Analyzing Gas Usage

I'm in the Detroit area...
I have compiled by gas usage from 9/1/04 to 2/3/06 by billing dates, as well as heating degree days for those same range of dates.
Since I have been refinishing my basement, AND turned the thermostat down from 70-71 to 67, I would like to anaylze how much more efficient my house is. The basement is no longer pouring cold air into the upstairs since I have insulated and drywalled the outside walls.
So how do I compare these with different heating degree days? If the heating degree days is 10% less this year from last, would you expect 10% less gas usage? Or is it not linear?
For a great example:
1/6/05-2/5/05 -- 30 days, 43.9 heating degrees PER day, 12.433 ccf/day 1/6/06-2/3/05 -- 28 days, 30.54 heating degrees PER day, 8.1 ccf/day
I should also note that I would estimate approx 0.65 ccf/day for non furnace related gas usage (that's how much was used during july/august when the heater was off completely).
Is this a good savings? It looks pretty good to me!
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One thing you have to be wary of is that some Gas Meter Readers get lazy in Winter and just sit at home and estimate your gas meter reading while watching game shows on TV or whatever. I have had it happen twice to me. Once at a large Mfg. Plant where I was the Accountant and noticed the gas billings seemed odd. When I brought it to the attention of the Manager of the Gas Company, they caught the lazy meter reader and he was fired. Rick

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Rick Shaw wrote:

Even if it says "actual"??

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Larry Bud wrote:

It is linear, but I'll bet you also have a water heater and that also uses gas and it does not change due to heating degree days. In fact it is a month or two behind the effect.
The other problem you may face is if you are looking at your bill rather than the meter. The bill is often estimated every other moth or so.

--
Joseph Meehan

Dia duit
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more heat/cool calculators at: http://www.hvacopcost.com /
double check your basement insulation and more at: http://www.buildingscience.com/resources/basements.htm
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I grew up there. Moved south as soon as I could...

2005: (12.433-0.65)ccf/day divided by 43.9 heating degree/days = 0.268 ccf/heating degree
2006: (8.1-0.65)ccf/day divided by 30.54 heating degree/days = 0.244 ccf/heating degree.

About a 9% savings.
-- Doug
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You are probably talking about "degree-days," ie degrees TIMES days vs degrees PER day. A degree-day is a 1 F temp diff maintained for 1 day... 43.9 DD would be a temp diff of 43.9 F maintained for 1 day.

That may not matter in January, if the heat ends up in the house.
If you know your furnace efficiency and therms/ccf and indoor electrical energy usage, you can estimate the thermal conductance of your house. For instance, with an 80% furnace and 1 therm/ccf and 600 kWh/mo (68K Btu/day) of indoor electrical use, the first record would correspond to a house with (100Kx12.433/0.8+68KBtu)/43.9F/24h = 1540 Btu/h-F and the second would make (100Kx8.1/0.8+68KBtu)/30.54F/24h = 1474, both being quite high, compared to a new energy-efficient house with a thermal conductance of 200-400 Btu/h-F.
Nick
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