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Discussion Starter · #1 ·
One of our HOA furnaces (80% ng) is not always firing on first try. Sometimes on second or third. Our service contractor said he discovered this in 2018 and recommended replacing gas valve and control board. Our Board did not want to spend the money and we did not have issues last winter although the building was mostly closed due to Covid so even if the furnace was not working probably no one would have noticed.

I asked why not troubleshoot to determine if it is valve or board. Contractor said they had talked with Lennox tech service and Lennox recommended changing out both board and valve. Cost would be over $1500. This put my BS meter on high alert. In the old days, I would simply have my VOM on the gas valve to see if it was getting voltage when it was supposed to open. If I get voltage and the valve does not open, replace valve not board. If no voltage, do some more troubleshooting.

What do you think? Thanks.
 

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Need to see which error code it's giving, if any and go from there.

It could be anything, like a fluttering pressure switch.
Changing parts without diagnosis is mal-practice.
 

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Discussion Starter · #3 ·
Need to see which error code it's giving, if any and go from there.

It could be anything, like a fluttering pressure switch.
Changing parts without diagnosis is mal-practice.
Good points. I never thought about the error code and our contractor did not mention it either. Not sure if it even threw a code. I will check tomorrow and post findings.

So, you are strongly disagreeing with changing out board and gas valve? I just picked up HVAC in the spring but have already uncovered three major and very expensive repairs that were either unnecessary or not done correctly. This is why I am recommending we change contractors.
 

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Good points. I never thought about the error code and our contractor did not mention it either. Not sure if it even threw a code. I will check tomorrow and post findings.

So, you are strongly disagreeing with changing out board and gas valve? I just picked up HVAC in the spring but have already uncovered three major and very expensive repairs that were either unnecessary or not done correctly. This is why I am recommending we change contractors.
I very strongly disagree with changing parts before diagnosing.

Everything needs to be ruled out.
If there's no code, just makes lit a little more tricky -> the code just points in the right direction.
Several issues can cause ignition failures - dirty burner crossover tubes, igniter resistance too high so it barely gets hot enough, dirty flame sensor, pressure switch not staying closed (which doesn't mean pressure switch is bad), manifold gas pressure set wrong.
 

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Discussion Starter · #5 ·
Right, they cleaned flame sensor and measured HSI resistance as part of their tuneup. Judging by the color I would say HSI is doing fine. Don’t think it is cross over because the burners either fire up or don’t. No indication that lead burner fires but flame can not propagate. Also, burners look great when they do fire. I need to know if gas valve is getting voltage when burners do not fire. If it is not, I can proceed to find cause. Will report back. Heck, maybe just a loose wire. I have seen that before.
 

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Long ago we tried changing the IBGT's on a drive, incoming side of a VFD. There was a set on the outgoing side as well. (6 step VFD's, LONG before PWM) We found in over time and a lot of effort that we needed to change them in sets.

The recommendation could be valid on that model and make of furnace. I have experienced many times where replacing one electrical part required replacing another if you wanted the best result. New ballast and old florescent lamps comes to mind.

I suggest you call Lennox and ask them. A bad valve could fry an new drive board. I do not know if the reverse is true. Electronics are fussy critters now days. You might want to start with the orginal problem when you call the mother ship.
 

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Right, they cleaned flame sensor and measured HSI resistance as part of their tuneup. Judging by the color I would say HSI is doing fine. Don’t think it is cross over because the burners either fire up or don’t. No indication that lead burner fires but flame can not propagate. Also, burners look great when they do fire. I need to know if gas valve is getting voltage when burners do not fire. If it is not, I can proceed to find cause. Will report back. Heck, maybe just a loose wire. I have seen that before.
Yes, don't throw parts at it without eliminating everything else first. Does this happen when it's very cold out, but not when it's warmer? Old HSI will do that. I measure the current, not the resistance. Typically if you're below 2A, it's time to replace. A few types run at 1.8A, but most run at 2.2A or higher. I write right on the furnace what the amp draw is, so it's referenceable.

As said above, it can be just any anything. I'm not liking this tech. Giving us all a bad name.
 

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To bad folks don't charge for what actually FIXED a problem instead of the labor of guessing and installing needlessly.

I prefer to diagnose and identify - but I'm old and have that experience with electronics.
 

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Discussion Starter · #9 · (Edited)
Yes, don't throw parts at it without eliminating everything else first. Does this happen when it's very cold out, but not when it's warmer? Old HSI will do that. I measure the current, not the resistance. Typically if you're below 2A, it's time to replace. A few types run at 1.8A, but most run at 2.2A or higher. I write right on the furnace what the amp draw is, so it's referenceable.

As said above, it can be just any anything. I'm not liking this tech. Giving us all a bad name.
You would hate him if I told you everything I have discovered. This unit is in the clubhouse attic and it was warm both days it was fired it up - first by me a couple weeks ago and yesterday by the tech. One good thing is that I have the same Lennox model furnaces in my house (Pulte put them everywhere). I can practice troubleshooting and get reference measurements at home. Eg, tech told me draft inducer was about to fail because it was drawing excessive current, 2.5 Amps vs 1.95 nameplate. Came home and amped both of my identical draft inducers and they both were drawing 2.5 amps. So yes, amp draw is high but also normal.

I will measure HSI amp draw at home and at clubhouse. But, why isn‘t resistance enough since I also know voltage? Because HSI resistance may go out of limit high when it heats up?

Edit. Just answered my own question. On my home Lennox, I have what appears to be a replacement HSI. Have lived here less than 3 years and furnace is 18-19 yo. The cold resistance is 15.2 ohms but it is drawing 1.2 amps while heating. Far from what super likes to see but it is working fine.
 

