Cost to operate natural gas furnace

jaysgardenDecember 23, 2010

Trying to gauge the cost(for natural gas) to run the furnace per hour. If anyone is good at this can you check my assumptions/calculations?

Here are my particulars:

1. I have a 120,000 BTU natural gas furnace.

2. Price per hundred cubic feet of natural gas from our supplier is $.70

Now the ciphering:

1.The amount of heat in one hundred cubic feet of natural gas is about 100,000 BTU

2.Divide the furnace input rating(in my case 120,000) by 1000 to get the number of cubic feet of gas the furnace will use in one hour.

=120 cubic feet

3.My supplier's bill is based on units of one hundred cubic feet(CCF) so I divide the above cubic feet by 100 to determine how many CCF the appliance will use in one hour. 120 divided by 100

=1.2 CCF per hour

4.My supplier charges $.70 per CCF so it costs me 1.2 times $.70 = $.84 per hour for natural gas for my furnace to run.


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First it does not matter how many BTU's your furnace is rated for. That amount is generally an average of a sampling of that companies product and not the amount that anyone's furnace will produce.

What you need to figure is how many CCF you use in a year. Then divide by the number of days in that year. Take the amount of yearly charges supplied by your supplier. Then figure out the daily rate of supplied gas times your usage.

Your usage goes up and down over the year and the amount used can vary depending on insulation, temp's on thermostat, type of house, how many doors and windows, etc. Our gas supplier's website provides the usage over the year and the amount's paid along with other information.

    Bookmark   December 23, 2010 at 11:43PM
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If I used 600 CCF's of gas in 2009 and divide that by 365 I get 1.64 CCF of gas per day. At $.70 per CCF thats $1.15 per day.

But if you have a natural gas oven, gas stovetop, gas hot water heater and gas furnace.

SO that $1.15 per day does not tell me anything about how much fuel the furnace consumes.

    Bookmark   December 24, 2010 at 7:06AM
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Part of the rating plate on the furnace states:

"Heating Input(BTU/Hr) 100,000"

How do you interpret this?

I interpret this as when all 5 burners are firing which they will since it is a single stage furnace, then the furnace will consume 100,000 BTU of heat which is roughly the equivalent of consuming 100 cubic feet of natural gas.


    Bookmark   December 24, 2010 at 8:02AM
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I watch or have watched many of the home improvement shows so my interpretation is the same as my previous answer. The amount on the rating plate is an average amount that that kind of furnace provides. It may or may not be what your furnace provides. I do wonder if the tag states output rather than input?

They find this by testing a sample of furnaces of that type pulled from the production line and averaging the amounts. Some will be higher some will be lower. Do a search on how products are rated. Almost all products are rated by this method. Most companies will adjust their product line if the average is too high or too low to maintain that rating amount.

You can of course go to a library and read about how ratings are achieved.

There should be some gauges that could be put on the gas line to tell exactly how much is used by the furnace but without one there is no way to tell how much this furnace or any other gas appliance uses. You should not put one of the gauges in yourself. If you do you can only hold yourself liable if your house explodes.

    Bookmark   December 24, 2010 at 10:22PM
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Although the company that made your furnace most likely did not test the actual furnace in your home, they tested one of the same model. The company knows by the gas valve and number of burners how much gas that model will consume. If the Input rating plate says 120,000 BTU/Hr then the furnace consumes close to that per hour.

Do not worry about gauges on the gas line to see how much gas the furnace uses. Heres what I would do:

My gas meter has a 1/2 cubic foot dial and a 2 cubic foot dial along with 4 other dials for meter reads. When the furnace is firing(make sure only the furnace is firing not hot water or oven) I'd watch the gas meter's 2 cubic foot dial for 5 minutes and note how many revolutions it makes in that 5 minutes. Say it makes 4.5 revolutions so the furnace consumed 9 cubic feet of gas in 5 minutes so in one hour the furnace would consume 108 cubic feet of gas(1.08Ccf). If you say your supplier is charging $.70 per CCF then your cost to operate the furnace per hour is $.70 times 1.08 CCF = $.75

Good Luck

    Bookmark   December 25, 2010 at 8:15AM
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My gas meter also has a 2 cubic foot dial. I set a video cam in front of the gas meter in the morning knowing the furnace would be on for at least 20 minutes to bring the house up to temp from the night's setback.

