Conduction heat loss

It is not possible to measure the actual amount of heat that is being lost from a whole house at any given time. The loss must be calculated from other data. What measurements do you think would have to be made? What factors does conduction heat loss depend upon?

The rate at which heat flows away from the outer shell of a house is proportional to:

a) the area of the shell;

b) the difference between the inside and outside air temperatures, t_t and t₀, that is, t_t - t₀;

c) the capacity of the material to conduct heat away and the thickness of the material.

It is sometimes necessary to measure experimentally the heat which flows through a sample piece of window or wall and then a calculation is made to determine the amount of heat that will flow through one square metre (1 m²) in one hour when the temperature difference on the two sides is one degree Celsius (1 degC). This quantity is called the 'U value' of the material of this thickness. The 'U value' is measured in kilojoules (kJ). (A kilojoule is the amount of heat required to raise the temperature of 0-24 kg of water through 1 deg C.)

The other factors to be considered are the total area and the actual temperature difference. The formula for total heat loss becomes:

H = UA×(ti-t₀) kJ/h,

where A is measured in square metres.

U values for typical constructions

External walls: Solid 023 m brick plaster covered 8-6

0-28 m cavity ventilated 6-8

0-28 m cavity unventilated 6-0

0-28 m cavity with foamed plastic in cavity 2-0

0-28 m cavity with light-weight block inner wall 4-6

Ground floor: Solid concrete and tiles 4-0

Wood floor on joists 7-0

Roof and top ceiling: Tiles laid on felt and battens 8-4

Tiles on battens 11-2

Tiles on boards and battens 6-0

Tiles on battens and felt with ceiling overlaid with 0.025 m

fibre glass or 0-050 m vermiculite 3-0

Windows: Ordinary 20-0

Double glazed 10-0

Doors: 0-025 m wood 10-0

Heat losses through party walls of semi-detached houses and the connecting walls, ceilings and floors of flats can be ignored if the adjoining houses or flats are heated.

Example 1

What is the heat loss per hour through a window 2m×1m if the inside temperature is 11 degC above the outside temperature?

H = 20×2×1×11 =440 kJ/h.

Example 2

Figure 1.1 shows the plans of a modern house. Assuming it is designed with an eye to comfort, using good insulation methods, the conduction losses will be as below, for an average temperature difference of 7 degC.

Area A×U×(t_t—t₀) = Mean hourly heat loss

External walls		×	4-6	×	7 =
Ground floor		×	4-0	×	7 =
Top ceiling		×	3-0	×	7 =
Windows		×	10-0	×	7 =
Doors	4×10-0×7 = Total hourly conduction loss H =
		9910 kJ

Exercise A

Give your answers in this and subsequent exercises in this chapter to two significant figures and then corrected to one significant figure, for greater accuracy is unreasonable. Why is that 1

1. What is the conduction heat loss per hour through each of the following?

a) An ordinary closed window 2 m x 1-5 m, if the temperature difference is 17 degC.

b) A 'tiles on battens and felt' roof 10 m x 8 m, if the temperature difference is 7 degC.

c) A wood floor on joists 10 m x 8 m, if the temperature difference is 10 degC.

d) The sides of a house 10mx8m by 5m high, if the walls are 0-28m cavity brick, ventilated, the door area is 3 m², the window area is 20 m² and the temperature difference is 10 degC.

2. What saving in heat is achieved per hour if:

a) the roof in 1 (b) has 0-025 m, fibre glass overlaid on the ceiling;

b) the window in 1 a) is double glazed;

c) the walls in 1 d) are filled with foam plastic?

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Heat losses	\|	Convection heat loss

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