Temperature demand

We can visualize the temperature demand nicely on a graph of external
temperature versus time (figure E.4). For a building held at a temperature
of 20 °C, the total temperature demand is the area between the horizontal
line at 20 °C and the external temperature. In figure E.4a, we see that, for
one year in Cambridge, holding the temperature at 20 °C year-round had a
temperature demand of 3188 degree-days of heating and 91 degree-days of
cooling. These pictures allow us easily to assess the effect of turning down
the thermostat and living without air-conditioning. Turning the winter
thermostat down to 17 °C, the temperature demand for heating drops from
3188 degree-days to 2265 degree-days (figure E.4b), which corresponds to a
30% reduction in heating demand. Turning the thermostat down to 15 °C
reduces the temperature demand from 3188 to 1748 degree days, a 45%

These calculations give us a ballpark indication of the benefit of turning
down thermostats, but will give an exact prediction only if we take into
account two details: first, buildings naturally absorb energy from the sun,
boosting the inside above the outside temperature, even without any heat-
ing; and second, the occupants and their gadget companions emit heat,
so further cutting down the artificial heating requirements. The temper-
ature demand of a location, as conventionally expressed in degree-days,
is a bit of an unwieldy thing. I find it hard to remember numbers like
“3500degree-days.” And academics may find the degree-day a distressing
unit, since they already have another meaning for degree days (one
involving dressing up in gowns and mortar boards). We can make this
quantity more meaningful and perhaps easier to work with by dividing it
by 365, the number of days in the year, obtaining the temperature demand
in “degree-days per day,” or, if you prefer, in plain “degrees.” Figure E.6
shows this replotted temperature demand. Expressed this way, the tem-
perature demand is simply the average temperature difference between in-
side and outside. The highlighted temperature demands are: 8.7 °C, for a
thermostat setting of 20 °C; 6.2 °C, for a setting of 17 °C; and 4.8 °C, for a
setting of 15 °C.

Leakiness – example: my house

My house is a three-bedroom semi-detached house built about 1940 (figure
E.7). By 2006, its kitchen had been slightly extended, and most of the
windows were double-glazed. The front door and back door were both
still single-glazed.

My estimate of the leakiness in 2006 is built up as shown in table E.8.
The total leakiness of the house was 322 W/°C (or 7.7 kWh/d/.C), with
conductive leakiness accounting for 72% and ventilation leakiness for 28%
of the total. The conductive leakiness is roughly equally divided into three
parts: windows; walls; and floor and ceiling.

Figure E.6. The temperature demand in Cambridge, 2006, replotted in units of degree-days per day, also known as degrees. In these units, the temperature demand is just the average of the temperature difference between inside and outside.
Figure E.7. My house.