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Tim H.
Tim H., HVAC Professional
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i have a lennox ghr 26 gas furnace. In the past year I have

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i have a lennox ghr 26 gas furnace. In the past year I have had two different service companies come. They combined have replaced 2 gas valves, a spark ignitor, a flame detector, a PC board, a regulator on the outside tank, said the venturi was rusted. blah, blah. The furnace still doesn't work. looking at the view port at least one venturi seems to blow back that is the flame is at the front of the venturi. I used a shop vac to suck on the in/out lines.
Hi, I'm Tim and I'm an HVAC expert here to assist you with your questions. Please provide all information I request as it makes it more difficult to troubleshoot something when I'm not there onsite. Essentially, you become my eyes and ears. Thanks for the opportunity to help. At the end of our dialog, you will have a chance to rate my service, but please wait until we are finished and you're satisfied you have all the information you need.
I'm sorry you are having trouble with your furnace. Sounds like you have some "parts changers" for technicians out there! Never a good sign. When you say venturis, are you talking about the burners?
Customer: replied 2 years ago.

there are three venturi (bugel shaped tubes) that the gas from the valve goes into. At one end are the orfices delivering gas. at the other are the spark igniter and flame sensor

Yes, those are the burner tubes. Are you saying the flame is wavering or whipping back on one of those burners?
Customer: replied 2 years ago.


That is not a good sign. It may mean a cracked primary or secondary heat exchanger. The heat exchangers can be seen on page 5, figure 1 of this manual. The primary is labeled "duralock" heat exchanger and the secondary is labeled condenser coil. These must be inspected for cracks, preferably by an expert who knows what he is looking for. When the primary or secondary heat exchanger develop a crack it cause the air blowing by the heat exchanger to disturb the blower flame. It can also cause the unit to make carbon monoxide.
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