week contributed 10 × 7 = 70 degree-days to the (ΔT ×duration) sum. I’ll
call the sum of all the (ΔT × duration) factors the temperature demand of
a period.

energy lost = leakiness × temperature demand.

We can reduce our energy loss by reducing the leakiness of the building,
or by reducing our temperature demand, or both. The next two sections
look more closely at these two factors, using a house in Cambridge
as a case-study.

There is a third factor we must also discuss. The lost energy is replen-
ished by the building’s heating system, and by other sources of energy
such as the occupants, their gadgets, their cookers, and the sun. Focussing
on the heating system, the energy delivered by the heating is not the same
as the energy consumed by the heating. They are related by the coefficient
of performance
of the heating system.

energy consumed = energy delivered/coefficient of performance.

For a condensing boiler burning natural gas, for example, the coefficient
of performance is 90%, because 10% of the energy is lost up the chimney.

To summarise, we can reduce the energy consumption of a building in
three ways:

1. by reducing temperature demand;
2. by reducing leakiness; or
3. by increasing the coefficient of performance.

We now quantify the potential of these options. (A fourth option – increas-
ing the building’s incidental heat gains, especially from the sun – may also
be useful, but I won’t address it here.)

Figure E.4. The temperature demand in Cambridge, 2006, visualized as an area on a graph of daily average temperatures. (a) Thermostat set to 20 °C, including cooling in summer; (b) winter thermostat set to 17 °C.
Figure E.5. Temperature demand in Cambridge, in degree-days per year, as a function of thermostat setting (°C). Reducing the winter thermostat from 20 °C to 17 °C reduces the temperature demand of heating by 30%, from 3188 to 2265 degree-days. Raising the summer thermostat from 20 °C to 23 °C reduces the temperature demand of cooling by 82%, from 91 to 16 degree-days.