Q5 HL 2008 U-Value It is proposed to replace the single glazing in a dwelling house with double glazing. (a) Using the following data, calculate the U-value.

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Presentation transcript:

Q5 HL 2008 U-Value It is proposed to replace the single glazing in a dwelling house with double glazing. (a) Using the following data, calculate the U-value of the: (i) single glazing; (ii) standard double glazing. Glass: single glazing thickness 5 mm Glass: double glazing thickness 4 mm Space between panes width 12 mm Thermal data of glazing: Conductivity of glass (k) 1.020 W/m °C Resistance of space between panes (R) 0.170 m2 °C/W Resistance of internal surface (R) 0.122 m2 °C/W Resistance of external surface (R) 0.080 m2 °C/W

Formulae: r = 1/k R=T x r U =1/Rt Material Conductivity (k) Resistivity (r) 1/k Thickness (m) Resistance (R) Single glazing Int surface Ext surface Double glazing space 1.02 0.9803 0.005 Rt = .008 0.012 0.0049 0.122 0.08 0.2069 0.007843 0.17 0.122 0.08 0.3798 Formulae: r = 1/k R=T x r U =1/Rt Single Glazing; U = 1 / Rt = 1 / 0.2069 = 4.83 W/m2/°C. Double Glazing; U = 1 / Rt = 1 / 0.3798 = 2.63 W/ m2/°C. Hence, the double glazing is almost twice as effective as the single glazing.

Q5 (b) A choice is to be made between the following types of double glazing: • standard double glazing; • low-emissivity (low-e) double glazing. Using the U-values obtained at (a) above and the following data, calculate the cost of the heat lost annually through each of the following: • single glazing; • low-e double glazing. U-value of low-e double glazing: 1.1 W/m2 °C Area of glazing: 25 m2 Average internal temperature: 18 °C Average external temperature: 5 °C Heating period: 11 hours per day for 40 weeks per annum Cost of oil: 80 cent per litre Calorific value of oil: 37350 kj per litre 1000 Watts: 1kj per second.

Cost of Heat Lost per year through; (i) Single Glazed Window Heat Loss: = U-Value x Area x Temp. Diff Temperature difference= 18 – 5 = 13 4.83 x 25 x 13 = 1569.75 Joules / sec Annual heating period: = 60 x 60 x 11 x 7 x 40 = 11088000secs (40 weeks a year, 7 days a week, 11 hrs a day, 60 mins an hour, 60 secs a minute) Therefore, 11088000 x 1569.75= 17405388000 joules (divide by 1000) = 17405388 Kj Note: Calorific Value of 1 Litre oil = 37350kj Therefore, 17405388 / 37350 = 466 litres of oil 1 Litre Oil costs -= 80c; Cost per annum: 466 x 0.80 = €372.80

(ii) Double Glazed Window 4.83 = 372.80 2.63 = X Cross multiply: 4.83X = 2.63 x 372.80 4.83X = 980.464 X = 980.464 / 4.83 X= €202.99 (iii) Low-e Double Glazed Window 4.83 = 372.80 1.1 = X Cross multiply: 4.83X = 372.8 x 1.1 4.83X = 410.08 X = 410.08 / 4.83 X = €84.90

(c) Using the information obtained at (b) above, recommend a preferred glazing type and give two reasons to support your recommendation. Low-e Double Glazing window type recommended Comparison of costs associated with heat lost for each window type for one year: (i) S.G. €372.80; (ii) D.G. €202.99; (iii) Low-e D.G. €84.90 • Four times more efficient than single glazing. • Two and a half times more efficient than double glazing. • Costs of low-e double glazed window type recouped over a shorter time (5-8 years) • Price of home heating oil is predicted to rise.