How Do I Compare Costs from One Furnace to Another?

As you know, we got a new, high-efficiency furnace last fall. Once winter is over here (which is the end of April, unfortunately), I'd like to do a cost comparison to see if the new furnace saved us money versus the old on we had last year, and if it did, how much it saved us.

I'm assuming that the basic information I need to compare the two years is the cost of natural gas each year as well as how much we used. If the calculation was that simple, I think I could do it easily. But it's not that easy, is it? Don't I need to somehow adjust for the average outside temperature from one year to the next (we kept the inside temperature relatively the same each year, so I'm calling that a wash)? For instance, what if one year was a lot colder than the other (which I think last year was colder than this year)? Then the furnace would have used more simply because of the weather, not because it was less efficient. So how do I make the adjustment?

I looked around on the web and couldn't find a calculator that would help me out, so I thought I'd ask all of you for your thoughts. There are a lot of smart people that read this blog and I suspect a few of them can set me straight. Am I thinking about the way to compare the two correctly? And is there an online calculator or guide that can help me make the comparison with little effort?

Comments

[jim]

FMF, yes you would need to account for how cold it was this winter versus last winter. One way to do that is to find the average temperature for this winter and for last winter. Here's a source for daily temperatures: Go to wunderground.com, search for your city, scroll down to 'weather history for this location', then click on 'yearly' view. They have the average temperature for each day. If you find the daily average temperatures that gives you a decent comparison point. If you go one step further you can calculate the 'degree days' for each month. Degree days is how much colder it is on a day from 65F. So if the av. temp today is 46F then there are 65-46 = 19 degree days.

You'd also need to take your gas bill and account for your non heating usage. So whats the average bill in summer or April? If take that as your base cost for gas and subtract it from your monthly bills to seee the extra you're paying for heat. e.g. if you pay $50 average for gas in April - Sept when the heat is off and then your January gas bill is $250 then you can assume that your heat cost for gas in Jan is $200 ($250-$50 base).

[Mat]

I'm not sure if there is an easy calculator out there for you, but if you or a friend are comfortable with statistics a basic linear regression should control for the variations in gas price and outside temperature. It's unlikely that you would have enough data points for a statistalcally significant result, but you might still be comfortable with the results.

(I should add, a statistically significant outcome would be a model with .05 p. But p is a bit of a sliding scale, all the more so outside of something requiring academic rigor)

-Mat

[jim]

You cna also find historical weather info at NOAA, like Grand Rapids, MI :
http://www.crh.noaa.gov/grr/climate/f6/
If you look up an individual month then the "HDD" # is the heating degree days.

[Bobbert]

I would aim to find the number of therms per degree it took to keep your house at the desired temperature on both models.

For each month, find the average daily temperature through Winter then see how much higher your average set daily temperature is.

Eg. Winter's average temp was 30 degrees, the house was an average of 64 degress, so on average you were keeping it 34 degrees above the ambient temp.

Then take the average number of therms used per day in winter and use that to find out how many therms it took per degree.

Eg. We used 420 therms in October through February, so 3.41 therms per day or 0.10029 therms per degree warmer.

Do the same for your old furnace and you should have a number that's comparable.

--

Summary:
On each furnace
Average House Temp - Average Daily Temp = Average Degrees per Day
Total Therms / Days = Average Therms per Day
Average Therms Per Day / Degrees per Day = Therms per Degree

Flaws:
Heat is lost more easily at higher temp contrasts. Warming 63 to 64 degrees when it's -20 degrees outside will be harder than when it's 62 outside. Hopefully the averaging will mitigate that.

Our gas bill includes a per-therm deliver charge and gas affordability program charge on top of the gas charge itself, don't forget to factor them in!

[Wayne]

I am not personally familiar with degree days. My dad delivered propane many years ago and they used the degree days to calculate how often to deliver to propane to each customer. You can go to www.degreedays.net and you can find a nearby weather station and have the degree day calculations for the last 24 months downloaded.

They have some explanation papers as well at the web site. Have fun calculating!

