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Old 11-19-2011, 12:36 PM   #1
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Lennox whoomps when it kicks on


I recently tried to start my furnace but the pilot light was out. I had a techician out who got it to light but took the furnace apart and tried to talk me into buying a new furnace. He left it in pieces and I had a friend put it back together and start it. It works O.K. but makes a big whoomp sound when it kicks on. What do I need to adjust and how? It is a Lennox Duracurve. thanks.

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Old 11-19-2011, 12:41 PM   #2
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Lennox whoomps when it kicks on


I have to ask, why did the tech you hire leave it in pieces? That is so unethical if it was working properly. Did he happen to say anything about a crack in the heat exchanger when he inspected it?

As for the sound, without being in front of it when it actually occurs I can't say, sorry.

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Old 11-19-2011, 01:19 PM   #3
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the Lennox dura-curve has a nick name with technicians......dura crack, and thats due to the fact that chances are very good that there are cracks in the heat exchanger were it curves.
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Old 11-19-2011, 02:11 PM   #4
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The technician did not say anything to me about the heat exchanger. He said the flame on the pilot was too low and started taking apart the furnace to clean it. He had talked us into considering an estimate on a new furnace. The best face that I can put on this is that he believed we would follow through and have a new furnace installed in a few days.
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Old 11-19-2011, 02:36 PM   #5
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Well, judging by what harley has said it's only a matter of time before you have no choice in the matter. A crack in a heat exchanger is very life threatening.
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Old 11-19-2011, 02:52 PM   #6
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Lennox whoomps when it kicks on


Thanks. We will look into it. But we were not having any problems with the furnace until this happened, I forgot to mention that the technician and our friend both said that we might need a new thermocouple. Could this be the problem? The sound that the furnace makes, sounds like it is taking a moment for the gas to ignite before it hits the pilot, then it ignites all at once. I'm not sure that it is doing this everytime it kicks on.
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Old 11-19-2011, 03:14 PM   #7
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Lennox whoomps when it kicks on


A thermocouple is the flame sensor. If the thermocouple wasn't sensing the flame then the unit would not fire, plain and simple.

A few things I'd ask your friend about concerning preventitive maintenance, dip switch settings for blower off. The longer the blower runs after the heat is cut off the less chance of a crack happening in the heat exchanger. The factory settings are usually either 60 or 90 seconds and can go to 120 so up to a full two minutes.

That and if your blower is set to low speed while on heat ( high is cool, low is usually heat) and you have a four speed blower with two wires "parked" on the board, see if he can up the speed to medium low while on call for heat.

If I had a furnace that was prone to cracking the heat exchanger those would be the things I'd look into. And make certain you have clean filters and evaporator coil for good air flow.
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Old 11-19-2011, 03:23 PM   #8
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Lennox whoomps when it kicks on


The burners on that unit must be perfectly aligned side by side or you won't get proper ignition as the flame does not run over from one to the other fast enough. They all must be level from front to back so one is not higher than the other. You have a dangerous condition called delayed ignition now. Am experienced Lennox tech may know how to setup those burners.
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Last edited by yuri; 11-19-2011 at 03:26 PM.
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Old 11-19-2011, 03:26 PM   #9
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Lennox whoomps when it kicks on


Quote:
Originally Posted by yuri View Post
The burners on that unit must be
oh goody, a fill in the blank game.

1.) potatoe
2.) cucumber
3.) lettuce
4.) ketchup
5.) cracked

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Old 11-19-2011, 06:31 PM   #10
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Lennox whoomps when it kicks on


The noice will be from a delayed ignition caused by dirty/ misaligned crossovers or a pilot flame that is improperly positioned too far away from the crossover.
Look at the burners when the main gas comes on. Do all the burners ignite at once or are some delayed a few seconds?
These lennox burners often develop delayed ignition from debris/dust/rust inside the venturi burner that diverts the main gas away from the internal crossover feeds. A good clean crossover can tolerate minor misalignment of the burners but a weak crossover can not.
When the burners are cool, insert a wire or brissle brush into the venturi burner ports up past the crossovers. Blowing air into those same venturi ports will help. When the crossovers are clean enough they should show a continuous ribbon of flame along their entire length when the burners are firing.
If not then cut a 1/2" by 4 " strip of sheet metal ( from a soup can?). With the burners off and cool again, this blade of metal can be bent to slip into the leading openings of the crossovers to clear really stubborn debris.
Check to confirm that all the burners are on the identical horizontal plane and that no burner is tipping over on one side.

The final possibility because you've had an F wit play with your furnace pilot is that the pilot assembly may not be in the correct position. The pilot flame should be in direct contact with the flame of the crossover. If not, write back to say so.

Last edited by how; 11-19-2011 at 06:44 PM.
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Old 11-19-2011, 06:47 PM   #11
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Lennox whoomps when it kicks on


mark if the tech said the pilot was low or weak that says its to weak to light burnners rapidly as is should. as mentioned alignement of burnners and pilot position is very critical....just keep in mind the age of the furnace and the history of the dura-curve with cracks to its heat exchanger...family safty is always 1st..its worth the money to have well known company service this unit if you plan to keep it...Ben

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