9.4 Seasonal Heating Degree Days

9.4 Seasonal Heating Degree Days

In previous examples, we are assuming that the outside temperature remains the same for all 150 heating days in a season. This is not realistic, but it explains the method to calculate the HDD. In a more realistic example, we need to find the temperature difference for each day and add all the temperature differences.

We will now look at Seasonal Heating Degree Days (HDD), which is the sum of temperature differences of ALL days - rather than just 1 day or 1 week - during which heating is required.

The table below provides Seasonal HDDs for selected places in the United States. The higher HDD indicates a higher heat loss and, therefore, higher fuel requirements.

HDD is used to estimate the amount of energy required for residential space heating during a cool season, and the data are published in local newspapers or on the National Weather Service website.

Annual Degree Days for Selected Places
PlaceDegree Days
Birmingham, AL2,823
Anchorage, AK10,470
Barrow, AK19,893
Tucson, AZ1,578
Miami, FL155
Pittsburgh, PA5,829
State College, PA6,345

Source: NOAA

Calculating Seasonal Heating Degree Days

To calculate Seasonal Heating Degree Days, use this formula:

Seasonal HDD =( (T b -T a )×No. Days in Month 1 ) +( (T b -T a )×No. Days in Month 2 ) +( (T b -T a )×No. Days in Month 3 ) 

Remember, in months where the average temperature is equal to or greater than 65, there will be no heating degree days, so the value for the month will be 0.

Example 1 - Calculate HDD for a Winter Month

Problem: Calculate the heating degree days for January in State College, PA, if the average temperature is 25°F.

Solution:

HDD = (Tb - Ta) × Number of Days

HDD = (65°F - 25°F) × 31 days

HDD = 40°F × 31 days

Answer:

HDD = 1,240 degree days

Example 2 - Calculate HDD for Multiple Months

Problem: Calculate the total seasonal HDD for a heating season with the following average temperatures:

MonthAvg Temp (°F)Days in Month
November40°F30
December30°F31
January25°F31
February28°F28
March38°F31

Solution:

November:  HDD = (65 - 40) × 30 = 25 × 30 = 750 HDD

December: HDD = (65 - 30) × 31 = 35 × 31 = 1,085 HDD

January: HDD = (65 - 25) × 31 = 40 × 31 = 1,240 HDD

February: HDD = (65 - 28) × 28 = 37 × 28 = 1,036 HDD

March: HDD = (65 - 38) × 31 = 27 × 31 = 837 HDD

Total Seasonal HDD:

Total = 750 + 1,085 + 1,240 + 1,036 + 837 = 4,948 HDD

Answer: 

The seasonal heating requirement is 4,948 HDD

Example 3 - Comparing Two Cities

Problem: Which city requires more heating energy: Birmingham, AL (2,823 HDD) or Pittsburgh, PA (5,829 HDD)?

Solution:

Pittsburgh HDD = 5,829

Birmingham HDD = 2,823

Difference = 5,829 - 2,823 = 3,006 HDD

Ratio = 5,829 ÷ 2,823 = 2.06

Answer: 

Pittsburgh requires 2.06 times more heating energy than Birmingham, or about 106% more heating.

Try This

Use this Energy Star Calculator to determine the number of Heating Degree Days (or Cooling Days) for any zip code.  

How many HHD did your location have last year?

State College, PA (Zip Code 16801) had 5,642°F for 2025. 

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