manual J provides the most widely used set of standards for HVAC sizing based on the specific needs of a homeowner, rather than on simple square footage calculations. This manual takes into account all the factors that may affect HVAC sizing, ranging from sun exposure to the average temperature range in an area.

HVAC sizing under ACCA Manual J starts with determining air volume needs based on square footage. Volume is then added or subtracted from the original calculation based on insulation, air leakage, and climate. Homes located in very hot areas will naturally require greater cooling loads than those in more moderate regions. Structures that are tightly insulated and have few air leaks will require smaller systems than buildings that are poorly insulated. Contractors also consider building orientation, shading, appliance loads, and many other individual factors when determining HVAC sizing.

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