# Furnace: Gas used per hour / Cost per hour

jaysgardenJanuary 6, 2011

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.

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joe_mn

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

January 6, 2011 at 2:03PM
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jaysgarden

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.

January 6, 2011 at 2:21PM
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brickeyee

Why do you even care?

If the bill is high turn the thermostat down.

January 8, 2011 at 12:37PM
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jaysgarden

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.

January 8, 2011 at 1:34PM
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brickeyee

"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.

January 9, 2011 at 9:36AM
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jaysgarden

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.

January 9, 2011 at 7:16PM
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brickeyee

"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.

January 10, 2011 at 4:36PM
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jaysgarden

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

January 10, 2011 at 5:40PM
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Plenipotentiary

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.

March 14, 2012 at 2:17AM
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mike_home

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.

March 14, 2012 at 9:17AM
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jaysgarden

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

March 15, 2012 at 8:06AM
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jaysgarden

@ mike_home:

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

March 15, 2012 at 8:26AM
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brickeyee

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?

March 15, 2012 at 4:16PM
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jaysgarden

@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.

March 16, 2012 at 9:19AM
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brickeyee

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

March 16, 2012 at 4:59PM
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jaysgarden

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?

March 16, 2012 at 5:10PM
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ScarletGray

It seems that some people have a poor understanding of the written English language. The poster said in his first post,

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

This I would assume is the poster's reasoning. I found the topic interesting to say the least.

March 16, 2012 at 5:52PM
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siupp

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.

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

November 23, 2012 at 6:59PM
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saltidawg

"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

November 23, 2012 at 9:34PM
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ScarletGray

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?

November 24, 2012 at 9:06AM
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scottsteele66

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

December 22, 2013 at 9:36AM
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jaysgarden

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.

December 23, 2013 at 7:34AM
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jonnyp

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 ?

December 26, 2013 at 8:37PM
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jaysgarden

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.

December 27, 2013 at 9:49AM
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ScarletGray

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

December 27, 2013 at 3:25PM
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