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Old 09-24-2012, 01:17 PM   #1
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natural gas heating value


Looking to clock my furnace against my gas meter for an op check I got curious about a number I saw on my most recent gas utility bill. The bill lists the conversion factor as 1.124... and interestingly enough if I count and time the meter rotations and apply the 1.124 conversion factor the BTUH inputs for my high and low fire stages are right on the money. But from an EPA document I found this: "The average gross heating value of natural gas is approximately 1,020 British thermal units per standard cubic foot (Btu/scf), usually varying from 950 to 1,050 Btu/scf." So how does my utility "juice up" the gas here to be a ~10% higher than average?

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Old 09-24-2012, 05:08 PM   #2
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Not adding as many other gasses to it as most do. On real cold winters, many will add butane to keep the pressure up.

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Old 09-24-2012, 05:16 PM   #3
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I thought it was the other way around beenthere. Natural gas 1000 BTU and when they juice it up the the BTU content goes up. Maybe I was told wrong.
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Old 09-24-2012, 05:30 PM   #4
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I thought it was the other way around beenthere. Natural gas 1000 BTU and when they juice it up the the BTU content goes up. Maybe I was told wrong.
No, your right, my head was somewhere else when I typed butane.
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Old 09-24-2012, 05:39 PM   #5
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Interesting... that makes some sense since the max plain natural gas according to the blurb above is 1.05.

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No, your right, my head was somewhere else when I typed butane.
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Old 09-24-2012, 07:36 PM   #6
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The conversion factor is what your gas utility uses to convert your meter reading to a billable unit (gigajoules) and takes into account temperature, pressure and the calorific value of the gas. if you want to know the c.v. of your gas, call the utility and ask. I suspect that you either didn't do the formula right, or your equipment is underfired. Also, if you have a 2psi delivery, you need to use a correction factor, otherwise you will end up with a lower than actual number. Can you post the formula that you used here?
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Old 09-24-2012, 07:39 PM   #7
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Beenthere and Marty, FYI, what you're referring to is called "peak shaving"
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Old 09-24-2012, 08:10 PM   #8
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You may want a peak at the attached specifically page 24

http://www.metroservicesinc.com/Comb...ng%20Guide.pdf

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Old 09-25-2012, 06:21 AM   #9
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Furnace is a York 2 stage 80/52 K BTUH input var speed 96% efficient unit. Yes, my house has a 2 psi system meaning the meter supplies 2 psi to the main distribution manifold in the house that then supplies all the loads at 9" wc (I measured this recently for another job I was doing).

I am not using any formula. Just timing the cfm dials on the meter and reading the numbers in the furnace manual table... the result of which the manual says to multiply by the conversion factor for the heating value of the gas to get the input rate. If I use the typical or average NG value of 1.02 I get input rates of about 71/46. If you multiply it by the stated heating value conversion factor in my gas bill of 1.124 it comes out to 80/52.

In a few weeks when it cools down enough here to run the furnace for awhile I'll do a full work up with temp rises, static pressures, and check the first and second stage output gas pressure settings.
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Old 09-25-2012, 06:32 AM   #10
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even if you come up with the proper answer to this problem ..you won't get nothing from the gas company.....we all get shaved...thats what they do..here in pa we get a delivery charge as much as the useage....if you want to beat them....BURN WOOD...
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Old 09-25-2012, 07:01 AM   #11
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Yes, I figure I am getting screwed somehow, but mostly I just want to make sure the furnace is dialed in. The manual gives specs for high and low fire gas valve regulator output (3.5" and 1.5" WC respectively). Not sure if those are intended to be absolute maxes or can they be exceeded if need be to get to the rated input BTUH? But really need an accurate heating value number in the first place....
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Old 09-25-2012, 09:52 AM   #12
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The gas company will give you the cv of the gas, they aren't trying to screw you. Peak shaving doesn't mean giving you less cv. They have to provide consistent delivery, otherwise everyone would have firing rate issues and that would be a potentially dangerous situation. If one is installing gas appliances, and they've never called the gas company for the cv, how are the properly commissioning those appliances? Answer: they aren't, and should have their ticket pulled.

The clocking formula is as follows:

Number of seconds in an hour X test dial size X cv X correction factor / seconds

The correction factor is to account for elevated pressure going through the meter and is used for 2psi and above. Provided you are at or near sea level, the number would be 1.136.

When using this formula, you must ensure that the test dial size and the cv are both in either metric or imperial.

Use the largest or smoothest running test dial and time the dial multiple times and use the average.

You probably did clock correctly, since your numbers came out in the right ball park, your formula is just different. It doesn't give you the cv of your gas, though.

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Old 09-25-2012, 10:11 AM   #13
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Thanks, Benny, I'll call them for the CV and run the math again. Understand about how to do the timing. Of course both my dials (meter has 1/2cf and 2 cf) are a bit sticky at some point in their rotation. I take a few measurements on the 2 cf dial and average them out. Can also cross check by extrapolating with the 1/2 cf dial (to the 1 cf or 2 cf chart) but that introduces multiplication errors.

I was wondering about how consistent the supplied heating value would be maintained. And wondering how tolerant is typical gas fired equipment to variations before there would potentially be safety issues?? 5%... 10%? Just curious....
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Old 09-25-2012, 10:19 AM   #14
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Heres a good explanation of the conversion for all variables, pressure, temperature and elevation.

www.peconet.com/products/RootsBulletins/rm-135.pdf


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Old 09-25-2012, 10:34 AM   #15
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Quote:
Originally Posted by raylo32
I was wondering about how consistent the supplied heating value would be maintained. And wondering how tolerant is typical gas fired equipment to variations before there would potentially be safety issues?? 5%... 10%? Just curious....
There shouldn't be much variation in the cv, 10% would be over 100 btuh and that is huge. Ask your utility for how much variation there is; my supplier doesn't vary much at all. When setting up an appliance, it can't be over fired, but can be under fired by up to 10%. Most techs I know who do their due diligence and actually commission an appliance, usually set them at 5% under. But, like I said, the cv should not vary drastically.

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