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Find the Correct Size of HVAC Systems

A correctly sized HVAC system can have a big impact on energy consumption and costs. Determine the right size to provide comfort and keep your client’s costs down.

Accurate sizing is a critical first step in the efficient operation of HVAC systems. Because proper sizing of a residential system is dependent upon multiple variables such as heat loss and gain, ductwork and sunlight, guesswork can lead to discomfort and energy usage issues on either end of the spectrum.

If undersized, the system typically will run flat out, yet not provide proper comfort. If oversized, the compressors could be "short cycling"—a sure sign of inefficiency, says Terry Townsend, president of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

"Oversizing and undersizing are prevalent in commercial and residential applications because people use the rule-of-thumb sizing," Townsend says, referring to general load calculations based solely on square footage.

Size Matters
Rule-of-thumb practices are discouraged, and ASHRAE maintains that comfort system performance is only as good as the accuracy of the heat-loss/heat-gain estimate. For instance, floor area to tonnage ratios for U.S. housing stock can range from less than 500 sq. feet per ton to more than 1,200 sq. feet per ton, depending upon exposure.

The U.S. Department of Energy (DOE) estimates that most current residential systems are oversized and would benefit from correct sizing. Correct sizing can reduce cyclic losses and improve part load humidity control, however, energy conservation can be compromised quickly if a new system's size is based on an old system's capacity. This mistake is more likely if the original system was not sized properly to begin with, or if a home's heating and cooling needs have changed since the installation.

According to the U.S. Census Bureau, residential footprints have trended higher over the past 30 years as a result of America's obsession to gain square footage through renovations and larger-scale new construction. In 1975, the average new single-family house was 1,645 sq. feet compared to 2,434 sq. feet today. Coupled with the preference shift from single-story to double-story houses, it's increasingly common for larger homes to have more than one HVAC system to handle the load.

Use "Manual J" Size
According to Jim Herritage, technical instructor for the Air Conditioning Contractors of America (ACCA), performing a detailed load calculation is where it all begins for retrofits or new construction. "Year-round comfort is the goal," he says. "In the summer, an air conditioning system not only cools a home's air (sensible cooling), it removes moisture (latent cooling). In the winter, the heating system must keep people comfortable without high utility bills."

When sizing a HVAC system to a residential property, ASHRAE recommends trained and qualified contractors use the industry standard for sizing residential systems: ACCA Manual J Residential Load Calculation Procedure. "Manual J is the official standard for residential load calculation and is required by many building codes around the country," Herritage says, referring to the ANSI-approved procedure now in its eighth edition (Manual J-8).

An important aspect to proper sizing is performing a room-by-room load calculation. The 2003 International Residential Code Commentary (M1401.3 Sizing) recommends a room-by-room calculation that allows designers to estimate the air-flow requirements for each area in the house. The estimate can be used in sizing the duct system for the types of heating and cooling units that use air as the medium for heat transfer.

New Construction vs. Retrofit
There are different sizing considerations for new construction versus retrofit, Townsend says. "In retrofits, where there are no other volumetric changes being made to the house such as an addition, or modifications to a basement or attic currently not conditioned, the capacity of the new unit will most likely stay the same as the old unit if no comfort problems have been experienced with the heating and cooling operations for the areas served," Townsend says.

Some differences may impact this. "With respect to an analysis of the envelope of the building where the insulation levels, the volumetric size, and internal loads, such as home theaters, home offices, commercial kitchens, indoor pools and hydro-jet tubs, may require a modification to the HVAC system," Townsend adds.

But don't forget the ducts. Adequately sized supply air and return air ductwork (ACCA Manual D) is essential for efficient and proper circulation of conditioned air. "Properly installed and maintained ductwork can last 20 years or more," Herritage says. "But time, heat and humidity can degrade a duct's insulation and contaminants can collect."

The DOE reports that if a duct system is improved with increased insulation and reduced leakage, and/or building loads are reduced through envelope changes, then the system could be downsized.

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