HDD (Heating Degree Days)

Definition

Heating Degree Days (HDD) is a measure of how cold a location is over a year, calculated by summing up the daily differences between the average outdoor temperature and a base temperature of 65°F. Each degree that a day's average temperature falls below 65°F contributes one HDD. For example, a day with an average temperature of 55°F contributes 10 HDD. This cumulative figure helps estimate the heating demand for a building, with higher HDD values indicating colder climates and greater heating needs. Homeowners often encounter HDD when evaluating the suitability and efficiency of heating systems for their specific region.

Why it matters

HDD is a critical factor in determining the appropriate size and efficiency of a heat pump for your home. A higher HDD value, such as Minneapolis's approximately 7,500 HDD, means your heating system will work harder and longer to maintain comfort compared to a region with lower HDD, like Atlanta's 2,800 HDD. Understanding your local HDD helps ensure you select a heat pump with sufficient heating capacity to keep your home warm without overworking the system, which can lead to premature wear and higher energy bills. For instance, a heat pump designed for a mild climate (low HDD) might struggle to heat effectively in a very cold climate (high HDD), potentially requiring supplemental heating and negating efficiency gains. Conversely, oversizing a unit for a low HDD climate can lead to short-cycling and reduced efficiency.

Where you'll see it

You'll typically see HDD referenced in climate data reports, energy audits, and sometimes on manufacturer's performance maps or regional sizing guides. It's not usually found directly on an AHRI certificate, which focuses on equipment ratings. Typical HDD values range from around 1,500 in very warm climates (e.g., southern Florida) to over 10,000 in extremely cold regions (e.g., northern Alaska). For most of the continental US, HDD falls between 2,500 and 7,500. When evaluating a heat pump, consider its rated heating capacity at specific outdoor temperatures (e.g., 17°F or 5°F) in relation to your local HDD. A heat pump's ability to provide adequate heat without relying heavily on auxiliary strips in a region with 6,000+ HDD indicates a robust cold-climate unit.

Common mistake

A common mistake is assuming that a high-efficiency heat pump rated for a mild climate will perform equally well in a severe winter climate with high HDD. While a unit might have an excellent HSPF rating, that rating is often calculated based on a national average climate, not necessarily your specific location's HDD. Homeowners might purchase a unit with a high HSPF, only to find it struggles to heat their home when temperatures drop significantly below freezing, leading to excessive reliance on costly electric resistance auxiliary heat. Always verify the heat pump's low-temperature heating capacity and performance curves against your local HDD to ensure it can meet your specific heating demands without excessive supplemental heating.

See also

• CDD (Cooling Degree Days)
• HSPF (Heating Seasonal Performance Factor)
• Balance Point
• Auxiliary Heat