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jaysgarden

Furnace: Gas used per hour / Cost per hour

jaysgarden
13 years ago

I've posted this topic before but has made some corrections and kind of consolidated all data.

I was trying to gauge the cost(for natural gas) when my furnace has run(consumed gas) for an hour's time. I did it two ways.

First I just took the BTU input rating of the furnace and did the calculations from there. Then someone pointed out that the BTU input rating may not be an actual value.

So secondly I needed to determine the amount of gas my furnace is actually consuming per hour.

Here are the particlars:

1. I have Goodman model GMPN100-4 furnace. (natural gas, single stage, pilotless and an input rating of 100,000 BTU per Hour)

FIRST WAY USING BTU INPUT RATING:

1.One cubic foot of natural gas has about 1,030 BTU.

2.Divide the furnace input rating(in my case 100,000) by 1030 to get the number of cubic feet of gas the furnace will use in one hour. So 100,000(BTU) divided by 1030(BTU per Cubic Foot) is about 97 Cub Feet.

3.My supplier's bill is based on units of one hundred cubic feet(CCF) so I divide 97 cubic feet by 100 to determine how many CCF the furnace will burn per

hour. This turns out to be 0.97 CCF

4.My supplier charges $.745 per CCF so it costs me 0.97 times $.745 = $.72 per hour for natural gas for my furnace to run.

SECOND WAY BY DETERMINING ACTUAL GAS CONSUMPTION:

As stated above someone pointed out that the BTU input rating may not be an actual value. So I wanted to determine how much gas the furnace was consuming per hour.

My gas meter 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 temperature from the night's setback. The furnace did run for about 20 minutes but I stopped my readings at 10 minutes. Here are some findings:

In 5 minutes the furnace consumed 7.6 cubic feet of gas.

In 10 minutes the furnace consumed 15.2 cubic feet of gas.

So 15.2 cubic feet of gas in 10 minutes extrapolated out to 60 minutes would be 91.2 cubic feet gas per hour or 0.912 CCF/hr.

At $.745 per CCF the cost is about $.68 per hour.

From the input rating data (100,000 BTU/hr) I calcualted .97 CCF/hr. So the actual gas consumption was a little bit less that the input rating.

Now some Notes:

1. My gas 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 CCF.

2. No other gas consuming appliances(oven, stovetop or hot water heater) were firing at the time of testing.

3. 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. In my case I took a 10 minute run cycle and multiplied that amount of gas usage by 6.

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

http://www.grainger.com/Grainger/REDINGTON-Hour-Meter-2PPV9?Pid=search

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.

Comments (41)

  • joe_mn
    13 years ago
    last modified: 9 years ago

    are you bored? read gas bill. amount of therms used per month. do the math.

  • jaysgarden
    Original Author
    13 years ago
    last modified: 9 years ago

    joe I kind of have been bored last month. Thanks for wondering but you are wrong in your conclusion. The purpose of my post was for how much gas the furnace uses per hour not how much gas the furnace uses per month.

    Tell me how to look at total usage on the gas bill and figure out HOW MUCH GAS THE FURNACE USES PER HOUR.

    Do you have each gas appliance separatley metered so you receive a bill for each appliance. If you do then looking at the bill would tell you how much you used in that billing cycle.

    I only have one gas meter at my domicile so I only receive one bill for all gas used in a billing cycle which is usually 30 or so days. Since I have a gas consuming furnace, gas consuming hot water heater, gas consuming dryer, gas consuming stovetop, gas consuming oven and two gas consuming infrared radiant heaters, theres no way I can look at the usage on my bill at determine how much gas MY FURNACE uses in a hour.

    Therefore I did what I did becuase I was kind of bored and posted it for others to ponder. Didnt think someone would suggest looking at the total usage on the bill and trying to figure out how much an appliance used in an hour.

    Thanks for the suggestion. I'll consider it knowing it will not work for what I want done.

  • brickeyee
    13 years ago
    last modified: 9 years ago

    Why do you even care?

    If the bill is high turn the thermostat down.

