# How to Calculate Heating and Cooling Degree Days

Degree days are used to estimate how many days each year heating or cooling will be needed. The length of the heating season is based on heat degree days or HDD. This value was often the basis of when the heat is turned on or off in buildings. The length of the cooling season is based on cooling degree days or CDD. According to "Fundamentals of Building Energy Dynamics" by Bruce Hunn, "energy use is equal to the degree days times the heating load coefficient for the building divided by the heating system times the heating value of the fuel (used)." Cooling degree days are also used to determine the likely cooling demand for which air conditioning or utility loads must be designed.

## Calculation for Heating Season in Standard Units

Degree days are a measure of the average number of days above or below the desired temperature.
1. Determine the average outdoor temperature per day during the heating season. This is called T-average.

2. Subtract the T-average from 18.3 degrees Celsius. This is the average heating required per day to keep the building at a comfortable temperature.

3. Multiply the average heating required per day by the number of days during the winter or heating season. The result is the heating degree days.

## Calculation for Heating Season in American units

1. Determine the average outdoor temperature per day during the heating season. This is called T-average.

2. Subtract the T-average from 65 degrees Fahrenheit. This is the average heating required per day to keep the building at a comfortable temperature.

3. Multiply the average heating required per day by the number of days during the winter or heating season.

## Calculation for Cooling Season in Standard Units

1. Determine the mean or average daily outdoor temperature during the cooling season. This is called T-average.

2. Subtract 18.3 degrees Celsius from the T-average.

3. Multiply the average cooling required per day by the number of days during the summer or cooling season.

## Calculation for Cooling Season in American units

1. Determine the mean or average daily outdoor temperature during the cooling season. This is called T-average.

2. Subtract 65 degrees Fahrenheit from the T-average.

3. Multiply the average cooling required per day by the number of days during the summer or cooling season.

## Warning

• According to "Modern Hydronic Heating," "estimating season's energy consumption using heating degree days assumes that buildings need heat input from their heating systems whenever the temperature outside drops below 65 degrees F. ... it is often incorrect for better-insulated buildings."