Oversizing of mechanical equipment has been a standard of installation for many years, but thanks to a growing emphasis on building efficiency and home performance, oversizing is (hopefully) seeing the end of its days.

Common reasons for oversizing include failure to perform the proper load calculations; compensating for leaky ducts; or contractors protecting themselves from callbacks during extreme weather. Whatever the reason, oversizing is always a no no—even for high-efficiency equipment. But there are many misconceptions floating around about why oversizing is bad, and many people lack a clear understanding of what’s really at stake.

Recent studies show that furnaces with an Annual Fuel Utilization Efficiency (AFUE) rating above 0.75 can be sized up to five times the load without a significant drop in efficiency. (Keep in mind, however, that the average load throughout the year is much lower than peak loads calculated by Manual J.) For example, let’s look at a 95% two-stage furnace. In second stage, the 60,000 BTU/h furnace will operate at steady state 95% efficiency when loads reach as low as 12,000 BTU/h. First-stage capacity of 39,000 BTU/h can operate efficiently for loads as low as 7,800 BTU/h.

Though efficiency may not be affected by oversizing, here are a few reasons why oversizing is still an issue:

1. Furnace prices increases with size, so installing the smallest possible unit will reduce the initial cost.
2. A furnace that is too large for the home will cycle on an off frequently, making the home less comfortable than an appropriately sized furnace.
3. Temperature swings from cycling may cause the homeowner to turn up the thermostat, which will result in higher energy bills.
4. Incorrect sizing can put stress on the system and shorten equipment life.

The bottom line: Sealing leaky ducts and using load calculations to choose the smallest possible furnace will assure maximum comfort and efficiency for your customers.