The simple answer is “Because They Will Reject Your Permit Application” as will your LEED, EnergyStar or HERSRater. Most HVAC installers and homeowners want to design for the hottest day in August and a 70 F indoor design temperature. This creates a situation where your HVAC system is oversized resulting in inadequate dehumidification, possible coil freeze and short cycling.
ACCA developed Manual J decades ago and many programs such as EnergyStar and LEED have required Manual J load calculations for years. The track record has been proven to be accurate and reliable. But still there are those who insist on upsizing heating and cooling equipment for that one hot week two years ago…
So let’s look at the numbers for wintertime. Table 1A in the ACCA Manual J book gives the ASHRAE 99% design temperature for all ASHRAE weather locations. Now in English this is the 30-year average for your ASHRAE weather location 99% of the time. So 1% of the time based on the 30-year average the outdoor temperature will drop below the outdoor design temperature. 1% = 88 hours a year.
And looking at summertime… Table 1A in the ACCA Manual J book gives the ASHRAE 1% design temperature for all ASHRAE weather locations. This is the 30-year average for your ASHRAE weather location. Again 1% of the time (88 hours) based on the 30-year average the outdoor temperature will it be warmer outside than the outdoor design temperature.
ALWAYS review your Manual J load calculation to ensure the designer did not artificially inflate the heat gain and heat loss results by selecting temperatures other than the recommended ASHRAE
design temperatures. Insist on an accurate Manual J based on ASHRAE approved design temperatures and size your heating and air conditioning equipment on those calculated results.
By doing this the sizing results will be accurate whether it’s a remodel, new construction, LEED, EnergyStar or CALGreen. Unfortunately, not all permit offices agree with the proven science and may have their own outdoor design temperatures so check what your permit office requires and pass that on to your Manual J provider to avoid an unnecessary rejection.
Size your equipment for design loads not the most extreme load possible. By using the accurate ASHRAE’s design temperatures your HVAC equipment will be “Just Right” providing optimal performance and comfort for years to come!