TO-22-C034 - Heating Degree Days Relevancy: Why a 55-Degree Balance Point is Better than 65 for Building Energy Analysis in Heating Dominant Climates

Heating Degree Days at 65F (HDD65) is still utilized by ASHRAE Standard 169 for determining Climate Zones and in other fundamental HVAC resources, such as Air Conditioning Contractors of America’s (ACCA's) Manual J and S calculations. The U.S. Energy Information Administration (EIA) also uses HDD65 and it even occurs within the Passive House Institute US (PHIUS) where heating is not in demand until Outdoor Air Temperatures (OAT) reach 45F. This paper explores and supports utilization of Heating Degree Days at 55°F (HDD55) for building energy analysis in heating dominant climates. This is supported by EnergyPlus modeling results for a sample of building types. A degree days overview on Energycap states that "The 65-degree balance point standard was developed 75 years ago to help the gas industry predict heating loads in residences. Studies back then showed that when the average daily temperature fell below 65F, residences began turning on the heat." Today, HDD65 doesn’t reflect structures built to modern energy codes, because internal loads are offsetting thermal envelope losses and when combined with the continued tightening of ventilation energy recovery requirements, heating is not necessary until much cooler than 65F. According to a Regional Technical Forum (RTF) presentation titled "Overview of Updated Commercial EnergyPlus Building Models and Alignment with CBSA Data" dated August 31, 2021, the 'apparent balance point range between 50-60F’ was the reasonable OAT before heating was utilized by occupants. When energy modeling to HDD65, decisions are being made relative to temperature ranges that have minimal impact on building energy use. For example, when the OAT is in the 50-65F range, this may mean more economizer hours are available without having to reheat the outdoor air or heat exterior spaces/operating zones. This would decrease the amount of energy the building utilizes, allowing for more accurately sized equipment. Similarly, utilizing Cooling Degree Day's at 50F (CDD50), as ASHRAE Standard 169 does to determine Climate Zones, to predict cooling loads isn’t appropriate as most building types don’t need cooling starting at 50F. Economizers are essentially mandatory for larger buildings which might need cooling at 50F OAT, but smaller buildings will possibly be in heating mode when the OAT is 50F. Utilizing CDD in the range of 60-70F would give a more accurate representation of required cooling loads and will also be a minor focus of this technical paper. New HDD and CDD figures should be produced for ASHRAE Standard 169, Technical Resource Manuals (TRM's), and other publications that utilize degree days. With this information, end users can better analyze the weather's impact on actual building energy usage and better determine equipment sizing and other impacts.