Gas Math

The question of the month is whether or not to upgrade to Natural Gas. To help with the calculation, I've discovered that Dorothy's Folder of Receipts contains receipts for filling the gas tank as well. It seems that during the winter months, Dorothy used an average of 2.6 gallons/day of heating oil, at an average cost of $2.89 per gallon.

A simple Google search gives me some handy information:
1 gallon fuel oil = 138,000 BTUs
1 Therm = 100,000 BTUs

Converted to Therms, Dorothy would have used an average of
[ (2.6 gallons/day) * (138,000 BTUs/gallon) * (1 Therm/100,000 BTUs) ]
= 3.59 Therms/Day

Now, I have my Puget Sound Energy gas bills from Ballardia Classic, and I can see that during the winter, we averaged 4.1 Therms/day, at a cost of $1.25/Therm. While Ballardia Classic is not Ballardia II, and has obvious differences such as size, insulation, and furnace efficiency, it does have one thing in common: its residents. I think we can safely conclude that as a fixed income retiree, Dorothy probably Conserved Energy. She probably closed doors and heated minimal areas of the house. She probably used the Trusty Wood Stove to supplement her fuel oil.

We would like to be as Energy Conscious as Dorothy, but well, we're not. So failing any other useful data, probably the bigger Therm number (ours) is more relevant.

Conveniently, PSE bills also break out consumption by month, and I can actually find all of the bills from last year in Ballardia Classic. At a glance, it looks like we have 6 months averaging 1 Therm/day, 4 months of 3.5 Therms/day, and 2 months of 4.5 Therms/day. So that all adds up to around
[(6*30*1+ 3*1)+ 4*30*3.5+2*31*4.5] fudging a bit on the # days per month =
882 Therms/year

To get the cost of Therms for oil, I get to do more math:
$2.89/gallon * 1 gallon/138,000 BTUs * 100,000 BTUS/Therm = $2.09/Therm.

Which, assuming equivalent efficiency, would be $1102 for natural gas, and $2090 for oil. Of course, I know that a new furnace would be 93% efficient, vs. my current relic which, the oil company has told me, is 83%efficient, so I really pay more like $2.30/Therm, or $2300 for oil

The cost of the Gas Project appears to be around $7000-8000; at this rate, it'll take almost 7 years for the project to pay for itself in fuel savings.
Makes me want to just get out the level and a couple of screws and just fix the Original Charming Mercury Thermostat.