It is possible to produce a summary of the thermal 'stresses' placed on a building within a particular climate by calculating the number of heating and cooling degree hours throughout the year. A degree hour is simply the number of degrees spent above or below a standard reference temperature during the course of 1 hour. Also, incident solar loads can be represented as degree hours using the sol-air temperature concept, as outlined in Szokolay's work in Climatic Data and its Use in Design (RAIA Press, 1988). Solar degree hours are effectively an excess cooling load on the building due to incident solar radiation.
The above image is an example degree hour graph. Cooling degree hours are shown in blue, heating in red and solar in yellow. Values are shown as the degree hour totals for each month of the year. This particular graph shows a maximum 6950 cooling degree hours in January, falling to almost zero in June and July. Heating degree hours, on the other hand, peak at around 5100 in July.
The Weather Tool displays this graph at the bottom of its monthly summary page.
Calculating Degree Hours
When ambient conditions outside a building are warmer than conditions inside, the building will generally gain heat from the outside via conduction and air infiltration through the fabric. When outside conditions are cooler, buildings will lose heat the same way. The amount of heat lost or gained will depend on the magnitude of the temperature difference between inside and out as well as the characteristics of the building (see the heat flow topic for more detail). However, to compare different climates or if the building is yet to be designed, it is possible to use standard degree day values to get a pretty good indication of heating/cooling stresses without having to perform detailed heat flow calculations.
Degree days are calculated by simply summing up the difference between the hourly dry-bulb temperature and a standard reference temperature over each month. Reference temperatures vary from country to country. For example, in the UK heating degree days are based on an outside dry bulb temperature of 15.5°C, whereas Australia uses 18°C and the United States use 65°F. Either way, the result is a value which represents the amount of time that the temperature was 1 degree lower or higher than the reference. Because heat flow is a product of temperature difference and time, the degree day summation calculation shows that 8 hours spent at a temperature 1 degree higher than the reference is equivalent to 1 hour spent 8 degrees higher.
Heating and cooling degree days are typically used by engineers and planning bodies to estimate the likely energy use of large numbers of buildings. They can also be used to estimate the likely air-conditioning requirements of a single building, but this will be a very crude approximation.
An Example Calculation
Ideally, degree day values are calculated from hourly recorded temperature data. Degree hours are calculated as the sum of the difference between the reference and recorded data, when for heating the recorded temperature is below the reference, and for cooling it is above. Taking Figure 2 below as an example, the following are the degree hour values for that day.
Daily Heating Degree Hours = S
dt = (18-15) + (18-14) + (18-14)
+ (18-13) + (18-12) + (18-12)
+ (18-12) + (18-13) + (18-14)
+ (18-16) + (18-17) + (18-17)
= 3 + 4 + 4 + 5 + 6 + 6
+ 6 + 5 + 4 + 2 + 1 + 1
= 49 degHr
Daily Cooling Degree Hours = S
dt = (21-20) + (23-20) + (23-20)
+ (22-20) + (21-20)
= 1 + 3 + 3 + 2 + 1
= 10 degHr
In this example all the temperature data has been integerised for convenience only. Actual recorded data may contain more accurate decimal values for temperature, in which case the more accurate values would be used.
The daily degree hours for each day of the month are summed together to give total monthly value. To get degree days, the monthly degree hour value is simply divided by 24, being the number of hours in the day.
It is also possible, though less exact, to use daily observations of minimum and maximum temperature to calculate daily values of heating and cooling degree days. The equation used to calculate heating degree days (HDD) for a single day is:
HDD = Tb -Tm if Tm < Tb HDD = 0 if Tm >= Tb
Tb = base temperature and Tm = average of maximum and minimum temperatures.
The equation for cooling degree days (CDD) is:
CDD = Tm -Tb if Tm > Tb CDD = 0 if Tm <= Tb
Solar Excess Degree Days
In addition, incident solar loads can be represented as degree hours using the sol-air temperature concept, as outlined in Szokolay's work in Climatic Data and its Use in Design (RAIA Press, 1988). Solar degree hours are effectively an excess cooling load on the building due to incident solar radiation. This calculation uses a formula based on latitude and the ratio of vertical to horizontal surfaces in a 'standard' building, as well as the following assumptions:
- All windows are fully shaded in the overheated period.
- A well designed building with north-south orientation and an aspect ratio of 1.4.
- Solar radiation on north and south walls will be negligible in the overheated period as the sun is near its zenith and a well-designed building will provide adequate eaves/shading.
- The roof is taken as a horizontal surface as opposing slopes will tend to compensate for each other.
- An assumed absorptance of 0.3 for roofs and 0.52 for the east and west walls, with surface film conductances of 22 and 18 W/mÃ‚Â² respectively.
Whilst they may sound a little general, as long as they are standardised, the result is a useful comparative measure of solar 'stress'.
- The Climates of Western Australia
- To see the comparison of degree hour values for a range of different climates.
- UK Degree Day Data
- UK Energy Efficiency Best Practice programme (EEBPP)
- US Heating Degree Days
- Climate Prediction Centre - Degree Day Statistics