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Old 02-16-2010, 04:38 PM   #1
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Ok here we go Make: Centurian , Model # GF8100C16MU118, Here's the deal this unit is maybe 3 years old, about 2 months ago I replaced the flame rollout switch and the control valve started malfunctioning a couple of days ago, it was't closing (seating) completely so I replaced it. I also checked the WC pressure, it was a little low 2.5, book called for 3.5 wc , so I bumped it up. Heres the problem Inshot Burners are getting way to hot, I have a pure blue flame with no roll out. But area where the top rollout is getting way way to hot and its tripping the flame rollout switch. The return filter is new, Induction motor is turning , vacuum line is clear, pressure switch working fine, hot surface igniter heating up, gas control opens, and flame shoots across inshot burner as it should, but gets extremley hot. The pan that holds and rollout switch get so hot you can't touch it, any suggestions?

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Old 02-16-2010, 04:44 PM   #2
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What type of manometer did you use to set the gas pressure. Home made? Do you work on gas for a living? Most of the time (99.5%) the valves are set accurately at the factory, not sure if your manometer is accurate and it may now be overfired. Post some pics of the burners etc and we can help better.

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Old 02-16-2010, 05:06 PM   #3
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What type of manometer did you use to set the gas pressure. Home made? Do you work on gas for a living? Most of the time (99.5%) the valves are set accurately at the factory, not sure if your manometer is accurate and it may now be overfired. Post some pics of the burners etc and we can help better.
Robinair manifold pressure test kit model 42160, range o-35 wc. Originally I left control valve at factory setting but problem with over heating so I thought it was possible the flame was lagging back. So I adjusted it to furnace specs. Yes I work on them for a living so dont be afraid of using technical terms. I'm leaning toward cracked heat exchanger but I just can't see that with the unit only being 2 or 3 years old.
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Old 02-16-2010, 05:11 PM   #4
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If there is a shortage of combustion air due to a partially blocked intake pipe you can get enough air/draft to prove the pressure switch but not enough oxygen for a proper flame and the flame temp will be too hot. On sealed burner boxes you have to follow the manafacturers procedure and set the pressure with the vacuum hose to the box from the gas valve disconnected and plugged on some units. That hose causes the valve to modulate/adjust its pressure due to the amount of combustion air/draft thru the exchanger. If you set the pressure with it attached you may get it wrong. Pretty advanced stuff even my apprentices and lots of techs don't understand.
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Old 02-16-2010, 05:20 PM   #5
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If there is a shortage of combustion air due to a partially blocked intake pipe you can get enough air/draft to prove the pressure switch but not enough oxygen for a proper flame and the flame temp will be too hot. On sealed burner boxes you have to follow the manafacturers procedure and set the pressure with the vacuum hose to the box from the gas valve disconnected and plugged on some units. That hose causes the valve to modulate/adjust its pressure due to the amount of combustion air/draft thru the exchanger. If you set the pressure with it attached you may get it wrong. Pretty advanced stuff even my apprentices and lots of techs don't understand.
I shorted you some info, I meant to state that this unit is 80+ and I had the cabinet door open so enough air was present. The manual the came with the furnace called for 3.5 on natural gas. The replacement was set at 2.8.
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Old 02-16-2010, 05:27 PM   #6
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Back in 20 mins. Supper time. Cracked heat ex. Is there flame disturbance when the fan starts?
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Old 02-16-2010, 05:28 PM   #7
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That does give me another idea though. This furnace was a change out, the installer, (Trust me, it wasn't me) put the new furnace in but didn't transition the metal vent pipe correctly, so in perspective it takes on the connection shape of a hot water heater vent pipe, basically the vent pipe off the furnace is just hanging loosely inside of the much bigger pipe exiting the house, perhaps it is stopped up , which would prove pressure switch , but in turn hitting blockage in main pipe causing heat to roll back, Would that be possible?
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Old 02-16-2010, 05:46 PM   #8
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You have a 100,000 BTU input furnace. So it needs 5,000 cubic foot of free air to have enough combustion air. is the area its installed in have that much volume?

