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## Home Heating Basics

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**National Average Home Energy Costs**EGEE 102**70 'F**30 F Furnace Why do we need Heating? EGEE 102**Heat Transfer**• Conduction • Convection • Radiation EGEE 102**Conduction**Energy is conducted down the rod as the vibrations of one molecule are passed to the next, but there is no movement of energetic EGEE 102**Convection**Energy is carried by the bulk motion of the fluid EGEE 102**Radiation**Energy is carried by electromagnetic waves. No medium is required EGEE 102**Degree Days**• Index of fuel consumption indicating how many degrees the mean temperature fell below 65 degrees for the day • Heating degree days (HDD) are used to estimate the amount of energy required for residential space heating during the cool season. • Cooling degree days (CDD) are used to estimate the amount of air conditioning usage during the warm season EGEE 102**How do we calculate HDD?**• HDD = Tbase - Ta • if Ta is less than Tbase • HDD = 0 • if Ta is greater or equal to Tbase • Where: Tbase = temperature base, usually 65 F Ta = average temperature, Ta = (Tmax + Tmin) / 2 EGEE 102**Heating Degree Days**• Calculate the number of degree days accumulated in one day in which the average outside temperature is 17ºF. Degree days = 1 day ( 65 – Tout) = 1 (65-17) = 48 degree days EGEE 102**Heating Degree Days in a Heating Season**• Calculate the degree days accumulated during a 150-day heating season if the average outside temperature is 17ºF Solution: Heating Season Degree days = 150 days ( 65 – Tout) = 150 (65-17) = 7,200 degree days EGEE 102**Degree Days for the Heating Season**EGEE 102**Class work**EGEE 102**Significance of HDD**• Mrs. Young is moving from Anchorage, Alaska (HDD =10,780) to State college, PA (HDD = 6,000). Assuming the cost of energy per million Btu is the same at both places, by what percentage her heating costs will change? Solution HDD in Anchorage, Alaska = 10,780 HDD in State College PA = 6,000 Difference = 10,780 - 6,000 = 4,780 Saving in fuel costs are EGEE 102**Home Energy Saver**• http://homeenergysaver.lbl.gov/ EGEE 102**Home Heating Costs in State College**Average House Energy Efficient House Total $1,891 Total $1,019 EGEE 102**Home Heating Costs**• Related to amount of insulation, material that resists the flow of heat • Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. The higher the R-value, the greater the insulating effectiveness. The R-value of thermal insulation depends on the type of material, its thickness, and density. • R-30 better than R-11 EGEE 102**Places to Insulate**• Attic is usually the easiest ad most cost effective place to add insulation • Floors above unheated basements should be insulated • Heated basements should be insulated around the foundaton EGEE 102**R-values for Building Materials**EGEE 102**R-Value for a Composite Wall**EGEE 102**Home Heating Energy**• Heat loss depends on • Surface Area (size) • Temperature Difference • Property of the wall ( R value) Outside 30¨F Inside 65¨F Q (Btus) 1 A (area) x Temperature Diff (Ti – To) = t (time, h) R EGEE 102**Q**Q t t Heat Loss Tcold Thot Heat Loss = Id Q/t is in Btu/h Area in ft2 Tin-Tout in °F Then the thermal resistance is R-value. The units of R-value are EGEE 102**Wall loss rate in BTUs per hour**• For a 10 ft by 10 ft room with an 8 ft ceiling, with all surfaces insulated to R19 as recommended by the U.S. Department of Energy, with inside temperature 68°F and outside temperature 28°F: EGEE 102**Calculation per Day**• Heat loss per day = (674 BTU/hr)(24 hr) = 16,168 BTU • Note that this is just through the wall • The loss through the floor and ceiling is a separate calculation, and usually involves different R-values EGEE 102**Calculate loss per "degree day"**• If the conditions of case II prevailed all day, you would require 40 degree-days of heating, and therefore require 40 degree-days x 404 BTU/degree day = 16168 BTU to keep the inside temperature constant. • This is the loss per day with a one degree • difference between inside and • outside temperature. EGEE 102**Heat Loss for Entire Heating Season.