7.2: Residential Heat Loss (II)

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Heating Degree Days

Local weather or climatic conditions are one of three factors that affects the amount of heat loss through conduction. When examining weather conditions, we look at both the inside and outside temperature of a home.

As shown in Figure 7.2.1, the inside temperature is usually taken as a standard comfort temperature of 65ºF. The outside temperature varies by the hour. Knowing this information can help us to understand two concepts:

Average outside temperature = Average of the maximum and minimum temperature during the day
Heating Degree Day (HDD) = The temperature difference through which air has to be treated, or how many degrees the mean temperature fell below 65ºF for the day. It is also an index of fuel consumption.

Figure 7.2.1 Temperatures inside and outside a house

Determining HDD

The formula for determining the HDD is

\[ HDD = T_{base} − T_a \]

where T_a is the average outside temperature given by

\[ T_a = \dfrac{T_{max} + T_{min}}{2} \]

To calculate HDD:

Determine the base temperature (i.e., inside temperature), which is usually about 65°F.
Find the day's average outside temperature using equation 7.2.2.
Use equation 7.2.1 to find the HDD.

Note

If the T_a is equal to or above 65 ºF, there are no heating degree days for that 24-hour period, or HDD = 0.

Example Problem

Example 1

Calculate the HDD for one day when the average outside temperature is:

(a) 13°F

(b) 2°C

Answer

Part (a)

Using equation 7.2.1,

\[ HDD = 65 - 13 = 52°F \nonumber\]

Part (b)

Convert the temperature to Fahrenheit to get 2°C = 35.6°F.

Then, using equation 7.2.1,

\[ HDD = 65 - 35.6 = 29.4°F \nonumber\]