Properly sized equipment will run nearly continuously on the coldest and hottest days of the year. Oversized equipment runs for shorter periods at higher intensity, compromising comfort, performance and equipment lifetime. It costs more up front. Higher capacity equipment also requires higher air flow, meaning larger ductwork and larger blowers.
Excessive oversizing was observed for 70% of study home furnaces and every study home air conditioner. Furnaces were sized an average of 158% of the minimum required size (maximum 238%) versus an industry recommended maximum of 140%. Furnace sizing practices did not appear to reflect the reduced heating loads due to insulated basements. Air conditioners were even more oversized, at an average of 208% of the minimum required size (maximum 322%) versus an industry recommended maximum of 115%. These data raised questions about sizing procedures and excessive safety margins.
With filters removed for testing, about one-quarter of study home furnaces operated near the limits or outside of manufacturer’s specifications for external static pressure and/or heat rise. In about half of the homes with air conditioning, air flows across indoor coils deviated considerably from the typical specification of 400 cfm/ton. Fourteen percent of air conditioners had air flows below 350 cfm/ton, while a third of the sample had flows exceeding 500 cfm/ton (with filters in place, external static pressure would increase somewhat, in turn decreasing air flows and increasing furnace heat rise).
* Oversized equipment was found in many study homes. Almost half of the furnaces exceeded the maximum recommended size. All of the air conditioners were excessively sized; the average unit was about twice as large as needed.