**• The typical heating requirement for a Pittsburgh heating season, September to May, is 5960 degree-days (a long-term average). Heat loss = Q/t = 404 Btu/degree day x 5960 degree days = 2.4 MM Btus The typical number of degree-days of heating or cooling for a given geographical location can usually be obtained from the weather service. EGEE 102**Numerical Example**EGEE 102**Heat loss Calculation**EGEE 102**Problem**• A wall is made up of four elements, as follows • ½” wood siding • ½” plywood sheathing • 3 ½ in of fibber glass • ½” of sheet rock • How many Btus per hour per sq.ft. will be lost through the wall when the outside temperature is 50F colder than inside? EGEE 102**Economics of Adding Insulation**• Years to Payback = C(i) x R(1) x R(2) x E------------------------------------- C(e) x [R(2) - R(1)] x HDD x 24 • C(i) = Cost of insulation in $/square feet • C(e) = Cost of energy, expressed in $/Btu • E = Efficiency of the heating system • R(1) = Initial R-value of section • R(2) = Final R-value of section • R(2) - R(1) = R-value of additional insulation being considered • HDD = Heating degree days/year • 24 = Multiplier used to convert heating degree days to heating hours (24 hours/day). EGEE 102**Pay Back Period Calculation**• Suppose that you want to know how many years it will take to recover the cost of installing additional insulation in your attic. You are planning to increase the level of insulation from R-19 (6 inch fiberglass batts with moisture barrier on the warm side) to R-30 by adding R-11 (3.5 inch unfaced fiberglass batts). You have a gas furnace with an AFUE of 0.88. You also pay $0.70/therm for natural gas. • Given • C(i) = $0.18/square foot; C(e) = ($0.70/therm)/(100,000 Btu/therm) = $0.000007/Btu; E = 0.88; R(1) = 19; R(2) = 30; R(2) - R(1) = 11; HDD = 7000 EGEE 102**Household Heating Fuel**EGEE 102**Typical Heating Furnace Efficiencies**EGEE 102**Comparing the Fuel Costs**EGEE 102**Fuel Costs**• Electric resistance heat cost =$0.082 (price per kWh) / [ 0.003413 x 0.97 (efficiency)] = $24.77 per million Btu. • Natural gas (in central heating system) cost =$6.60 (per thousand cubic feet) / [ 1.0 x 0.80 (efficiency)] = $8.25 per million Btu. • Oil (in central heating system) cost =$0.88 (price per gallon) / [ 0.14 x 0.80 (efficiency)] = $7.86 per million Btu. • Propane (in central heating system) cost =$0.778 (price per gallon) / [ 0.0913 x 0.80 (efficiency)] = $10.65 per million Btu. EGEE 102**Heating Systems**EGEE 102**Heating Systems**• Some hot water systems circulate water through plastic tubing in the floor, called radiant floor heating. EGEE 102**Electric Heating Systems**• Resistance heating systems Converts electric current directly into heat • usually the most expensive • Inefficient way to heat a building • Heat pumps Use electricity to move heat rather than to generate it, they can deliver more energy to a home than they consume • Most heat pumps have a COP of 1.5 to 3.5. • All air-source heat pumps (those that exchange heat with outdoor air, as opposed to bodies of water or the ground) are rated with a "heating season performance factor" (HSPF) EGEE 102**Geothermal Heat Pumps**• They use the Earth as a heat sink in the summer and a heat source in the winter, and therefore rely on the relative warmth of the earth for their heating and cooling production. Additional reading http://www.eren.doe.gov/erec/factsheets/geo_heatpumps.html#sidebar EGEE 102**Benefits of a GHP System**• Low Energy Use • Free or Reduced-Cost Hot Water • Year-Round Comfort • Low Environmental Impact • Durability • Reduced Vandalism • Zone Heating and Cooling • Low Maintenance EGEE 102**Solar Heating and Cooling**• Most American houses receive enough solar energy on their roof to provide all their heating needs all year! • Active Solar • Passive Solar EGEE 102**Passive Solar**• A passive solar system uses no external energy, its key element is good design: • House faces south • South facing side has maximum window area (double or triple glazed) • Roof overhangs to reduce cooling costs • Thermal mass inside the house (brick, stones or dark tile) EGEE 102**Passive Solar**• Deciduous trees on the south side to cool the house in summer, let light in in the winter. • Insulating drapes (closed at night and in the summer) • Greenhouse addition • Indirect gain systems also such as large concrete walls to transfer heat inside EGEE 102**Passive Solar Heating**EGEE 102**EGEE 102**Source: Global Science, Energy Resources Environment