In 5 minutes the funace consumed 7.6 cubic feet of gas.
In 10 minutes the funace consumed 15.2 cubic feet of gas. I also timed how long it took per revolution of the 2 cubic foot dial. That was determined to be 1 minute 19 seconds to 1 minute 20 seconds.

So 15.2 cubic feet of gas in 10 minutes extrapolated out to 60 minutes would be 90.8 cubic feet gas per hour which is a little over 93000 BTUs.

    Bookmark   December 28, 2010 at 6:52AM
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In my last post I Stated:
"So 15.2 cubic feet of gas in 10 minutes extrapolated out to 60 minutes would be 90.8 cubic feet gas per hour which is a little over 93000 BTUs."

15.2 cub feet gas times 6 = 91.2 cub feet gas....not 90.8 cub feet gas as originaly stated.

This comes to about 94,000 BTU.

    Bookmark   December 28, 2010 at 9:14AM
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I'm confused by this post -- the cost/hour to run your furnace isn't going to be relevant at all in the big picture. My gas furnace runs at 6 a.m. to bring the temp up to 20c, then only runs periodically throughout the day. So I'd guess that during a 24 hour period (and depending on the temps outside) it runs for maybe an hour or 1.5 hours. Personally I don't care about the details (which you obviously do!) because what matters is my bill - which, including gas hot water, is $800-$1,000/year, depending how cost the winter was.

    Bookmark   December 29, 2010 at 10:44AM
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I'm confused as to why you are confused.

The purpose of the post was to determine how much natural gas my furnace consumes per hour and what the cost is.

I ultimately determined the furnace consumes aboout 92 cubic feet of natural gas per hour. My natural gas supplier charges about $.70 for one hundred cubic foot of gas.

So it cost me about $.65 per hour in natural gas for the furnace to run.

    Bookmark   December 29, 2010 at 9:51PM
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I have lots of questions. Most furnaces will not run at the same level for an hour. If my furnace, except for the coldest days, ran for a solid hour I would be having the tech come and check what was stuck. Most furnaces come on burn until the designated temp is reached and turn off. They then cycle back on when the temp drops. If necessary this can happen several times an hour if your house is drafty. Was your furnace blasting the entire time. It sounds more like the amount of gas used was an average for that time and not the amount of gas that the furnace would use if it was going the complete time.

My furnace came with the option of having a pilot light or not pilot light. You don't state which yours has. The pilot light on it and any other appliance with a lit pilot light will use a small amount of gas continuously. Because of the pilot lights is why I mentioned the gauge in the line being the only way to check rather than watch the meter. Without setting the thermostat to the highest setting so that your furnace would run for that hour what did you do to be certain it was running at it's max.

What other gas appliances do you have? If furnace is your only gas appliance then the meter would be your gauge but you would still have to run your furnace at full blast for an hour to figure the cost for an hour.

I am all most certain that Ottawavalley could add additional things that are confusing.

    Bookmark   December 30, 2010 at 10:19PM
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maifleur: The point of my post was not to run the furnace for a hour which I could of easily done if I wanted to feel like I was in the tropics. The point of my post was to figure out how much it costs for natural gas for the furnace to run for an hour. To do that I first had to calcualte of my gas the furnace consumed per hour.

I left out anything that was not pertinent. Hence the absence of other gas consuming products running at the time of my reading like pilots, ovens, water tanks, lights.

FYI the furnace does not have a pilot. It has a hot surface ignitor for ignition. I didn't add that in my original post because I knew there was no gas consumption c/w a pilot. Also the furnace is single stage.

As far as your other gas consuming appliance question, at the time of my readings I know there was no other gas consuming product firing at the time.

Also I think a furnace would consume the same amount of gas whether it fired for 60 minutes straight OR sixty one-minute cycles.

Thanks for your critique though.

I never stated that my furnace ran for an hour straight. I took my readings in the morning after raising the thermostat from the night's setback. The furnace ran for 20 minutes in the morning to satisfy the thermostat setting. Per my data, the furnace consumed 7.6 cubic feet of gas every 5 minutes for that 20 minutes.

So I took 10 minutes gas consumption(15.2) multiplied by 6 to get an hours worth(91.2 cub feet).

Or I could of taken 5 minutes gas consumption(7.6) and multiplied bu 12 which is also 91.2 cub feet.

Or I could of taken the gas consumed in the 20 minutes(30.4) and multiplied that by 3 which is also 91.2 cub feet.

