Owens Corning
Energy Saver Worksheet: Setback Thermostat
Insulate NowThe worksheet calculates the potential savings using a setback thermostat. We all know that it takes less energy to keep your house at 60 degrees during the winter than at 70 degrees, but how does that relate to your heating bill? To calculate the saving exactly requires a heating engineer and a thorough survey of your house.

We've created a simplified worksheet where you record the time your furnace runs at its normal temperature setting. You record the time your furnace runs at the temperature you plan to set the thermostat back to. The assumption being that the difference in the running time of your furnace is about equal to the energy saved.

You may want to try different combinations of temperature setbacks and durations. If your house empties out every morning don't forget to figure in a setback period for those daytime hours when there's no one there.

Choose an average temperature winter day to time your furnace because if you use a very cold day the running times won't represent your typical savings. Wait at least a half hour between thermostat settings for the house temperature to stabilize before you start timing. If you have a total electric house about 30% of your total electric bill goes to heat water and provide light. If you heat water with the same fuel that you heat your house about 20% of the bill is for heating water so subtract the amount from the total on the worksheet.

For example, if you heat your house and make hot water with oil and your total oil bill for the year is $1500 you can expect to save about $132 a year if you set back your thermostat 10 degrees each night for 8 hours.

Here's how we got the figure using the worksheet:

Since 20% of oil is used to heat water multiply $1500 by .8 to get your $1200 annual heating cost.

Let's say you timed the furnace and it ran for 30 minutes when the thermostat is set at 70 degree and for 20 minutes when you set it back to 60 degrees. With this information, find the total minutes your furnace runs without a setback: 30 minutes times 24 hrs. or 720 in. Then calculate the time the furnace runs with a setback: 30 minutes for 16 hrs. and 20 minutes for 8 hrs. or 640 minutes.

ind the difference between these running times and then figure the savings by subtracting the setback running time from the non setback running time. Then divide the answer by the non setback running time to find the percentage of savings.

Apply the 11% savings to your total bill (1200 X .11) to find your potential savings of $132.

Cost Of Energy

Electric:

$ _______________ / ______________ = (1) $ _____________

Electric Bill Kilowatt Hours Cost/KWH

Oil:

$ _______________ / ______________ = (2) $ _____________

Oil Bill gallons delivered Cost/Gal

Natural Gas:

$ _______________ / ______________ = (3) $ _____________

Gas Bill Therms Used Cost/Therm

Enter (1),(2) or (3) Cost of Energy (4) $ ______________

Savings for Setting Back the Thermostat

(1) $_______________ If you heat house and heat water

Total $_____________ times .8 = (1)$___________

electric/gas/oil Total Total Heat
electric/gas/oil

If you heat house, heat water and light

$_____________ times .7 = (1)$___________

Total Total Heat
electric

Regular Thermostat Setting __________ degrees

Lower Thermostat Setting __________ degrees

Hours you plan to keep thermostat set at lower setting (2) ______ Hrs

Hours furnace will run a normal setting (24 Hrs - (2) ______ Hrs) = (3) _______ Hrs

Furnace running time Regular Thermostat Setting during

1 hour period = (4) ______ Minutes

Furnace running time Lower Thermostat Setting during

1 hour period = (5) ______ Minutes

Furnace running time no setback

(4) __________ X 24 hours = (6) ___________ Min

Minutes No setback run
time

Furnace running time with setback

(5) _________ X (2) ___________ = (7) _____________ Min

Min/hr run time Hrs at Low Min at Low

Low Temp Temp Temp


(4) _________ X (3) ___________ = (8) _____________ Min

Min/hr run time Hrs at Reg Min at Reg

Reg Temp Temp Temp


Total furnace running time with setback

(7) plus (8) (9) ___________ Min


Running time Saved (10) ___________ Min

(6) minus (9)


Percent Savings:

(10) _____________ / (6) _____________ = (11) _____________%


Estimated annual savings

(11) _____________% X (1) $ ____________ = (12)$ _____________


Analysis


Figuring this savings without getting into heavy math that probably will not yield anything meaningful I have come up with a simple concept: The amount of energy used to heat a house is directly proportional to how long the heating system runs. This is true for any type system, and state of insulation, windows etc. The longer the furnace runs, the more energy is used, the less it runs, the more you save. Each house is unique as far as energy efficiency goes, but this approach measures actual energy consumption in that particular house.


Given two target temperature settings of say 70 degrees normal setting and a 60 degrees setback setting, the homeowner is asked to time how long their furnace runs at each temperature. The difference in running time is basically the savings. This ratio can then be applied to the seasonal heating bill to figure the potential savings.


Total heating bill $1200.00

Outside temp 40 degrees

Setback to 60 degrees for 8 hours

High Furnace on Time (70 degrees) 30 min/hr

Low Furnace on Time (60 degrees) 20 min/hr

No Setback Energy Use = (30 min/hr * 24) = 720 min

High Temp Energy Use = (30 min/hr * 16 hr) = 480 min

Low Temp Energy Use = (20 min/hr * 8 hr) = 160 nin

Savings = 720 - (480 + 160) = 80 nin

Estimated Savings % = (80/720)100 = 11%

Estimated Dollar Savings = $1200 * .11 = $132

Assumptions:

Heat saved during house cool down to lower setting is equal to heat required to reheat house. (basically a wash)

Energy use is proportional to the time the furnace runs.

The energy saved between the regular temperature and the set back temperature is proportional at different outside temperatures.

Formulas:

High Furnace on time = time furnace runs per hour at high setting

Low Furnace on Time = time furnace runs per hour at setback setting

No Setback Energy Use = High Furnace on Time * 24

High Temp Energy Use = High Furnace on Time * hours at high temp

Low Temp Energy Use = Low Furnace on Time * hours at setback temp

Savings = No Setback Energy -(High Temp Energy + Low Temp Energy)

Estimated Savings % = Savings/No Setback Energy

Estimated Dollar Savings = Heating Bill * Savings %


written by Gene and Katie Hamilton

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