Just a wide guess. This furnace is on a 1800 sq ft house.

Yes, that type of flue hook up can cause some of the trouble your having.
Do you have a combustion analyzer, to see how well it is or isn't burning. Using one will tell you if its getting enough combustion air.

look to see if the high limit switch has been changed out to a higher temp then should be in there.

What is the temp rise across the heat exchanger? What CFM is the blower moving? What speed is the blower set to?
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Old 02-16-2010, 05:49 PM   #9
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Did you check for dead bird(s) or bird bits in the ventor fan. With a large chimney they easily get in and fall down. I would meter clock the furnace to make sure the firing rate is proper AND do a temp rise thru it to see if it is overheating. Don't have many York/Colemans where I am mostly Lennox /Carrier. Should have a proper B vent to prevent downdrafting and nuisance press switch tripping.
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Old 02-16-2010, 06:21 PM   #10
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You have a 100,000 BTU input furnace. So it needs 5,000 cubic foot of free air to have enough combustion air. is the area its installed in have that much volume?

Just a wide guess. This furnace is on a 1800 sq ft house.

Yes, that type of flue hook up can cause some of the trouble your having.
Do you have a combustion analyzer, to see how well it is or isn't burning. Using one will tell you if its getting enough combustion air.

look to see if the high limit switch has been changed out to a higher temp then should be in there.

What is the temp rise across the heat exchanger? What CFM is the blower moving? What speed is the blower set to?
I don't know the blower cfm, however board was flashing 4 times, by the code on panel, this meant limit switch was tripping.. I don't have a combustion analyzer. This furnace was working fine until I added the replacement gas control valve. Should I try backing off the pressure lower than 2.8 wc?
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Old 02-16-2010, 06:26 PM   #11
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NO!! Then you will get flashback in the burners from too much air mixed with the gas/bad mixture. If it is cycling on the limit you got airflow and overfiring problems.
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Old 02-16-2010, 06:27 PM   #12
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When you adjusted the gas pressure, you did so because you found the furnace to be overfired by clocking the meter, right? I've found a lot of furnaces to be grossly overfired while still being set at 3.5"w.c.
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Old 02-16-2010, 06:27 PM   #13
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Blower doesn't dissrupt flame. so your right probably wouldn't be heat exchanger.
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Old 02-16-2010, 06:32 PM   #14
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May have the wrong orifices and be overfiring. Meter clock it to find out fer sure. Let me know if you need a formula for that. Usually in the install manual of most new units.
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Old 02-16-2010, 06:33 PM   #15
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Quote:
Originally Posted by Livewire78 View Post
I don't know the blower cfm, however board was flashing 4 times, by the code on panel, this meant limit switch was tripping..

Limit tripping means the furnace is over heating.




I don't have a combustion analyzer. This furnace was working fine until I added the replacement gas control valve.

If it was working fine before you changed the control valve.
Why were you there. And why did you change the valve?
It was not working fine.

Should I try backing off the pressure lower than 2.8 wc?
No. Don't lower the manifold pressure. Causes other headaches.

Clock meter like Yuri said.

The furnace is more likely then not, grossly oversized.
The duct system is undersized.

If you can't get them to replace that furnace with the correct size.
Then make the duct work and supply registers and grille corrections needed to make that furnace safe and usable.

Take the temp rise across the heat exchanger. Suppose that its inputing 100,000BTU and giving you 80,000BTU out.

If its moving 1140CFM, then your temp rise is 65. Which gives you about 5 of safety before exceeding max allowable temp rise.

If your temp rise is above 70, its too high.


Output BTU divided by (temp rise times 1.08) equals CFM.

EG: 80,000/(80X1.08=86.4)=926CFM

The temp rise alone would be too high. But, you would know the CFM its moving. And be able to determine how much more air it needs to be able to move to be back at safe temp rise.




PS: What size house is this furnace in?

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