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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)

(ii) What is the area of the glass part of the wall of the room? (1 mark)

(iii) How much energy is lost through the walls every second when the temperature difference between the outside and inside is 10oC? (2 marks)

(iv) How much energy is lost through the window every second when the temperature difference between the outside and inside is 10oC? (2 marks)

(v) What is the total amount of energy lost through the wall and window? (1 mark)

(vi) Explain why double glazing reduces the amount of energy escaping through the windows. (2 marks)

(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)

(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)

(Total 14 marks)