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Old 09-15-2010, 04:29 PM   #1
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Gas furnance over-firing, how do I correct?


Got a new furnace, and my checking shows its over-firing by about 25%.
Called Northwest Natural Gas. I'm told the energy content is ~1020-1040 BTU/ft^3

The furnace is rated 60,000 BTU input, and there is a section about checking furnace input and there's a chart that shows at 1040BTU/ft^3, it should consume a cube in 63 seconds.
It's consuming it in 50 seconds, so its seriously over-firing.

The installer just hooked it up, didn't do anything with the gas valve.

Does this need to be corrected? Is this within the range that's correctable using the pressure adjustment?

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Last edited by HVAC_NW; 09-15-2010 at 07:59 PM.
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Old 09-15-2010, 06:22 PM   #2
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Gas furnance over-firing, how do I correct?


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Originally Posted by HVAC_NW View Post
Got a new furnace, and my checking shows its over-firing by about 25%.

The manual states, at 1040BTU/cu.ft, it should consume a cube in 63 seconds. It's consuming it in 50 seconds, so its seriously over-firing.

The installer just hooked it up, didn't do anything with the gas valve.


If all the installer did was to '' hook it up '' you need a new installer. How did you check it to know it's over fired ??

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Last edited by kenmac; 09-15-2010 at 06:36 PM.
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Old 09-15-2010, 06:24 PM   #3
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Gas furnance over-firing, how do I correct?


What is the gas companies BTU per cubic foot value?

Are you sure its not 950 BTUs per CF of gas.

Yes, that is correctable with a gas pressure adjustment, and a temp rise, and CFM check.
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Old 09-15-2010, 07:32 PM   #4
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Gas furnance over-firing, how do I correct?


I know the energy content of gas, because, I called the gas company. They said it varies from month to month, but its in the range of 1.01 to 1.06 in the last year or so.

Last edited by HVAC_NW; 09-16-2010 at 09:11 PM.
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Old 09-15-2010, 07:43 PM   #5
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Gas furnance over-firing, how do I correct?


is this the formula you used ?? 3600 x the dial size (1/2, 2ft, etc,) divided by seconds per revolution .add 10% more for 2# system. Don't know if you have 2 # gas systems there or not
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Old 09-15-2010, 07:45 PM   #6
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Gas furnance over-firing, how do I correct?


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is this the formula you used ?? 3600 x the dial size (1/2, 2ft, etc,) divided by seconds per revolution .add 10% more for 2# system. Don't know if you have 2 # gas systems there or not
Once you know the energy content of the gas, and you measure the cubic feet going into furnace, that alone is enough to calculate the BTU input.

Northwest natural gas says user end of gas is at 6.5" W.C.
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Old 09-15-2010, 08:03 PM   #7
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Gas furnance over-firing, how do I correct?


Did you repeat the test at least 3 times? Gas meter dials sometimes stick and jump a bit so getting it down to a second or 2 is difficult. Let it go round 2X and divide the time by 2 for more accuracy. The installer should check the manifold pressure. It may be overfired from the factory. Have never seen one need new orifices. You don't live in a high altitude area do you? May need de-rating if you do. Check the rating plate for that info.
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Old 09-15-2010, 08:14 PM   #8
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Gas furnance over-firing, how do I correct?


Put your manometer on it, and see if 3.2" gets your were you need to be.
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Old 09-15-2010, 10:28 PM   #9
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Quote:
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Did you repeat the test at least 3 times? Gas meter dials sometimes stick and jump a bit so getting it down to a second or 2 is difficult. Let it go round 2X and divide the time by 2 for more accuracy. The installer should check the manifold pressure. It may be overfired from the factory. Have never seen one need new orifices. You don't live in a high altitude area do you? May need de-rating if you do. Check the rating plate for that info.
Meter moved smoothly. I measured four cubes, divided by four
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Old 09-15-2010, 11:09 PM   #10
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Gas furnance over-firing, how do I correct?


Call the installer back out and have him check both the line pressure and manifold pressure. That should be part of any furnace start up because regulators and gas valves are not always set correctly.
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Old 09-15-2010, 11:15 PM   #11
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Old 09-16-2010, 01:35 AM   #12
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Gas furnance over-firing, how do I correct?


Do you have an agreement on paper that shows a warranty?
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Old 09-16-2010, 03:25 AM   #13
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Old 09-16-2010, 03:32 AM   #14
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Gas furnance over-firing, how do I correct?


You should really consider changing your screen name. Your gonna give the PROs the impression you are a tech and will either ignore or hammer you for asking elementary heating questions.

But tell me about the written agreement. What does it say...leave nothing out. Post it if you can.

You have rights as a consumer and I don't like when nice folk like you might be getting screwed.
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Old 09-16-2010, 09:20 PM   #15
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Gas furnance over-firing, how do I correct?


I got the install tech to come back with a manometer. Manifold pressure was pushing 5" WC. Valve manual says factory default is 3.5" WC. I'm not sure how it was this far off. Clocked with utility meter and it appears to be in spec now. I don't think the upper limit for that valve even goes to 5". I don't know WTF happened here.

It's not quite heating season yet. Glad I caught it before it got used beyond testing.

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