  • jaysgarden
    Original Author
    13 years ago
    last modified: 9 years ago

    hey brick...you missed the point of my post. Did you somehow draw the conclusion that my gas bill was high? If so why? I never stated I had a high gas bill.

    All I wanted to know was how much gas the furnace consumed per hour. And it turn I saw how that figure related to the BTU input rating of the furnace.

    Thanks for reading though.

  • brickeyee
    13 years ago
    last modified: 9 years ago

    "All I wanted to know was how much gas the furnace consumed per hour. And it turn I saw how that figure related to the BTU input rating of the furnace."

    The value on the nameplate is likely to be as accurate as anything you can try to calculate short of putting a demand meter on the furnace.

  • jaysgarden
    Original Author
    13 years ago
    last modified: 9 years ago

    I found that the value on the rating plate was off by 7%

    No need for a specific demand meter just for the furnace when you can isolate the furnace's need and read the main gas meter. This is what I did.

  • brickeyee
    13 years ago
    last modified: 9 years ago

    "I found that the value on the rating plate was off by 7% "

    So what?

    It is a demand driven system.

    All that tells you is that the design point is not going to be met.

    If the system was designed to operate to 15 F it will not hold the design point at 15 F and the temperature in the house will fall.

    The errors in the heat loss from a structure (especially on older structure) swamp a 7% error in the heater capacity.

    I still fail to see the purpose of all this.

  • jaysgarden
    Original Author
    13 years ago
    last modified: 9 years ago

    brick

    I'm going to state verbatim what it says on the rating plate and you tell me how you interpret it.

    Heating Input(BTU/Hr): 100,000 Natural Gas, 100,000 LP gas

    Thanks

  • Plenipotentiary
    12 years ago
    last modified: 9 years ago

    I rarely participate in any internet forum. The very low standard of thought and courtesy exemplified in the responses to your post are not something I would participate in by choice. Though, thankfully, here, the hostility here is limited to the making of dismissive comments.

    I registered for this forum just to make sure that you know that, for each of the comments propelled by some dark little personal need, there are probably hundreds who don't comment for reasons similar to mine. Just look at the Google rank for this page.

    I found your post useful in the extreme. Preliminary to installing a solar pool heating system, for a meaningful financial analysis, I need to know how much of our gas bill derives from the gas pool heater. Specifically, how does the nameplate value correspond to actual use? Perhaps it is only a nominal or comparative value like the EnergyGuide tags on home appliances and equally useless for predicting actual consumption.

    Your result and a couple of others converge on Actual Consumption=95% Rated Peak Consumption. Knowing this has allowed me to proceed with design and decision without doing the test myself, which would be unusually difficult due to some peculiarities of our installation and family dynamics.

    I am amazed by the generosity of people who experiment and publish their results in fora like this, or who put together instructional videos and post on YouTube without any attempt to profit from it.

    Thank you.

  • mike_home
    12 years ago
    last modified: 9 years ago

    Jay,

    I would not consider a difference of 7% between the input rating and the amount measured on the meter to be significant. Here are my thoughts as to explain the discrepancy:

    1. Furnace input ratings are usually nice round numbers. Perhaps Goodman has rounded up the acutal number measured under test conditions. A measured value greater than 95,xxx BTU could be rounded up to 100,000.

    2. Natural gas is a mixutre of several gases. The meter measures gas flow, but who knows what the gas company is sending you on any particular day.

    3. The gas pressure may be lower than the optimum setting or the gas valve itself may need an adjustment for the air and gas mixture.

    4. The gas meter itself may have a reading error.

    You can see if each of these items contributed a few percentage points that it can easily add up to the 7% you calculated.

  • jaysgarden
    Original Author
    12 years ago
    last modified: 9 years ago

    Plenipotentiary thanks for your response. I did notice that all of the responses preceding yours were of the nature of WHO CARES and ARE YOU BORED and SO WHAT.

    I tried to play along with those comments with some tactful responses.

    By the Google rank of the page do you mean that it shows up first when you use the proper search terms?