[jeffinwesternwa]

my gas bill breaks out monthly use and avg temps, it's just easy to look at this years vice last year (they put a chart in) and you can see if use is up/down. I keep tStat @ 65'F when home and 60'F @ nights/out but, use an efficient gas fireplace in evenings that gets main living area to 68-70'F fast. I heated last year for about $500! That's Hot water too! Natural Gas is THE way to go. good insulation, common snese w/ temps and an electric blanket helps (electric avgs $1~ a day winter and summer for me). home built in 2008, small, one story in the SW WA, Portland OR metro area....Suggest to your gas co they use a bill like NW Natural Gas Co does!

[MC]

My gas and electric bills from year to year are within $10 of each other--& I live in snowy, cold Minnesota. The total therms (for natural gas) are also identical, although they do vary month to month within a year.

Evidently, year to year differences in seasonal temp changes have a minor effect on annual heating costs.

So, if you just compare total costs from one year to the other you will probably get very close to exactly how much your new furnace is saving you. ie no need for a complicated temperature integration.

[DCS]

I've got a better way if you're heating with DTE natural gas. Their web site has a "MyEnergy Analyzer" feature that you can use to compare your bills. You can only compare one month at a time to one other month, and once you pick a month you can only compare it to the previous month or the month about a year ago at roughly the same time. It won't let you select a range unfortunately.

It will compare the bills and note any rate changes, difference in days billed, weather differences (including showing you heating-degree days), and differences in gas volume usage. It even calculates CCF/heating degree day, which is really what you're looking for. No calculator required.

To use it you log into your account (or set one up if you don't have one - it only takes a copy of the paper bill and a minute or two) and then go to "Analyze My Bill" on the left side.

[Rebecca]

Duke Energy has a fantastic website with tons of historical data on your usage.

[FMF]

Thanks, everyone!

DCS -- I do have DTE and will use that option. Thanks!!!

[James S]

http://www.microsoft-hohm.com/

[theCase]

If you make the assumption your house heating DEMAND remains the same and one knows the relative efficiency one could do this as an easy approximation.

Lets say the old furnace is 70% efficient and you spend $1000/year for heating.

$1000 * 70% = $700, meaning your house BURNED $1000 dollars worth of gas and DELIVERED $700 worth of heat.

Since your house will still require the same $700 worth of heat after the new furnace is installed. just work it backwards. Assume the new furnace is 85% efficient, then $700 / 85% = $823.50. meaning one could expect a savings of $176 per year. Simplistic yes, but essentially correct.

[MasterPo]

You also have to somehow adjust for the inevitable increase in the cost of fuel.

Even if your new burner uses less gas, if the cost per unit goes up you really haven't saved anything in real cash-in-hand terms.

[Mark]

@MasterPo: You HAVE saved in real cash even if the price per unit goes up. The units you didn't buy would have been paid for at the higher price.

[MasterPo]

Mark - MasterPo hears that argument all the time, and it's still BUNK! (no insult intended)

Reducing possible future costs is NOT the same as reducing what you pay now.

If you pay $100 a period for something, do something, and now pay only $75 that's $25 more in your pocket to spend.

But if you pay $100 a period for something, do something, and still pay $100 there's no difference. Even if the rest of the world is paying more than you it doesn't put anything additional into your pocket.

[mbhunter]

What about heat loss from your house? I'd think a house would get leakier over time.

[Old Limey]

@theCase
I fully agree with your approach, it is by far the easiest and does not require that you know anything at all about the temperature conditions.
My old furnace was 70% efficient. My new, top of the line, Carrier furnace is 99.6% efficient and also has a variable speed motor that saves electricity and also makes a huge difference in quietness and maintaining the exact temperature specified on the programmable thermostat.
My home heating costs have therefore been reduced by (1 - (70/99.6)) = 0.297, i.e. by 29.7%.
I also increased my roof insulation to the maximum by having fiberglass blown in on top of the existing insulation. We already had double pane glass in all windows and doors.

In order to obtain a dollar amount you would have to examine your monthly bills but they also include the gas used by your other gas appliances and there's no easy way of separating the furnace usage from everything else. My baseline rate for natural gas has only increased about 5% since 2003 and for a 2,300 sq. ft. home, my highest total gas bill this year, to keep the temperature at 70deg during the day, and my wife happy was only $128 in January, here in Silicon Valley.