Heating degree days
Heating degree days (HDD) are a metric used to estimate the energy demand for heating buildings by quantifying the temperature difference between the outside air and a specified base point temperature. Established by the American Gas Association in 1927, HDD serve as a cumulative measure over a set period, indicating how much heating is necessary when outdoor temperatures drop below this base point, commonly set at 15.5 degrees Celsius (60 degrees Fahrenheit) in some regions. The calculation of HDD helps assess annual heating needs; for example, if a building requires 450,000 kilowatt-hours (kWh) for heating over a year with 3,000 HDD, the heating efficiency can be determined as 150 kWh per HDD.
Different regions employ varied methods for calculating HDD, including distinct base temperatures and seasonal versus annual assessments. For instance, the United States typically uses a base temperature of 65 degrees Fahrenheit (18 degrees Celsius), while the United Kingdom employs 15.5 degrees Celsius. Additionally, HDD can highlight the impact of energy efficiency measures, such as insulation improvements, by comparing heating energy usage before and after upgrades. Historical HDD data is also pivotal in analyzing climate trends and making informed decisions about future energy demands, thereby contributing to broader discussions on climate change and energy policy.
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Heating degree days
Summary: Heating degree days are used to estimate the energy demand for heating habitable spaces in buildings.
The American Gas Association developed heating degree days in 1927 as a method to allow for the normalization of energy consumed in the process of heating buildings. They are the cumulative measure over a period of time of the temperature differential between the outside air temperature and the base point temperature for a building, and can be measured in degrees Celsius (C) or degrees Fahrenheit (F). The base point is a fixed notional temperature above which heating is not required in the building.
Since heating demand depends on how cold the outside weather is, when the temperatures outside fall lower than the base point, heating degree days measure cumulative temperature shortfall for which demand for heating fuel is generated.
Assuming the base temperature is taken at 15.5 degrees C, if the outside temperature is 13.5 degrees C for 12 hours in a day, and 14.5 degrees C for the remaining 12 hours, then the degree day calculation for that day will be
2 degrees C x 0.5 day + 1 degrees C x 0.5 day = 1.5 degree day.
If such yearly calculations result in heating degree days in year 1 = 3,000 degrees C, this means that energy is required to heat a building by 3,000 degrees C over the whole year.
If 450,000 kilowatt-hours (kWh) is the energy used over the whole year only for heating the building, then the heating efficiency is said to be 450,000/3,000 = 150 kWh per degree day. In other words, 150 kWh of energy is required to heat the building by one degree Celsius. This type of calculation may be used to estimate the heating fuel demand over days, weeks, or months. If the measure of heating degree days in year 1 is more than in year 2 for the same location and base point, it means that the weather was colder in the year with more heating degree days than in the second year.
Heating degree days are also used to monitor the performance of energy efficiency measures carried out in a building. If a building is upgraded by adding insulation to cavity walls and roofs, then the heating efficiency (kWh per degree day) calculation for the building before and after improvements can quantify the savings made in the energy used to heat the building. For example, if 450,000 kWh are used for heating 3,000 degree days in year 1, then heating efficiency in year 1 is 150 kWh per degree day. If 360,000 kWh are used for heating 4,000 degree days in year 2, then the heating efficiency in year 2 is 360,000/4000 = 90 kWh per degree day. In this example, the energy efficiency can be said to have improved by 60 percent after adding insulation to the building envelope. Degree days vary from year to year and can be normalized using this method.
Base point assumptions for degree days also need to take into account internal heat gains of a building, which are affected by the occupancy types and patterns of the building. Commercial office buildings have a different heating requirement than industrial buildings, and therefore should have different base temperatures. In addition, the internal heat gain of a building may be affected by the building envelope’s thermal properties as well as natural factors like solar heat gain, wind direction, and others, all of which typically vary throughout the year.
Variations
The United States, the European Union (EU), and most individual European countries utilize their own calculation methods to determine heating degree days. Differences include the use of different base and threshold temperatures, as well as whether heating degree days are determined for an entire year or simply for the season in which heating is used. In the United States, for example, the average (high minus low) temperature is subtracted from a base temperature of 65 degrees F (18 degrees C) to determine the required amount of heating energy. Monthly and annual summaries of heating degree day data are then compared to long-range base data covering 30 or more years.
A different base temperature of 15.5 degrees C (60 degrees F) is used in the United Kingdom (UK). In the UK, a building is generally heated to 19 degrees C, but the base point is assumed at 15.5 degrees C to account for internal heat gains from people and equipment of about 3.5 degrees C. Ireland uses the same base point as the UK, except the use of a base point of 18 degrees C for hospitals while Germany and countries using German benchmarks use a normalized degree day index with an average value equal to 1.0.
Certain types of occupancies like commercial office buildings are only heated to full temperature during working hours, but the standard degree day data almost always covers a continuous 24-hour period and includes temperatures at night, which are largely irrelevant to a building that is only heated in the daytime. More specifically, such degree day data will represent the total heating demand from that area, which is a combination of working and nonworking hours including holidays and weekends and may be more relevant to such combinations of commercial and residential buildings rather than as a specific building-type indicator of heating energy demand. On a larger scale, historical degree days are used to record climatic variations over many years. Such data forms part of the evidence for any scientific arguments about climate change and global warming. Degree days are based on a notional base temperature that is relevant to habitable buildings, and find relevance in many fuel suppliers’ policy processes to predict demand for fuel in future winter months. Heating degree days are also used to track energy usage, which is necessary for combatting global climate change.
Bibliography
Degree Days for Energy Management (CTG004). London: Carbon Trust Publication, May 2010.
Ristinen, Robert A., et al. Energy and the Environment. 2nd ed. Hoboken, NJ: John Wiley & Sons, 2006.
"What Are Heating and Cooling Degree Days?" National Weather Service, www.weather.gov/key/climate‗heat‗cool. Accessed 1 Aug. 2024.
"What Is a Degree Day? EIA, 2022, www.eia.gov/energyexplained/units-and-calculators/degree-days.php#. Accessed 1 Aug. 2024.