ABBEYnet

Calculation of Heat Loss

Q1. A bungalow has an un-insulated timber and felt flat roof 125m²in area. Using the following data, calculate the annual cost of heat loss through the roof.

Data:
Time Span: 12 hours per day, 7 days per week for 40 weeks of the year.
Average Internal Temperature: =18°C
Average External Temperature: =06°C
U value of roof =1.9W/m²°C
Calorific value of Oil =37350 KJ per Litre
Cost of Oil =€0.23c per Litre
(1 Watts = 1 Joule per Second)

Calculation:
Average Temperature loss = 18°C - 06°C = 12°C
Heat lost per meter² =19m²°C x 12°C = 22.8 W/m²__
Heat lost through whole roof = 22.8 W/m² x 125m = 2850 Watts__

(1 Watt = 1 Joule per second)
Therefore 2850W = 2850J/s

Time for heat loss = 60(s) x 60(m) x 12(h) x 7(d) x 40(w) = 12,096,000 seconds

Total Heat Loss for the Period = 2850 J/s x 12096000 s = 34,473,600,000 J
(= 3.44 x 10¹º J)
= 34,473,600 KJ

Calorific Value of 1 Litre of Oil = 37350 KJ per Litre

Therefore, the number of Litres lost = 34,473,600 KJ
37350 KJ = 922.99 L

Value of Oil lost 922.99 L x €0.23c = €212.29

Q2. A House has a 25% heat loss through the roof; the annual cost of heating the house is €600. The installation of 150mm attic insulation costs $200 and reduces heat loss through the roof by 60%. Calculate the payback period.

The cost of the heat lost is = €600 x .25 = €150

Reduction in heat lost due to instillation of insulation is 60% of €150 = €90

The length of time to repay the outlay cost = Capital Cost =€200
Annual Savings €90

= 2.22 years

Q3. A house looses 35% of heat through the external walls. Loss can be reduced by 60% by insulation the walls. Capital cost is €500. Calculate the payback period.

Q4. 15% of heat lost through external doors and windows. Loss can be reduced by 50% by draught proofing. If the Capital cost is €80, calculate the payback period.

Method of Heat Transfer:

· Conduction
· Convection
· Radiation

Page 117 of your Construction Studies Book