Sizing Your System Properly

The standard practice for sizing air conditioning and heat pump systems is to do what is called a heat load calculation on the structure. To do this a manual called Manual J is used. Manual J will help the air conditioning company size the air conditioning equipment based on the outdoor design temperatures, attic and Wall insulation, window types and locations, elevation of the home, and other structural and environmental factors.

ASHRAE recommends that homes in the Western New York area be designed at 89 degrees. When sizing a home for comfort, a qualified air conditioning contractor will never design a home based on a extreme high temperature. To size the area for the very few hours of additional cooling that those extreme temperatures may require will only reduce the comfort control and increase cooling costs during the bulk of the year. It is also important to note that the hours where load exceeds Manual J are less than 1/2% of the summer cooling hours.

So many do not understand that a perfectly sized air conditioner will run continuously for the very few hours during the summer when the temperature is above the design temperature. To design the system to cycle during that extreme condition would leave the homeowner with a unit over sized during 99.95% of the time. To do this would only create the conditions stated above.

Another important fact that many do not understand is, contrary to so many who believe that the contractor undersized their home, if Manual J was used to design the comfort system, Manual J consistently oversized by about 20%. they call this a built-in safety factor.

Manual J is an accurate tool for calculating loads and provides a reasonable margin of safety without increasing unit size.

With the help of Manual J, the air conditioning contractor will look at the capacity of (or near) the design temperature, not the nominal rating

Ratings are at 95 degrees F. 
Western New York area design temperature is 89 degrees F.
That is why an air conditioning contractor who just goes from square footage to design a system, or suggests you need a larger unit because of square footage alone, may not have his or her customer’s best interest at heart. If the square footage that the contractor is doing his calculation on is incorrect, it can only compound the problems.

There have been studies done that show that as many as 53% of home air conditioners are oversized by as much as a ton or more compared to Manual J. Units are often 60% to 80% larger than they need to be because someone thought they needed a larger unit. A larger unit will not only cost more during installation but also during normal operation.