Summary of Temperature Data

This graph shows the average daily minimum and maximum temperatures for each month in Armidale. The averages are calculated from long term weather data measured at the 2ARM weather station in Armidale (up to 1996).

Chart of Min/Max temperatures

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Heating Degree Days

One way to estimate the heating requirements of a house is to first calculate the number of Heating Degree Days (HDD) for each month. The Heating Degree Day unit is the measure of degrees that the mean daily temperature is below a specified base-line threshold temperature. Usually 18.3°C (65°F) is taken for the base-line, but 15°C is also commonly used, where the householders are comfortable with lower indoor temperatures.

To calculate the number of heating degree days in a month, take the average of the mean daily maximum and minimum outdoor temperatures for the month, subtract from the base-line temperature, and multiply by the number of days in the month. If the value is less than zero, use zero, ie no heating is required. A graph of the number of heating degree days for each month in Armidale is shown below, using base-line temperatures of 18.3°C and 15°C. The total number of heating degree days for the year is 1833 for 18.3°C base-line, and 1069 with 15°C base-line temperature.

Heating degree days are reasonably good indices of the heating requirements of buildings. Over the long term, solar and internal heat gains will offset losses when the mean daily outdoor temperature is 18.3°C. Heating requirements are proportional to the number of heating degree days during the heating season. To estimate the energy required for heating, the number of degree days is multiplied by the energy use coefficient (heat loss rate coefficient), which depends on the energy efficiency of your house, and the size and design of the area being heated.
The scale on the right of the graph below uses a nominal coefficient of 100 kJ / °C-day / m2 and assumes a floor area of 150 m2. The graph indicates a peak energy use of between 125 to 175 MJ per day, averaged over the month of July. The total energy used for heating over the year is 27.5 GJ, or down to 16 GJ using the lower 15°C heating degree days. In a well insulated house, and only heating rooms as required, the energy required should be significantly less.

For more details about evaluating your home energy rating, have a look at the First Rate software demo package.

Chart: Heating Degree Days

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