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Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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My Lennox Elite furnace (1998 manufacture) has presented the

Resolved Question:

My Lennox Elite furnace (1998 manufacture) has presented the same problem for several years. Sporadically, the furnace will not fire, when this happens the induced draft fan runs until I shut the unit down. The code lights indicate that the pressure switch has been open more than x times during the heating cycle. I have replaced the pressure switch twice. After the last replacement, 6wks ago, the unit malfunctioned within hours, I "reset" the switch by sucking on and blowing into the hose connected to the switch. The unit ran for over a month with no malfunftiuons. Why is the switch opening too much and causing the unit to malfunction? It is supposed to get into the teens for several nights and silly me I'd like to sleep instead of babysit the furnace. I can always get it to restart by the blowing/sucking method, but it really SUCKS to have to do this! The induced draft fan was replaced in '05 due to excessive noise. Is this a control circuit board issue rather than a switch issue?
Submitted: 6 years ago.
Category: HVAC
Expert:  Douglas replied 6 years ago.
a couple questions....

Is this furnace vented with plastic or metal?
tell me about the vent. length, number of elbows that kind of stuff.

I see alot of pressure switches get changed when they are merely responding to borderline pressure. Without actually knowing the vacuum by measuring it, how can they determine the pressure switch is the problem? I submit that it is probably a vent problem or a drain problem.
Customer: replied 6 years ago.
The exhaust is side wall through 143.5" of 2" PVC with 4 -45s and one 90 degree fiiting. (If I did the math right that is equal to 27'.) The intake is side wall through 100" of 2" PVC with 6 -45s and one 90 degree fitting.(equal to 28.3"). This is a retrofit from an old Lennox Pulse. There had been two mufflers int the piping. That may account for some of the extra fittings. I check the drain regularly and occasionally blow out the drain hose with compressed air. It is clear at this time. Borderline pressure would explain the sporadic occurance, but how do I prevent it?
Expert:  Douglas replied 6 years ago.
I need to find the venting tables... does an 80k Lennox allow 2" vent pipe? (really, a new vent system should have been installed to the new furnace vent tables). I hear it alot... "We do it all the time and it worked before"... Rest assured engineering wouldnt have specified rigid vent tables if it were not important.

Let me see if I can find venting tables. We will need the actual model number unless you have a copy of the installation instructions nearby.
Customer: replied 6 years ago.
I should have listed this before G26Q3-75-4 Serial #5890B 16565 75,000 Btu.The vent table included in the Installation Instructions shows that with a 30' run 2" pipe is acceptable for 50, 75 and 100,000 Btu units. 3" is not required until 60' run.
Expert:  Douglas replied 6 years ago.
Ok... Good. This may just be a case where the tech needs to connect a magnahelic or manomter to the pressure switch line to see if there is any other problems. Additionally, a slow drain can cause intermitant pressure switch trips as can condensation build up in the tube to the pressure switch. I have also seen deposits form in the oriface to the pressure switch tube at the inducer assembly. You could take that tube loose and rod the hole out.

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