Questions on Heat Transfer
Q2.
The U-value of a material tells you how much energy passes through the material every second, for every square metre of material, for every degree difference between the inside and outside temperature.
The table below shows the U-values for materials used in various parts of a house:
Material
|
U-value /W m-2 oC-1 |
brick cavity wall |
1.5 |
brick insulated cavity wall |
0.5 |
single glazed windows |
5.0 |
double glazed windows |
3.0 |
(a) Imagine your bedroom has an uninsulated cavity outside wall which measures 2.0 m by 4.0 m. Within the wall is a double glazed window which measures 1.0 m by 1.0 m.
(i) What is the area of the brick part of the wall of the room? (2 marks)
Brick area = area of wall - area of window 
= 2.0 x 4.0 - 1.0 x 1.0
= 7.0 m2
(ii) What is the area of the glass part of the wall of the room? (1 mark)
1.0 x 1.0 = 1.0 m2
(iii) How much energy is lost through the walls every second when the temperature difference between the outside and inside is 10oC? (2 marks)
Energy Transfer per second = Area x Temperature difference x U-Value
= 7.0 x 10 x 1.5
= 105 J
(iv) How much energy is lost through the window every second when the temperature difference between the outside and inside is 10oC? (2 marks)
Energy Transfer per second = Area x Temperature difference x U-Value
= 1.0 x 10 x 3.0
= 30 J
(v) What is the total amount of energy lost through the wall and window? (1 mark)
105 + 30 = 135J
(vi) Explain why double glazing reduces the amount of energy escaping through the windows. (2 marks)
The air trapped between the two sheets of glass is a very good insulator. It transfers energy at a slower rate than glass . It therefore reduces the rate at which heat energy is able to be transferred through the window to the outside.
(b) In a typical three-bedroomed semi-dateched house, the cost of Do-it-Yourself double glazing is about £1400. If the heating system in the house uses gas, the householder could expect to save about £50 a year on heating costs.
(i) How many years would it take for the householder to cover the cost of installing the double glazing from the savings on heating bills? (2 marks)
£1400/£50 = 28
It would take 28 years for the househlder to recoup his investment.
(ii) Quite a lost of householders choose to fit replacement double glazed windows, but fewer put in cavity wall insulation. Suggest a reason for this. (2 marks)
Windows often need replacing anyway because of rotting frames so the householder upgrades to improve heating bills and the general look of the house whereas cavity wall insulation, although it saves money on fuel bills is not such a visible improvement to the property.
(Total 14 marks)
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