CDD (Cooling Degree Days)
Annual sum of degrees above 65°F. Houston ~2,800 CDD vs Boston ~650. Drives cooling load — matters for SEER2 weighting.
Definition
CDD, or Cooling Degree Days, is a measure of how much and how long the outdoor temperature was above a certain threshold, typically 65°F, over a year. It's calculated by summing the daily differences between the average daily temperature and 65°F, but only for days when the average temperature is above 65°F. For example, a day with an average temperature of 75°F contributes 10 CDD. Homeowners encounter CDD because it's a key indicator of the cooling demand in a specific location, directly influencing the energy required to keep a home cool. A city like Houston might have around 2,800 CDD annually, while Boston has closer to 650 CDD.
Why it matters
CDD is crucial because it directly correlates with your home's cooling load and, consequently, your electricity bill during warmer months. In areas with high CDD, like the Sun Belt, a significant portion of your annual energy consumption will be dedicated to air conditioning. This makes the efficiency of your cooling system, measured by metrics like SEER2, particularly important. For instance, a heat pump with a higher SEER2 rating will provide greater savings in a high-CDD climate compared to a low-CDD climate, as it will operate more frequently. Understanding your local CDD helps you prioritize system efficiency and size your equipment appropriately. Over-sizing a system in a low-CDD area or under-sizing it in a high-CDD area can lead to discomfort and higher costs.
Where you'll see it
You typically won't see CDD listed directly on a heat pump quote or an AHRI certificate, but it's the underlying climate data used to calculate the weighted efficiency ratings like SEER2. Manufacturers use national average CDD data, or sometimes regional data, to determine these ratings. While you won't see a 'CDD value' for a specific unit, you should be aware of your local CDD when comparing systems. A higher local CDD (e.g., above 2,000) indicates a greater need for high-efficiency cooling. If you live in a region with very low CDD (e.g., below 500), the incremental benefit of a super-high SEER2 system might be less pronounced compared to a moderate one, though other factors like heating performance (HSPF2) become more critical. Be wary if a contractor doesn't discuss your local climate's cooling demands, especially in warmer regions.
Common mistake
A common mistake is assuming that a high SEER2 rating alone guarantees optimal performance without considering the local CDD. Homeowners might invest in a premium 20+ SEER2 system in a mild climate (low CDD) where the system rarely runs at full capacity, thus not realizing the full energy savings potential. Conversely, in a high-CDD climate, underestimating the importance of SEER2 and opting for a lower-efficiency unit (e.g., 14 SEER2) can lead to significantly higher operating costs over the system's lifespan. The efficiency benefits of a high SEER2 system are most pronounced where the cooling load is highest, directly correlated with the local CDD. Always balance the system's rated efficiency against your specific climate's cooling demand.