I can confidently say that if I were to run the furnace for an hour straight, the gas consumed would of been in the ballpark of 91.2 cub feet. WHich was the point of my post. To see how much gas was consumed in an hour and cost.

    Bookmark   December 31, 2010 at 8:36AM
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I am fascinated by your approach to collecting this information, especially videotaping the gas meter.

May I ask, what are you going to do this information?

    Bookmark   January 1, 2011 at 9:22PM
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jaysgarden, I should have been more clear. While I admire your scientific approach, I am curious about how you'll use the info. Determining the cost to run your furnace for an hour won't help you figure how much it will cost to heat your home for a year. That is dependant on the weather, how insulated your home is, and what temp you keep the house at.

Also, I see from my own gas bill that the Gas Supply Charge for a month is $35.19... but there's a Customer Charge of $18, Delivery to you $19.17, Transportation to Enbridge $12.86, and HST $9.74.

My gas bill gives much more info than I'd ever use -- Gas used this period (m3), billing period days, Total effective gas supply rate (=13.7818cents/m3). Couldn't you use your gas bill to capture the actual data more easily?

    Bookmark   January 2, 2011 at 7:15AM
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I'm not going to do anything breakthru with my findings. I was just curious to see how much gas the furnace consumed during an hour's run time and then the cost per hour.I understand the cost is dependant on the price of gas .

I did make a misstake when I noted above in the first posting that my furnace is 120,000 BTU. The furnace is 100,000 BTU. At first I was satisfied with taking the BTU input rating and doing my calculations from that.

But maifleur made a good point that the BTU rating may not be an actual value. So thats when I needed to determine the amount of gas my furnace is actually consuming per hour. It ended up coming out to be about 94,000 BTU.

ottawavalley...I understand that outside temp, thermostat setting and how well my home is insulated are factors as to HOW OFTEN the furnace will run. That wasn't my concern. I only wanted to know how much gas was consumed when the furnace ran for an hour. How that hour was achieved was not important. It could of been 60 continuous minutes or six 10-minutes cycles.

My gas bill gives total units used in hundred cubic feet.
As far as just looking at the bill for consumption, since I have a gas oven/cooktop, gas hot water tank and the gas furnace, just looking at the bill would not tell me how much gas the furnace used. Also my supplier reads the gas meter every other month. The months in between they estimate usage.

Sounds like on your gas bill everyone gets a piece of the pie with all the different charges. My bill is broken down to a charge for gas consumption, a customer service charge of like $21 a month and of course the tax on the consumption charge. So it's pretty straightforward how much the gas costs per hundred cubic feet. This month gas was $.745 per hundred cubic feet.

I've hooked up the following 24V hour meter to the gas valve:

The meter progresses in tenths of an hour increments anytime the gas valve is open. I can tell how long the furnace runs each day, month or season. This helps with filter changes also. I've found that about every 125 hours the filter is dirty enough to be replaced.

I appreciate the comments.

    Bookmark   January 2, 2011 at 8:36PM
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Here is a relatively easy way to figure out how much it costs to heat your home with natural gas.

Find your utility bill. Locate the total CCF that you have used for the year. (1044ccf)

Calculate the average cost of gas per CCF.
For the current month, add up All the charges(some are not provided by ccf, but include everything) billed
( $82.05), then divide by the ccf used for the month. This gave me a cost per ccf of $1.05

Multiply these together to get your annual cost of heating your home ($1,098)

Dont forget this part distribution within your building. This is usually hot air, hot water, or steam. Mine is hot air. No jokes please

I need electricity to run the blower fan. If this were the only electrical thing you used it would be easy, so I used an approximation. For the same months the gas was used, I found that the extra electrical useage totaled 4000 KWH (kilowatthours). The cost per kwh is $.18; so the electrical expense comes to $720.

Bottom line
Gas $1,098
Electric $ 720

Total $1,818 per year

Advanced calculation:
Calculate how much time you use the heating. Go to a website calculating Heating Degree Days, . Search for a location near your home.
Mine is 6,000 heating degree days for the year(based on a 65 degree base).
Divide your annual cost by the HDD. This gives me $.303 per HDD
This can be used for daily or monthly estimates. When your average temperature is 35 degrees, your HDD is 30 (65base minus 35degrees). Thus your heating expense for that day is $9.09 ($.303 times 30 HDD)

    Bookmark   December 18, 2011 at 2:29PM
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