You are shopping for a new appliance. You look at the various models, comparing features and efficiencies to try to pick out the best one for you. Then the salesperson comes up to you and says, “For only a little extra money, we can actually decrease the efficiency of this unit for you.” Sounds pretty tempting, doesn’t it?
In essence, many people do choose to spend more money to get less efficiency when shopping for a residential furnace, boiler, chiller, or rooftop unit by purchasing a unit that is too large for their home or facility.
Size and Efficiency
Not all efficient furnaces are created equal. For example, a 92% thermal efficiency rating is the efficiency rating for continuous operation at full load condition. This will be the most efficient operating point for the unit. A 92% AFUE (Annual Fuel Utilization Efficiency) rating includes expected operation at less than continuous, full-load conditions and includes cycling losses. Each time the furnace cycles off, the various components within the furnace, such as the heat exchanger, cool down, and heat is lost to the outdoors.
How can I know what size to buy?
Sizing a furnace is typically done by the seat of the pants. For cold climates, residential furnaces were often sized at 40-45 BTU/hr per square foot of living space. However, this often leads to systems being substantially oversized, especially for new homes. A load calculation should be completed instead. A load calculation is a relatively detailed analysis based on actual features of your home or building.
Another effective way to determine if the size of your furnace is in the ballpark is to use a heating degree-day calculation. For our climate, this can be simplified to:
Load (Btu / hr) = 50 x heating therms x Efficiency
Where heating therms can be determined by subtracting twelve times July usage from the annual therm consumption.
For example, if the annual gas usage is 1,500 therms and the July usage is 25 therms, the heating load is 1,200 therms. If the existing furnace is 82% efficient, the correct size for this system is around 50,000 BTU/hr. Note this is for estimating purposes only, a warm winter or night set-back will result in a smaller calculated system size than actually needed.
A Real Life Example
A furnace that is appropriately sized should run most of the time during the coldest part of the year. The percent of time that my furnace operated is plotted on the chart. These three days were some of the coldest of the year, with temperatures ranging from about -10F to 10F. Except when coming off night set-back, the furnace only operated an average of about 55%-60% of the time.
At 41 BTU/hr per square foot, this furnace was sized correctly based on the rule of thumb. Based on the measured data and a heating degree analysis, a better size would have been around 30 BTU/hr per square foot.
If you are buying a new furnace, it will be worth your while to have a heating load calculation performed. An oversized furnace costs more to purchase, and it will cost more every day it runs. If you plan to build a new home, you should insist on it.