# E Heating II

A perfectly sealed and insulated building would hold heat for ever and

thus would need no heating. The two dominant reasons why buildings

lose heat are:

**Conduction** – heat flowing directly through walls, windows and

doors;

**Ventilation** – hot air trickling out through cracks, gaps, or deliberate

ventilation ducts.

In the standard model for heat loss, both these heat flows are proportional

to the temperature difference between the air inside and outside. For a

typical British house, conduction is the bigger of the two losses, as we’ll

see.

### Conduction loss

The rate of conduction of heat through a wall, ceiling, floor, or window is

the product of three things: the area of the wall, a measure of conductivity

of the wall known in the trade as the “U-value” or thermal transmittance,

and the temperature difference –

power loss = area × *U* × temperature difference.

The U-value is usually measured in W/m^{2}/K. (One kelvin (1 K) is the

same as one degree Celsius (1 °C).) Bigger U-values mean bigger losses of

power. The thicker a wall is, the smaller its U-value. Double-glazing is

about as good as a solid brick wall. (See table E.2.)

The U-values of objects that are “in series,” such as a wall and its inner

lining, can be combined in the same way that electrical conductances

combine:

There’s a worked example using this rule on page 296.

### Ventilation loss

To work out the heat required to warm up incoming cold air, we need the

heat capacity of air: 1.2 kJ/m^{3}/K.

In the building trade, it’s conventional to describe the power-losses

caused by ventilation of a space as the product of the number of changes

*N* of the air per hour, the volume *V* of the space in cubic metres, the heat

capacity *C*, and the temperature difference ΔT between the inside and

kitchen |
2 |

bathroom |
2 |

lounge |
1 |

bedroom |
0.5 |