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Old 02-16-2010, 06:42 PM   #16
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JUST TO CLEAR THIS UP. The reason I changed the control valve is because it was not closing completely. Gas was still seeping thru when furnace was off. Causing major combustion problems due to too much fuel being present. Prior to changing out control valve the furnace was running correctly. The manual on the furnace called for 3.5 WC. So that is what I set it at.

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Old 02-16-2010, 06:45 PM   #17
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I makes the complete cycle, the blower kicks on for about 2 minutes max. then shuts down due to high heat. It did the same thing before I adjusted the control valve.
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Old 02-16-2010, 06:49 PM   #18
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Originally Posted by Livewire78 View Post
JUST TO CLEAR THIS UP. The reason I changed the control valve is because it was not closing completely. Gas was still seeping thru when furnace was off. Causing major combustion problems due to too much fuel being present. Prior to changing out control valve the furnace was running correctly. The manual on the furnace called for 3.5 WC. So that is what I set it at.
Your slightly missing the point.

It wasn't working ok. Or the valve wouldn't have been leaking through.

The excessive heat from the burners is probably what caused the valve seat to no longer seat properly.
Its a cascade effect. Your working on curing the symptoms. Now its time to cure the problem.



Clock the meter as Yuri said. if it comes out to 100CFH, then do the tests/checks that I said. That will lead you to the cure for the problem.


PS: What size house is this in.
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Old 02-16-2010, 06:50 PM   #19
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If it is/was overfired/overheated for a while the heat exchanger may be cracked. If you look at the top or by the burners and see a large black spot they will crack there from overfiring/overheating, especially the ICPs. Does not take long for a heat exchanger to get damaged. Sometimes the only way to know is to yank the exchanger out. Google cracked heat exchanger images.
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Old 02-16-2010, 06:53 PM   #20
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Originally Posted by Livewire78 View Post
I makes the complete cycle, the blower kicks on for about 2 minutes max. then shuts down due to high heat. It did the same thing before I adjusted the control valve.
The blower shouldn't shut down because of high heat.
As long as the limit is open, the blower should stay running to cool the heat exchanger down.

Unless you mean the blower motor is shutting down because its getting too hot.
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Old 02-16-2010, 06:55 PM   #21
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Your slightly missing the point.

It wasn't working ok. Or the valve wouldn't have been leaking through.

The excessive heat from the burners is probably what caused the valve seat to no longer seat properly.
Its a cascade effect. Your working on curing the symptoms. Now its time to cure the problem.



Clock the meter as Yuri said. if it comes out to 100CFH, then do the tests/checks that I said. That will lead you to the cure for the problem.


PS: What size house is this in.
Got what your sayin now. House is bout 1800 sq ft. I'm fixin to sound like an idiot but what do u mean clock the meter?
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Old 02-16-2010, 06:59 PM   #22
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The blower shouldn't shut down because of high heat.
As long as the limit is open, the blower should stay running to cool the heat exchanger down.

Unless you mean the blower motor is shutting down because its getting too hot.
I was just sayin that a little bit after the blower runs the rollout switch opens and kills the flames.
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Old 02-16-2010, 07:00 PM   #23
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Ask and you shall receive:http://tech.lennoxintl.com/PDFs/505254a.pdf
Read page 27
Later:http://tech.lennoxintl.com/PDFs/505065c.pdf
Page 39
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Old 02-16-2010, 07:07 PM   #24
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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?
Do u have a link to this formula? Are talking about Air temp across the exchanger minus house air temp to get your value?
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Old 02-16-2010, 07:08 PM   #25
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Thankyou for the link!
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Old 02-16-2010, 07:11 PM   #26
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I was just sayin that a little bit after the blower runs the rollout switch opens and kills the flames.

usually. if the roll out trips shortly after the blower runs. You got a cracked heat exchanger.

Since you posted that.

Check the heat exchanger before you go any further.
Might be able to see if you pull teh blower and look up, or open the supply plenum and look down. Or you may have to pull it to check it.

After you find the crack/hole(yep, I'm confident its shot). Convince the customer to take the heat exchanger credit. And apply it toward the right sized furnace.

That means you should do a load calc to determine what size they really need.

Do throw any more parts on this thing. Cracked heat exchangers are a serious thing.
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Old 02-16-2010, 07:13 PM   #27
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The exam is tomorrow. LOL
Load Calculator Get the right sized unit in there as well.
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Old 02-16-2010, 07:23 PM   #28
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Do u have a link to this formula? Are talking about Air temp across the exchanger minus house air temp to get your value?

Yes. You measure return air temp at the furnace return, and supply temp in either plenum(not the best place) or in the supply within 3' of the plenum.

Then use the formula I posted.
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Old 02-16-2010, 07:27 PM   #29
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usually. if the roll out trips shortly after the blower runs. You got a cracked heat exchanger.

Since you posted that.

Check the heat exchanger before you go any further.
Might be able to see if you pull teh blower and look up, or open the supply plenum and look down. Or you may have to pull it to check it.

After you find the crack/hole(yep, I'm confident its shot). Convince the customer to take the heat exchanger credit. And apply it toward the right sized furnace.

That means you should do a load calc to determine what size they really need.

Do throw any more parts on this thing. Cracked heat exchangers are a serious thing.
Just curious have you seen a heat exchanger crack when the unit was'nt but a couple of years old? I told the home owner to get with the guy that originaly installed it and see if it was still under warrenty
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Old 02-16-2010, 07:30 PM   #30
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Just curious have you seen a heat exchanger crack when the unit was'nt but a couple of years old? I told the home owner to get with the guy that originaly installed it and see if it was still under warrenty
Yep.

Burned through. others that the crimp rings popped.

You should have taken/written down the serial number.
if your a contrator/service tech. You can get them a warranty credit the same as any company..

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