An ongoing topic of contention between HVAC Contractors, and well themselves, is the correct outdoor design temperatures in cooling & heating when calculating the ACCA Manual J heat loss and heat gains. I recently heard of a contractor that was questioned by their local building inspector about their use of a much lower (0F) / higher (95F) than design outdoor temperatures.

Their reply: “I have been designing using these temperatures for decades” was not sufficient.

The key to trusting the use of correct design temperatures is an understanding of ‘why’ this number:

Winter: ACCA Manual J (International Residential/Mechanical/Energy Conservation Codes), Table 1A (1B for a couple micro climates) uses the 99% design temperature for heating. This loosely means, that on average, 99% of all the heating hours in the Winter for this city is at this temperature or above. In other words, if we design to the 99% temperature, then we are oversized 98% of the year! This also means for that 1% of the year below the design temperature, the system will operate constantly and at highest capacity, to try and maintain temperature.

Summer: The same approach applies to the 1% design temperature for cooling. One percent of the year, on average, the mercury in the thermometer reaches above the Summer Design Temperature from ACCA Manual J Table 1A. During this time, the system will operate at highest capacity, constantly, to try and maintain design temperature within the space.

Using temperatures below the 99% design temperature for heating, or above the 1% in the Summer, will artificially inflate the size of the equipment for what? To be oversized 99.99% of the year? In case you were not aware, when using ACCA approved Manual J V8 software, there is enough rounding to oversize equipment by 20% (statement from Hank Rutkowski, P.E. ACCA during Residential EPIC Course) even on the most aggressive calculation. Then, contractors have the ability to continue to oversize within code requirements set forth in ACCA Manual S ‘Residential Equipment Selection’.

Yes, it is true - if you undersize the system then as the installing contractor you will get the phone call on the coldest/warmest days of the year.

Worse - if you oversize all of your equipment, particularly variable speed systems that are supposed to save the homeowner energy, then you will receive a lot more phone calls on all of the mild days of the year too! Plus, when the homeowners get their monthly utility bills...