# Gas furnance over-firing, how do I correct?



## HVAC_NW (Oct 15, 2007)

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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## kenmac (Feb 26, 2009)

HVAC_NW said:


> 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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## beenthere (Oct 11, 2008)

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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## HVAC_NW (Oct 15, 2007)

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.


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## kenmac (Feb 26, 2009)

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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## HVAC_NW (Oct 15, 2007)

kenmac said:


> 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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## yuri (Nov 29, 2008)

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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## beenthere (Oct 11, 2008)

Put your manometer on it, and see if 3.2" gets your were you need to be.


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## HVAC_NW (Oct 15, 2007)

yuri said:


> 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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## Marty S. (Oct 31, 2009)

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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## HVAC_NW (Oct 15, 2007)

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## hvaclover (Oct 2, 2008)

Do you have an agreement on paper that shows a warranty?


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## HVAC_NW (Oct 15, 2007)

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## hvaclover (Oct 2, 2008)

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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## HVAC_NW (Oct 15, 2007)

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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## yuri (Nov 29, 2008)

I would check it once a week for the first 2 months. That valve may still be faulty and wander out of range. 4" is the max I have seen and it may lose its adjustment on its own. I would be uncomfortable with what it is doing.


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## HVAC_NW (Oct 15, 2007)

yuri said:


> I would check it once a week for the first 2 months. That valve may still be faulty and wander out of range. 4" is the max I have seen and it may lose its adjustment on its own. I would be uncomfortable with what it is doing.


I don't mess with gas stuff, but meter clocking method is good enough, right?


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## hvaclover (Oct 2, 2008)

HVAC_NW said:


> I don't mess with gas stuff, but meter clocking method is good enough, right?


You should out of respect for our industry and the dignity of the PROs here drop the hvac from you screen name.

_Hvac_ in your name misleads other diy posters in thinking you are a PRO. And yet they read this thread where you did not install your own furnace and didn't have the right tools to check the gas press. You make us look bad.

If you don't mess with gas you are no HVACer.

my 2 cents
clover


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## beenthere (Oct 11, 2008)

HVAC_NW said:


> I don't mess with gas stuff, but meter clocking method is good enough, right?


If it was good enough. We wouldn't have combustion analyzers. Or check temp rise across heat exchangers.


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## HVAC_NW (Oct 15, 2007)

hvaclover said:


> You should out of respect for our industry and the dignity of the PROs here drop the hvac from you screen name.
> 
> _Hvac_ in your name misleads other diy posters in thinking you are a PRO. And yet they read this thread where you did not install your own furnace and didn't have the right tools to check the gas press. You make us look bad.
> 
> ...


My registration date precedes yours by an entire year,son. It's what I happened to pick in 2007. I haven't logged in a while. I added a user title to clear any confusion and this is a *DIY forum*. You are not the one to tell me what my user name should be.


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## beenthere (Oct 11, 2008)

HVAC_NW said:


> It's what I happened to pick in 2007.



Your screen name, while confusing. Doesn't violate any for um rule I know of.


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## yuri (Nov 29, 2008)

Meter clocking is okay but I would google U-tube manometer. Can make one yourself. Still best to get a Pro to check it but I am concerned you monitor it for awhile for safety. Working with gas is DANGEROUS and not recommended for DIYers but we can't stop people from doing it. I am sure electricians and car mechanics feel the same way about their trade.:wink:

In reality the manifold gas pressure will vary/modulate(+- .5"WC) on a high efficiency furnace. Vacuum thru the burner box/heat exchanger varies with outdoor temp and venting setup and sucks on the gas valve to alter the firing rate. Special procedure required and experience to know how to workaround and with that.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Your screen name, while confusing. Doesn't violate any for um rule I know of.


It still does not make it right, but I won't make an issue of it.
You are right about making it confusing, though.


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## kenmac (Feb 26, 2009)

Don't forget SON . He was here a year b-4 you:laughing:


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## hvaclover (Oct 2, 2008)

kenmac said:


> Don't forget SON . He was here b-4 you:laughing:


Yeah your right he was here before me..

But I can't be his SON!:no: My Daddy wasn't afraid of gas and he knew how to repair.

Another reason he can't be my Daddy is because my Daddy would never have to ask for help on this site to make a repair.


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## HVAC_NW (Oct 15, 2007)

Anyways, dead-on calibration is impossible, because heating value changes from time to time due to variation in fuel composition, but here is the outcome. All measurements were taken based on time it takes to go through four cubic feet of nat. gas, then divided by four to improve accuracy. 

Furnace rated input: 60,000BTU/hr 
Before adjustment: 74,400BTU/hr
After adjustment: 58,400BTU/hr 

Am I correct to assume that a few percent under fire is better than 25% over-fire? 

There is no other gas appliances here, so it was the only thing burning gas. I don't see why energy content of fuel * rate of flow isn't enough to calculate BTU input. 

Yuri: what's odd...
(I called service for this, didn't do it myself) 
The valve pressure adjustment was not responding at all, for a while. It had to be unscrewed a lot before it became responsive at all, but at that point, it was responding perfectly. I'll check again when heating season comes in, but I'm not messing with gas line. I'm going to clock it, and if its skewed significantly, then I'm going to make a service call. I don't mess with gas pipes, period.


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## hvaclover (Oct 2, 2008)

What will give you an almost perfect burn is a combustion analyzer.


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## yuri (Nov 29, 2008)

Turning the screw in increases the pressure but it sounds like it was bottomed/maxed out by a pizzed off worker. Meter clock it and you should be OK.


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