    Thanks

  • jaysgarden
    Original Author
    12 years ago
    last modified: 9 years ago

    @ mike_home:

    Your points 1,3, and 4 are valid reasons for the difference. Good points.

  • brickeyee
    12 years ago
    last modified: 9 years ago

    You still have not bothered to tell anyone why you are so concerned with per hour usage.

    Or do you simply enjoy wild goose chases?

  • jaysgarden
    Original Author
    12 years ago
    last modified: 9 years ago

    @brickeye: did not know your approval of an explanation was needed.

    Maybe it was for a science fair.
    Maybe it was for a design decision.
    Maybe I'm raising wild geese and want to keep them warm in the barn and need a cost analysis.

    For whatever reason, just as you can't understand why I posted this, I cannot understand why you continue to read/post on a topic you have stated has no purpose?

    Good day.

  • brickeyee
    12 years ago
    last modified: 9 years ago

    Then we will now to ignore your questions as just idle speculation with no useful end point.

  • jaysgarden
    Original Author
    12 years ago
    last modified: 9 years ago

    brickeye that is your opinion which you are entitled to. By the way why are you so concerned with what I do with my idle time?

  • siupp
    11 years ago
    last modified: 9 years ago

    Very informative and useful post. Perfect for determining whether I should use multiple electric space heaters (wattage consumption calculated with help of Kill-A-Watt) or simply turn on the gas furnace and heat up the whole house. Thanks Jay.

    And yes this page ranks up top in my google search result.

    Why people are so ignorant to say that this post serves no purpose is completely beyond me.

  • SaltiDawg
    11 years ago
    last modified: 9 years ago

    "Why people are so ignorant to say that this post serves no purpose is completely beyond me."

    So you register as a new forum "member" to say this?

    Thanks for contributing. lol

  • ScarletGray
    11 years ago
    last modified: 9 years ago

    saltidawg say:
    "So you register as a new forum "member" to say this?"

    hey what about the 52 other words siupp said? Dont they have any meaning or are worthy of a register?

  • scottsteele66
    10 years ago
    last modified: 9 years ago

    Hi Jay,

    how long did it take the meter test dials to make one revolution when the furnace alone was in operation with the burner on? .5ft dial? 1ft Dial? 2ft Dial?

    Thanks, I missed this question on a test and would like the formula for finding out such info?

    Scott

  • jaysgarden
    Original Author
    10 years ago
    last modified: 9 years ago

    scottsteele66: I dont know how my gas consumption was on a test of yours but here is my data.

    The 0.5 cubic foot dial took about 20 seconds to make one revolution whereas the 2 cubic foot dial took about 80 seconds.

    You should of been able to figure this out from the data in the first post which was :"In 5 minutes the furnace consumed 7.6 cubic feet of gas."

    With that being known, there were about 3.8 revolutions of the 2 cubic foot dial (7.6 / 2) and about 15 revolutions of the 0.5 cubic foot dial (7.6 / 0.5).

    So take the 5 minutes, which is 300 seconds and divide 300 by 3.8 revolutions of the 2 cubic foot dial and that gives you about 80 seconds per revolution of the 2 cubic foot dial.

    Similarly take the 300 seconds and divide it by 15 revolutions of the 0.5 cubic foot dial and that gives you 20 seconds per revolution.

  • jonnyp
    10 years ago
    last modified: 9 years ago

    So what's the deal, you're still at it 2 yrs later. I stumbled across this post and I am curious. Why do you need specific consumption of your gas heating appliance ?

  • jaysgarden
    Original Author
    10 years ago
    last modified: 9 years ago

    I only post something when someone else comments or asks a question.

    As for your question, I just wanted to know how much I was paying for natural gas each hour the furnace would run.

  • ScarletGray
    10 years ago
    last modified: 9 years ago

    I wonder why it matters to some people why the poster did or needed to do this. Regardless of why, the findings were legitimate.

