Question

Question 3 (12 marks)

• a.

  • i.Explain in your own words what is meant by the U-value of a piece of building fabric.

    (2 marks)

  • ii.An insulated wall in a new house consists of four layers. Table 1 gives their thicknesses and thermal conductivities.

Table 1

Layer	Thickness/mm	Thermal conductivity/W m−1 K−1
Brick	115	0.76
Foam insulation	110	0.025
Aerated concrete block	105	0.12
Dense plaster	13	0.56

• For each layer, calculate the thermal resistance in m² K W^–1.

  (4 marks)

  • iii.If the thermal resistance of an outside layer of air is 0.04 m² K W^–1 and of an inside layer of air is 0.13 m² K W^–1, show that the overall U-value of the wall is just under 0.18 W m^–2 K^–1.

    (3 marks)

• b.Explain briefly how the design of a condensing gas boiler differs from a non-condensing one. Why is the difference between the higher and lower heating value of a fuel such as natural gas important in determining the increased boiler efficiency?

  (3 marks)

Answer 1