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Furnace Efficiency

Furnace Efficiency. Bryce Cox Dallin Bullock. Problem. My gas bill is very expensive My furnace claims an efficiency of 78% but it appears to be less efficient. Objective. Using recorded high and low temperatures calculate the heat flux from my apartment in the month of March 2008

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Furnace Efficiency

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  1. Furnace Efficiency Bryce Cox Dallin Bullock

  2. Problem • My gas bill is very expensive • My furnace claims an efficiency of 78% but it appears to be less efficient

  3. Objective • Using recorded high and low temperatures calculate the heat flux from my apartment in the month of March 2008 • Use information on gas bill to determine natural gas used for heating • Compare calculated use to measured and compare efficiencies

  4. Assumptions • Radiation is negligible • Furnace keeps temperature inside apartment a constant 70º F • All neighboring apartments also at constant 70º F • Heat transfer through only two walls

  5. More Assumptions • Average convective heat transfer coefficient of 10 W/m^2·K • Inside wall surface temperature is equal to temperature inside apartment

  6. Set up • Generate value for conductive resistance in walls • Find values for thermal conductivity of wall materials

  7. Solution • Total resistance ofcomposite wall is1.054 m^2·K/W • Resistance of windowsis 0.105 m^2·K/W

  8. Solution • From recorded high and low temperatures we estimated average day and night time temperatures • We then used the average temperatures to calculate heat flux for each day and night

  9. Results • We found the total energy lost by heat transfer for the month was 3,371,151,069 Joules • Converted to British Thermal Units: 3,195,234 BTU • 3.195 DecaTherms (Unit used by gas company)

  10. Results • My gas bill states that I used 7.2 decatherms in March. Estimating that 1.5 decatherms a month is used by the water heater and oven then 5.7 DTH were used by the furnace

  11. Results • The furnace has a published efficiency of 78% • According to our calculations this means the furnace should use 4.1 DTH • We found that it actually used 5.7DTH which is about 56% efficiency

  12. Conclusion • The discrepancy in efficiency is due to many possible factors: • Dirty filter • Restricted air flow (poor duct arrangement) • Dust buildup in combustion chamber

  13. Recommendations • Change filter every 3 months • Clean ducts if needed • Make sure vents are not obstructed

  14. Appendix • Resistance per area equations:

  15. Appendix • Heat transfer equation:

  16. Appendix • Thicknesses • Brick 102mm • Wood siding 8mm • Studs 130 mm(40 mm wide, .65 m apart) • Gypsum 12mm • Glass 7mm

  17. Appendix • Total wall area: 34.82 m^2 • Total window area: 5.63 m^2

  18. Appendix • References: • BYU Geography Department – Weather statistics • http://www.pgjr.alpine.k12.ut.us/science/james/provo2008.html • Trane Heating Systems – Furnace Information • http://www.trane.com/residential/products/Furnaces/XR80.aspx

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