  • Dot Neveragain
    8 years ago

    Hello, very probably this post is no longer being monitored, but I would still like to ask a question, in hope some knowledgeable person might see it (it did come in at the top of my hits to a query about gas usage). My question was, what formula would I use to see how much it would cost to raise the communal furnace in our condo by two degrees. We have an ongoing struggle about it without any data enlightening the dialogue. Our senior citizens are bitterly cold with the setting at 68, but our board president says we must save money, etc. without knowing how much money we are actually saving. I realize one would have to factor in the temperature and I imagine the size of the building, not to mention the cost of gas. I would not need an exact figure, though, just a hint. Could one just divide the total amount of one's gas bill by say the temperature of 68, get a figure of X amount of heat per degree, then multiply that by say 70, a better temperature for senior citizens, to see how much more gas would be used (from there it's easy to get a cost)?

  • mike_home
    8 years ago

    Dot,

    I would like to respond to your question, but we should not hijack this old thread. Can you start a new posting and I will respond. You will also be more likely to get responses from others.

  • Dot Neveragain
    8 years ago

    Thanks, but I can't tell where to start a new thread!

  • tigerdunes
    8 years ago

    Quite easy. Top of main page.

    link attached...

    Garden Web HVAC Home Page

  • t z
    8 years ago

    This is a great post even years later. Anyone who understands energy consumption knows getting everything into a unit cost is critical when finding energy usage savings. For a furnace cost/hour is perfect as your thermostat can tell you how many hours it runs per day. To me this seems obvious but lots of people replying to this don't seem to get it for some reason. Thanks OP!

  • parkcityric
    7 years ago

    I know this is a really old post but thanks for doing it. I have a similar furnace (Goodman 110k btu) and a nest thermostat that gives a history in how long the furnace ran in a 24 hour period. I was looking for a way to easily estimate how much it cost in gas to run per hour. (Or what I might have saved by trimming down on the run time) useful for me!

  • mike_home
    7 years ago

    Your furnace burns 110K BTU per hour so that is equal to 1.1 therms per hour. Look at your gas bill and find you how much a therm costs. Multiply the cost by 1.1 and you will have your hourly cost.

  • PRO
    Austin Air Companie
    7 years ago

    Please stop it... that teeth chattering noise is driving me nuts. (It's supposed to be in the 80's today --- Katy, Tx)

    ~ALARMING PERSPIRATION BULLETIN~

    I've been running the AC for at least a few days now.

  • Manohar Srikanth
    4 years ago

    Ok, I landed on this thread because I could not find any other discussion for what seems to be a very important topic and should have been more common on the internet.


    I ran into this problem because I have been secretly wondering if the gas company is ripping me off with fake charges or if there is any leak. I recently changed the gas furnace which was 40 year old, with a newer one that is variable capacity. I was told the old one could be 50% efficient because it got old, and the new one we changed is grater than 95% efficient. So, I was hoping to see a dramatic reduction in gas usage during winter months.


    Come winter, I see that I am using about the same average therms per day (in comparison to the old furnace last year). This was shocking because I was hoping yo see a dramatic reduction.


    Then I did some googling and landed on this page. Fortunately, my thermostat provides performance report that show how long it ran the furnace each day, and also tells at what capacity it ran (for instance it says 20% capacity, which means of the 85000BTUs it only used 20% gas demand, this is an advanced feature of the variable capacity furnace). If I account for run time and % capacity, I arrive at very low therms usage per day. When I look at the Gas provider website which also gives therms used each day, there is a dramatic difference. I see 2 to 4X more therms used per day than what my calculation shows.


    I should probably buy a flow meter and have it installed by a professional to track down the issue.

  • armoured
    4 years ago

    @Manohar, this may seem a stupid question but to rule out any confusion - do you have access to the actual gas meter to check the numbers? Do you know if your utility is using actual meter readings or estimated usage?

    Many utilities will use an average figure - sometimes smoothed over the entire year, i.e. they average consumption so that monthly bill is the same even in summer - and then correct with actual usage readings periodically (possibly even only once a year).

    The upshot being that if so, they have just continued to bill you based on previous usage and may not reflect your current consumption at all.

