Presentation on theme: "U-values. Convection Conduction Radiation Heat flows from a hot place to cooler place."— Presentation transcript:
Convection Conduction Radiation Heat flows from a hot place to cooler place
All buildings waste heat Different materials lose heat at different rates Homes need to be insulated to reduce heat loss U-values compare rate of heat loss through different materials. They measure how quickly heat travels through a material
Measured in W /(m 2 o C) W-Watts (heat loss) m 2 -area of material o C-temperature difference
25Wats Area of material 2m 2 Temperature difference 20 o C U value = 25 / (2x20) U=0.625 W/(m 2 o c) This is a low U value, a good insulator.
High value U-values lose heat more quickly Low value U-value lose heat more slowly Metals have high U-values (good conductors) Insulators have low U-values, (ie foam)
Low U values
High U values
Pay back time = initial cost / annual saving Buying insulation costs money But you could save in the long term Over time the money you save on bills equals the money spent, this is payback time.
1.Copy and complete the table below: P Payback Time and Energy Efficiency InsulationCostAnnualSavingPayback Time (Yrs) Loft Insulation £400£80 Double Glazing £2,000£50 Cavity Wall Insulation £600£60 Draught Excluder £40£20 5yrs 40yrs 10yrs 2yrs Double Glazing a.Which type of insulation costs the most? b.Which type of insulation is the most effective? c.Which type is the most “cost effective”? d.Which type pays for itself after 40 years? Loft Insulation Draught Excluder Double Glazing
useful output efficiency X total input 2.An ordinary light bulb is 8% efficient. If it uses 1,000J of electrical energy how much energy is wasted as heat? Payback Time and Energy Efficiency useful output energy =efficiency total input energy X useful output energy =0.08 X 1,000 useful output energy =80J waste energy = 1, waste energy = 920J
useful output efficiency X total input 3.A gas boiler wastes 260J for every 1,000J of energy it is supplied with. What is the efficiency of this appliance? Payback Time and Energy Efficiency efficiency = total input energy useful output energy efficiency = 1, efficiency = 0.74 efficiency = 0.74 X 100 = 74% How could the waste energy be reduced? Insulate the boiler and hot water tank, lag the pipes and service the boiler
Heat flows from a hotter place to a _________ place. The heat flow is ________ if the material has a lower U-value. Insulating the wall reduces heat losses because it _______ the walls U value Use these words: cooler, reduces, slower