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Heating and Efficiency

Heating and Efficiency. AP Env Sci - Home Heating Activity. Heat Needed for a Home or Other “Space”. Heat expressed in Btu - British Thermal Units - heat 1 lb water 1 degree F. H=heat needed in Btu H= K is thermal conductance What are factors that determine K. H = K T t

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Heating and Efficiency

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  1. Heating and Efficiency AP Env Sci - Home Heating Activity

  2. Heat Needed for a Home or Other “Space” • Heat expressed in Btu - British Thermal Units - heat 1 lb water 1 degree F. • H=heat needed in Btu • H= K is thermal conductance • What are factors that determine K. • H = K T t • T = temp. diff. Between inside and average outside • t - time increment

  3. What factors determine K • K is thermal Conductance • On total size of home in square footage • Construction material • number of windows and efficiency of windows • insulation in walls and ceiling • Other conservation measures

  4. What is a Degree Day (DD)? • 650 F is considered the standard temperature • t is measured in days • T - is difference between average temperature outside and 650F. • Example: average temp outside is 500F for one week, How many heating degree days that week? • DD= Tt = (65-50) 0F X 7 days = 105 degree-days

  5. Calculation of Thermal Conductance • To compare houses of different sizes, must eliminate the factor of square footage from K • k = K/A, where A is the area to be heated in square feet • SO, k is the thermal conductance per sq foot • So, H= k x A x DD • So, what factors determine H, the amount of heat needed to reach an average of 650F? • Size, the outside temperature, and the efficiency of the home (k) • If k = 10 BTU/sq foot - DD, what does this mean?

  6. Calculation Example • What is the heating requirement in BTU for a home of 1300 sq ft, 6000 DD for entire heating season, with a k value of 10 Btu/sq ft-DD?

  7. Calculation Example • What is the heating requirement in BTU for a home of 1300 sq ft, 6000 DD for entire heating season, with a k value of 10 Btu/sq ft-DD? • 10 Btu/sq ft-DD X 1300 sq ft X 6000 DD = 7.8 X 107 Btu

  8. Fuel Requirements • Two factors • Type of Fuel • Efficiency of furnace or other heating device • Hdelivered = the amount in heat in Btu delivered to the space in the home • Hpurchased = amount of energy content in the fuel • ε = efficiency of the heating unit • Therefore, Hpurchased = Hdelivered/ ε

  9. Calculations of heat needed • Say gas furnace is 70% efficient • to deliver the 7.8 X 107 Btu, how much BTU in oil would you need to “purchase” (that is, actually put into your furnace)?

  10. Calculations of heat needed • Say gas furnace is 70% efficient • to deliver the 7.8 X 107 Btu, how much BTU in oil would you need to “purchase” (that is, actually put into your furnace)? • 7.8 X 107 Btu / .70 = 1.14 X 108 Btu

  11. How much actual fuel needed? • Calculate the amount of natural gas required • For natural gas: 1030 Btu/ft3 • Note:gas companies typically use mcf (1000 ft3) or ccf (100ft3) • At cost of gas at _______ how much is the cost to heat this home? • (1.14 X 108 Btu) X (1 ft3/1030 Btu) = 1.11 X 105 ft3 • ( 1.11 X 105 ft3 ) X $1.25 / 100 ft3 = $ 1387.50

  12. How can cost be reduced? • Higher efficiency furnace • Redo calculation with 90% efficiency furnace • 7.8 X 107 Btu / .90 = 8.7 X 107 Btu • 8.7 X 107 Btu X (1 ft3/1030 Btu)= 8.4 X 104 ft3

  13. How can cost be reduced? • Higher efficiency furnace • Redo calculation with 90% efficiency furnace • (1.14 X 108 Btu) / .90 = 1.27 X 108 Btu • ( 1.3 X 105 ft3 ) X $1.25 / 100 ft3 = $ 1050

  14. Important question - What determines k? • (k represents the efficiency of the home) • Factors that could improve k are: • Improve thermal efficiency (k) of building - HOW? • Say efficiency is improved by 20%, what would you save?

  15. Payback periods - Improve Furnace • N (years) = cost of improvement X years/saving each year • =cost of improvement/savings per year • Calculate the payback period for improving the efficiency of your furnace in the above example, assuming a more efficient furnace would cost $2000.

  16. Payback periods - Improve k - efficiency of home • Assume previous problem 20% improvement in k? • What would you do to accomplish this, and at what cost? (assume original 70% efficient furnace) • Say 10 new windows, at cost of $250 per window. • Say, improve insulation - total cost of $600, you do labor.

  17. Improved Efficiency by 20% • (.2) (10 Btu/sq ft-DD) = 8 Btu/sq ft-DD • 8 Btu/sq ft-DD X 1300 sq ft X 6000 DD = 6.24 X 107 Btu • 6.2 X 107 Btu / .70 = 1.14 X 108 Btu

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