    If you are sure they are using actual readings based on current consumption, the answers to your questions will obviously be different.

  • Manohar Srikanth
    4 years ago

    @armoured that is a good point, I loosely assumed that meter reading and online reporting are in strong alignment (updated almost daily basis). I do have access to the meter, and I plan on recording it for a few days to rule out any confusion. BTW, now that you mention it, it is possible they may be just "predicting/smoothing" the usage rather than taking real meter readings every day. This may be done to avoid too much communication between the meter and the central system, or the meter is simply not capable of transmitting at that rate (I am not familiar with how the data is transmitted). The electricity usage reporting on the other hand show more accurate reflection of the usage pretty much on hourly basis.

  • armoured
    4 years ago

    It very much depends. Automated readings are, I think, far more common in electric utilities than in gas - some gas metres have no data transmission capability at all. So actual physical readings by an employee are not unheard of, and some utilities do annual averaging because customers don't like their bills fluctuating. At any rate, your monthly bill should at minimum indicate the meter reading and cubic feet/meters/therms used. It may also have a statement somewhere noting the basis for the bill - most recent reading, estimated usage, historical usage or some other term, although the specific meaning may not be obvious unless you look at the small print or footnotes.

    At any rate you do not need to monitor daily (can't hurt of course) - you should be able to compare your recent gas bills to the current reading. I've had this happen (I thought the info was being reported to the utility by a family member and instead they were just using estimated usage). It should be immediately obvious - i.e. the utility bill might show 50,000 units on meter on estimated basis and your actual current reading is substantially below that. (Obviously allow for your usage in the current month).

    On the positive side if this is the case you can ring up the utility and ask them to adjust based on numbers you report or ask to have the meter read by an employee, if you've overpaid in previous months you'll have a negative balance and get a month or two with no bills.

    Unfortunately this may make estimating how much the new efficient unit is saving you a bit more difficult.

  • Brian Groh
    2 years ago

    Fantastic work.


    Now that I have a "Nest" thermostat, it keeps track of useage and I can use the work you've done here as a template for estimating the cost of my hourly useage and the overall impact my furnace use has on my monthly winter bills.


    Baffled by the nay-sayers. It's like saying "Why bother figuring out how your car works? Just drive it." The more we know about how our appliances work, the more efficient we can use and maintain them and get ahead of repairs. But... that's obvious to most of us.





  • 24SEVEN 365
    4 months ago
    last modified: 4 months ago

    Cheese and rice! How many times is your furnace going to shut off in that hour? I know, I know you just wanted to see what it costs for one hour.

  • greenhousedc
    2 months ago

    @jaysgarden this has been an exemplary post! Thank you so so much for sharing. I have a 250,000 BTU Hayward natural gas pool heater and I run it for 10 hours straight in the winter to get the pool up to temp if I want to swim. So I wanted to know how much it cost per hour to run. I did my math from the other direction (working from gas costs towards cost/BTU) and your math (working from BTU back to gas costs) helped me validated my calculations, which I found hard to believe at $2.30/hour to run.


    Calculations based on Michigan Consumers Energy Costs per Mcf in December 2023:


    $5.2191 Gas Distribution

    $0.4164 Other Surcharges

    $3.492 Gas Cost Recovery

    $9.1275 Total Natural Gas Costs


    $0.365513434 Tax

    $9.493013434 Total Cost per Mcf


    1 Mcf = 1,039,000 BTU

    0.000000962464 Mcf/BTU (1 / 1,039,000)

    $ 0.00000913668 Cost per BTU ($9.493013434 * 0.000000962464)


    For a 250,000 BTU heater:

    $2.28 / hour to run (250,000 * $0.00000913668)



    Feel free to check my math :)

  • sktn77a
    2 months ago

    I divided my month's gas usage (105 therms) by my monthly bill ($150) to get my actual gas rate - inclusive of all the miscellaneous charges and taxes - and got ~$3.50 an hour for my 250,000 BTU gas pool heater.


    Sigh.... I remember when it only cost a buck an hour to heat the pool!

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