Finding the right-sized gas furnace for a home often runs into the Goldilocks problem — some are too large, and some are too small. Getting the right one requires a combination of knowing the proper criteria and the experience to put the information into a workable package. Software helps take the guesswork from the problem, but the final determination is still up to the professional who installs the furnace.
The size of the home is the first, obvious factor in determining the furnace size, but there are many other considerations also. Whether a home is open — with a few large rooms — or closed, with many small rooms, makes a difference. The orientation of the home needs to be looked at, as a home that has living areas facing south will heat more during the winter from the sun.
Climate and Construction
The home’s climate also determines the size of the furnace. A Northern California home with mild winters requires less heating power than a Midwest or New England home. The quality and amount of insulation makes a difference, along with the home construction. A brick home is naturally more insulated than a home with wood siding. Even the shrubbery around a home can lessen the requirements for a larger furnace. A home that is well-protected from winter winds by landscaping will require less heat.
Furnaces have come a long way in efficiency over the past few decades. A high-efficiency furnace generating 100,000 British thermal units (Btu) with a 93 percent rating will produce significantly more heat than the same-sized, less-expensive furnace with an 80 percent efficiency rating. By buying a furnace with greater efficiency, it may be possible to use a smaller unit and receive the same amount of heat.
Because a too-large furnace wastes resources, energy and money, many municipalities require that contractors use appropriate software to determine the best furnace size for a new home. A variety of software packages are available, but they all work similarly. The data, such as the square footage, is input into the program, and the answer is computed. A replacement furnace is often sized to match the existing one if the homeowners are comfortable with the service it provides.
A “working” estimate is figured out by knowing your climate, the square footage of the home, the size of the furnace and its efficiency. A standard that is used by heating and cooling equipment companies is to provide at least 25 to 30 Btu of heat per square feet in the home for a moderate to warm climate. A mid-sized home of 2,000 square feet would need approximately 50,000 to 60,000 Btu to heat it properly. With a less efficient furnace operating at 80 percent efficiency this would require a 60,000- to 72,000-Btu furnace. With a high-efficiency furnace operating at 95 percent, the home only needs a 52,500- to 63,000-Btu furnace.