92 gas furnace only ignites with exhaust disconnected

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Old 12-20-10, 11:49 AM
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92 gas furnace only ignites with exhaust disconnected

The furnace code indicates it's the pressure switch. However, when I disconnect the exhaust pipe during the start up cycle, the unit will ignite and run fine. When I reconnect it, it shuts down after awhile. There are no obstructions in the vent pipes. Any thoughts?
 
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Old 12-20-10, 03:14 PM
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unit

are you sure that the vent is 100 percent open? it could be a dirty heat excanger,or cracked heat exchanger...the switches do not go bad that often.if you have an intake pipe,make sure it is open
 
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Old 12-20-10, 04:12 PM
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Thanks for your response. Yes, the vent is absolutely clear. The unit is almost new, so I don't think the heat exchanger would be the issue. The intake pipe is also clear.
 
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Old 12-20-10, 04:42 PM
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problem

is this a brand new installation? you must check and follow the manufacturers installation instructions on length of pipe,maximum number of elbows allowed,and actuaal pipe size.....it looks like the vent pipe is too small....too long of a distance,with too many elbows......if you take the exhaust pipe off,it has no restrictions.....if you hook it up and it odes not work.....it is not installed properly.....who installed it?....
 
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Old 12-20-10, 04:45 PM
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check this also

how is the condensate trap installed? is it installed per the mfgs instructions? next time please give more specific info,such as manufacturer,age of unit etc....:NO NO NO:
 
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Old 12-20-10, 05:19 PM
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Originally Posted by seabee570 View Post
.....if you take the exhaust pipe off,it has no restrictions.....if you hook it up and it odes not work.....it is not installed properly.....who installed it?....

Well, not quite. You can have other parts of the furnace plugged up but the furnace may still operate if you remove the inlet and.or outlet air pipes. Having the furnace start working helps confirm that there is a problem, but not necessarily where it is.
 
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Old 12-20-10, 05:23 PM
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i first said that the heat exchanger could be dirty...if it is new...there is not much else...so i did say that other parts could be plugged,he then said it isd a newer furnace so sp you are wrong.....funny how no reply. from rmmag...i think that htis is a do it yourself installation gone wrong
 
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Old 12-20-10, 05:27 PM
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Well, not quite. You can have other parts of the furnace plugged up but the furnace may still operate if you remove the inlet and.or outlet air pipes. Having the furnace start working helps confirm that there is a problem, but not necessarily where it is.

it could be a dirty heat exchanger,or cracked heat exchanger..(seabee570)
 
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Old 12-20-10, 05:33 PM
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please do not quote me without reading my previous postson this topic..i first said that the heat exchanger could be dirty.rmag said it was new...now you quote me saying that other parts could be plugged up....what sense does that make...i have installed many 90 plus furnaces,and I have seen a few with the described conditions,and they were installed wrong.......
 
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Old 12-21-10, 10:56 AM
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My apologies for the delayed response - out for dinner. However, after looking at the postings (the bickering and the condescending remarks directed towards me), I thought I'd sit this round out. It seems someone let my ex-wife into this forum.

This is an Armstrong high efficiency 90+ condensing gas furnace. It's about 4 years old. However, it's never been used. It's in a house that a contractor lost while under construction. I took over the project and I'm keeping it as my residence. (I'm a general contractor). There are three furnaces. This is the only one giving me trouble. Again, the symptom is that it will not ignite with the exhaust vent connected. When disconnected, it will ignite and run. The vents are clear. The condensation and pressure switch hoses are clear, not damaged, not trapped, etc. The condensate trap is installed properly. I have my ideas about the problem, but did not want to prejudice your opinions by giving mine.

Are there any men out there who have some ideas? Thanks very much for your help.
 
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Old 12-21-10, 12:22 PM
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furnace

you have to get the install instructions,you have to make sure that the vent piping is installed per the manufacturers instructions. not every furnace is installed exactly the same. you cannot exceed the length specified in the manual.you need to verify this.is you find that you are close to the maximum distance on piping,it might have to be increased.what make and model? measure how far the pipe goes from the furnace to the outside,count the number of elbows and 45's,then check the manual.
 
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Old 12-21-10, 12:38 PM
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Thank you for the response. Yes, my thoughts were that the issue was with the pipe size, length, and/or 90's. This unit was installed facing away from the outside wall. Therefore, they had to do a 180 with the vent pipes to get to the outside wall. I'll re-do the venting according to specs and go from there. Thanks for your help.
 
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Old 12-21-10, 01:24 PM
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one other thing

if you can,use long sweep 90 degree elbows.....
 
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Old 12-21-10, 02:26 PM
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make sure the discharge pipe is sloped back to furnace and not trapping condensate
 
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