The first thing I want to do is adjust the figures for the different numbers of days in each billing cycle. That one day difference is important. The easiest way I believe is to divide the number of therms used by the number of days to give an average therms per day figure.

Using your numbers for December 2006:

119 therms / 31 days = 3.8 therms per day

Now to make this relevant to the average temperature that month, I would divide this derived figure by the average temperature to get an average therms per day per degree average temperature.

3.8 therms per day / 28F = 0.14 therms per day per degree F

Doing the same calculations for December 2007 gives me the figure 0.28 therms per day per degree F.

This method should give you a rough comparison, but it's not ideal in some ways. I don't like using averages, they can be heavily skewed over wide ranges. Also, what we are comparing is your gas consumption against the average temperature. Your heating does not however heat that way, it raises the temperature in your house to whatever the thermostat is set at above that temperature, which means we're missing a more useful number from our calculations.

For example, when it was on average 16F, and your thermostat is set at 70F, your heating had to raise the temperature in the house on average by 54F. This number has a direct relationship to how much work your central heating did, and so would be a better way to compare the numbers.

If I used the assumed figure of 70F for the thermostat, I get the numbers:

December 2006: 0.09 therms per degree F your house temperature is raised.

December 2007: 0.08 therms per degree F your house temperature is raised.

If however your thermostat was reduced to 65F during December 2007, the figure would become 0.09 therms per degree your house temperature is raised.

As you can see, the amount of work performed by your central heating is very important to these figures and could turn them around entirely. Unfortunately, if your house is anything like ours, the thermostat setting changes several times a day. Traditional mercury thermostats are also exceptionally inaccurate. Some by 5F or more, which is perhaps why some people report that they only heat their house to 64F!

We might also have to factor in other appliances that use gas in your house?

To really be sure if there is a statistical difference we would have to collect all the data you have for your gas usage, the weather and any other data you have and perform some type of statistical test on it, which would give us a probability of the difference in gas consumption being by chance. It's kind of extreme, I certainly haven't bothered to statistical test any of my own bills yet, but it would give you as close to an accurate answer as possible.

Anyway, I hope this highlights the difficulty we have deriving useful figures for energy use outside controlled environments. As the variables increase and the accuracy of the figures we do have are questioned, attempts to turn them in to something useful become more difficult.

Thank you for that great explanation. After I thought through this a bit, I had a feeling it was going to be more difficult than I had originally thought because as you stated there are a number of variables. This does give me something to work with though, so I really appreciate your response.

Kevin

Have you ever thought of moving south? My freezer is set at 28* not my house! I enjoy my extra summer here in Florida when the rest of the country is cold. Come on down the houses are many the taxes are high and the pay is low, but it could become paradise to you

Pete

Maybe you misunderstood. The 28 F temp is outside, not inside. That would be a bit chilly even for me.

Thanks for the invite, but I can't imagine a worse place to live for me. We might have some cold winters, but you have that heat and humidity for a good hunk of the year. Nothing against FL, but I love the seasons and the huge swings in temps. It makes life interesting.

Right now it's a balmy 27 F, but by tonight we should have temps in the -15 F range with wind chills in the -40 F. range. A good night to watch a movie, eat something good and look through the garden catalogs. Winter can be very cozy.

K

Hi Kevin/Pete, this year I am trying to do the same sort of heat saving calcs after some minor things I have been doing around the house for energy saving. I am not really looking for dollars and cents, but that I am moving things in the right direction.
So I looked at my Dec08 bill, which had 250 therms used over 213 last year, I acknowledge the temp is variable thru the day and we have lowered all setting from last year. So we never go above 70F. So a difference of 37 therms.
I talked to a neighbour and their figures were Dec08 205 used over 143 last year. We have the same billing period of 35 days. So a difference of 62 therms.
This would indicate a substantial saving by what we have done since last year. I know there are many many variables here, but I am trying to get a picture.
Would this method give a reasonable indication.

do you realy need to calculate? November is easy but December was on average 12c lower and yet only 16 therms different, where as Nov was 26 therms difference between 06 and 07 but the average temp was the same, ball park it! imagine what it would have been! I am pretty good at math but trying to work this out is pointless as the average temp was 12 difference it doesn't matter what the highs and lows where, if you kept the inside of your house at an average temperature what ever that was, you are on the right track, what I am trying to say is your first post I think it was, from England was correct you had to heat you house more so it used more oil/gas but probably not as expensively as it would have done the year before.
on that note I missed most of the winter here in CT being at home in the UK, But was amazed the week after I arrived in CT it snowed and was a foot deep, but NONE of the houses around me by midday had any snow on there roofs, whilst in the UK we has a stupid couple of inches and the snow on roofs lasted for days, I was so concerned, I ventured out to examine our roof, dispite not having boots! and was relieved to find we had snow! Phew! I don't understand how people want to pay to heat the sky, and the added bonus of snow on you roof is it forms it own insulation level, thus keeping you house even warmer win win in my book.

FYI to heather snow on roofs does insulate however it can become so heavy that the roof collapses. Some older houses have systems to remove the snow from the roof. Look for wires along the edges of the roofs. Some people also use what is called in some areas snow rakes.