United States Heating Degree Day map, 1961-1990

United States Cooling Degree Day map, 1961-1990

Heating degree day (HDD) is a measurement designed to quantify the

demand In economics, demand is the quantity of a good that consumers are willing and able to purchase at various prices during a given time. The relationship between price and quantity demand is also called the demand curve. Demand for a specific item ...

for

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

needed to heat a building. HDD is derived from measurements of outside air

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

. The heating requirements for a given

building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fu ...

at a specific location are considered to be directly proportional to the number of HDD at that location. Related measurements include the cooling degree day (CDD), which quantifies demand for

air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...

Definition

Heating degree days are defined relative to a base temperature—the outside temperature above which a building needs no heating. Base temperatures may be defined for a particular building as a function of the temperature that the building is heated to, or it may be defined for a country or region for example. In the latter case, building standards or conventions may exist for the temperature threshold. These include: The base temperature does not necessarily correspond to the building mean internal temperature, as standards may consider mean building insulation levels and internal gains to determine an average external temperature at which heating will be required. Base temperatures of 16 °C and 19 °C (61, 66 °F) are also used. The variation in choice of base temperature implies that HDD values cannot always be compared – care must be taken to ensure that only HDDs with equal base temperatures are compared. There are a number of ways in which HDD can be calculated: the more detailed a record of temperature data, the more accurate the HDD that can be calculated. HDD are often calculated using simple approximation methods that use daily temperature readings instead of more detailed temperature records such as half-hourly readings, the latter of which can be used to estimate an

integral In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along wit ...

. One popular approximation method, that used by the U.S. National Weather Service, is to take the average temperature on any given day (the mean of the high and low temperature) and subtract it from the base temperature. If the value is less than or equal to zero, that day has zero HDD. But if the value is positive, that number represents the number of HDD on that day. (For cooling degree days, the process works in reverse: the base temperature is subtracted from the average, and if ''this'' value is positive, that number represents the CDD.) This method works satisfactorily if the outside air temperature does not exceed the base temperature. In climates where this is likely to occur from time to time, there are refinements to the simple calculation which allow some 'credit' for the period of the day when the air is warm enough for heating to be unnecessary. Thi
more accurate algorithm
enables results to be computed in temperate climates (maritime as well as continental) throughout the year (not just during a defined heating season) and on a weekly as well as monthly basis. HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements. In the course of a heating season, for example, the number of HDD for

New York City New York, often called New York City or NYC, is the most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the most densely populated major city in the Un ...

is 5,050 whereas that for Barrow, Alaska is 19,990. Thus, one can say that, for a given home of similar structure and insulation, around four times the energy would be required to heat the home in Barrow than in New York. Likewise, a similar home in

Miami Miami ( ), officially the City of Miami, known as "the 305", "The Magic City", and "Gateway to the Americas", is a coastal metropolis and the county seat of Miami-Dade County in South Florida, United States. With a population of 442,241 at ...

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

, whose heating degree days for the heating season is 500, would require around one tenth of the energy required to heat the house in New York City. However, this is a theoretical approach as the level of insulation of a building affects the demand for heating. For example, temperatures often drop below the base temperature during night (daily low temperature in diurnal variation), but because of insulation, heating is unnecessary. In the end of spring and in the beginning of fall or in the winter depending on the climate, sufficient insulation keeps the indoor temperature higher than the outdoor temperature with little or no heating. For example, in southern California, during winter heating is not necessary in Los Angeles and

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if the insulation is sufficient to take into account the colder night temperatures. Also, buildings include

thermal mass In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...

such as concrete, that is able to store energy of the sun absorbed in daytime. Thus, even if the heating degree days indicate a demand for heating sufficient insulation of a building can make heating unnecessary.

Example of use

HDD provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year). In conjunction with the average U-value for a building they provide a means of roughly estimating the amount of energy required to heat the building over that period. One HDD means that the temperature conditions outside the building were equivalent to being below a defined threshold comfort temperature inside the building by one degree for one day. Thus heat has to be provided inside the building to maintain thermal comfort. Say we are given the number of heating degree days ''D'' in one year and we wish to calculate the energy required by a building. We know that heat needs to be provided at the rate at which it is being lost to the environment. This can be calculated as the sum of the heat losses per degree of each element of the buildings' thermal envelope (such as windows, walls, and roof) or as the average

U-value In the context of construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. R-value is the temperature difference pe ...

of the building multiplied by the area of the thermal envelope of the building, or quoted directly for the whole building. This gives the buildings' specific heat loss rate ''P''_specific, generally given in watts per kelvin (W/K). Total energy in kilowatt hours (kW⋅h) is then given by: :

Q = P_\text \times 24 \times \frac \;

W⋅h As total energy consumption is in kilowatt hours and heating degree days are o. days×degreeswe must convert watts per kelvin into kilowatt hours per degree per day by dividing by 1000 (to convert watts to kilowatts), and multiplying by 24 hours in a day (1 kW = 1 kW⋅h/h). Since a 1 °C temperature change and a 1 K change in absolute temperature are the same, these cancel and no conversion is required. Example: For a typical

winter day with high of 40 °F and low of 30 °F, the average temperature is likely to be around 35 °F. For such a day we can approximate the HDD as (65 − 35) = 30. A month of thirty similar days might accumulate 900 HDD. A year (including summer average temperatures above 70 °F) might accumulate an annual 5000 HDD.

Problems

Calculations using HDD have several problems. Heat requirements are not linear with temperature, and heavily insulated buildings have a lower "balance point". The amount of heating and cooling required depends on several factors besides outdoor temperature: How well insulated a particular building is, the amount of

solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...

reaching the interior of a house, the number of

electrical appliance A home appliance, also referred to as a domestic appliance, an electric appliance or a household appliance, is a machine which assists in household functions such as cooking, cleaning and food preservation. Appliances are divided into three ty ...

s running (e.g. computers raise their surrounding temperature) the amount of

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ho ...

outside, and what temperature the occupants find comfortable. Another important factor is the amount of

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

indoors; this is important in determining how comfortable an individual will be. Other variables such as precipitation, cloud cover, heat index, building albedo, and snow cover can also alter a building's thermal response. Another problem with HDD is that care needs to be taken if they are to be used to compare climates internationally, because of the different baseline temperatures used as standard in different countries and the use of the Fahrenheit scale in the US and the Celsius scale almost everywhere else. This is further compounded by the use of different approximation methods in different countries.

Conversion

To convert °F HDD to °C HDD: :

^C\, \mathit = \frac \times \, ^F\, \mathit

To convert °C HDD to °F HDD: :

^F\, \mathit = \frac \times \, ^C\, \mathit

Note that, because HDD are relative to a base temperature (as opposed to being relative to zero), it is incorrect to add or subtract 32 when converting degree days from Celsius to Fahrenheit or vice versa.

References

External links