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You would hate him if I told you everything I have discovered. This unit is in the clubhouse attic and it was warm both days it was fired it up - first by me a couple weeks ago and yesterday by the tech. One good thing is that I have the same Lennox model furnaces in my house (Pulte put them everywhere). I can practice troubleshooting and get reference measurements at home. Eg, tech told me draft inducer was about to fail because it was drawing excessive current, 2.5 Amps vs 1.95 nameplate. Came home and amped both of my identical draft inducers and they both were drawing 2.5 amps. So yes, amp draw is high but also normal.

I will measure HSI amp draw at home and at clubhouse. But, why isn‘t resistance enough since I also know voltage? Because HSI resistance may go out of limit high when it heats up?

Edit. Just answered my own question. On my home Lennox, I have what appears to be a replacement HSI. Have lived here less than 3 years and furnace is 18-19 yo. The cold resistance is 15.2 ohms but it is drawing 1.2 amps while heating. Far from what super likes to see but it is working fine.
Yea that's low but it depends on the material type. That's why I try to get a reference baseline. The resistance changes non linearly with heating. Very small differences when they are cold, can mean a much longer warm up time, enough for the trial for ignition to fail. You can also see the amp draw change from cold to hot.

Venter motors can by weird like that. Being low current doesn't help, as it's on the low end of most meters. Often I'll wrap the wire a few times around my amp clamp and divide the measurement by the number of loops. It can give a more accurate reading. But +10% out is usually perfectly normal for them.
 

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Discussion Starter · #11 ·
Ok, here is the scoop. Nice morning to take a climb into our clubhouse attic! Before I called for heat I disconnected leads to gas valve and connected VOM. Wanted to test circuit board while unit was cold after a dormant period and without exercising GV. Furnace sequenced fine with same 1.2 amp draw on HSI. Got my first 28.5 vac on schedule then dropped out since no gas and unproven flame sensor. She cycled 5 times just like it should before locking out. Diagnostic code was alternating slow flash which could mean 4 different conditions but most likely just ”burners failed to ignite”. Connected leads to GV and called for heat. Did not fire first or second time. Fired third time but GV slow responding. Subsequent calls for heat fired first time. I would say board is fine but GV is having trouble opening, especially after being dormant. Does better after exercise. One of the 4 error conditions is “pressure switch opened 5 times during a heating cycle”. If the pressure switch were opening everytime, I do not believe the furnace would ever fire.

Oh, another little tidbit. That furnace had its control board replaced 4/18 by the same tech who advised me to replace both board and GV at the same time per Lennox tech service. Guess he did not think I would discover the new board. The invoice says back of board showed wear and damage plus relay was opening and closing. I was not involved back then so do not know the real scoop. However, these guys are quick to replace control boards which ultimately turns out to be unnecessary.
 

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Ok, here is the scoop. Nice morning to take a climb into our clubhouse attic! Before I called for heat I disconnected leads to gas valve and connected VOM. Wanted to test circuit board while unit was cold after a dormant period and without exercising GV. Furnace sequenced fine with same 1.2 amp draw on HSI. Got my first 28.5 vac on schedule then dropped out since no gas and unproven flame sensor. She cycled 5 times just like it should before locking out. Diagnostic code was alternating slow flash which could mean 4 different conditions but most likely just ”burners failed to ignite”. Connected leads to GV and called for heat. Did not fire first or second time. Fired third time but GV slow responding. Subsequent calls for heat fired first time. I would say board is fine but GV is having trouble opening, especially after being dormant. Does better after exercise. One of the 4 error conditions is “pressure switch opened 5 times during a heating cycle”. If the pressure switch were opening everytime, I do not believe the furnace would ever fire.

Oh, another little tidbit. That furnace had its control board replaced 4/18 by the same tech who advised me to replace both board and GV at the same time per Lennox tech service. Guess he did not think I would discover the new board. The invoice says back of board showed wear and damage plus relay was opening and closing. I was not involved back then so do not know the real scoop. However, these guys are quick to replace control boards which ultimately turns out to be unnecessary.
Check the voltage over the pressure switch during startup. Just to be sure. At this point it does sound like a gas valve. Make sure that the heat exchanger is good before spending money on it.

A 3 year old control board is not likely an issue and wouldn't be my first guess. I guess he's outta there as soon as you can make it so.
 

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Discussion Starter · #13 ·
Will do on the pressure switch and report back. Yes, our current contractor is kaput. The only reason he did fall checkup is because we already paid for it. Our Board is ok with a change.
 

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When the furnace is an 80%, the heat exchanger can be inspected from the blower compartment, just have to pull the assembly.
If it's good you'll have to get the valve changed i guess, if not, weigh labor cost of warranty hxt replacement with that of a new furnace.
 

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When the furnace is an 80%, the heat exchanger can be inspected from the blower compartment, just have to pull the assembly.
If it's good you'll have to get the valve changed i guess, if not, weigh labor cost of warranty hxt replacement with that of a new furnace.
It's not the best inspection though. I wish they designed resi furnaces with access panels.
 

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It's not the best inspection though. I wish they designed resi furnaces with access panels.
Neither is inspection from the top (if there's no spacer between furnace and coil), or through the limit opening "the best". Using one method only, parts of it won't be visible or close enough to properly inspect.
 

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Neither is inspection from the top (if there's no spacer between furnace and coil), or through the limit opening "the best". Using one method only, parts of it won't be visible or close enough to properly inspect.
I use a scope and I still don't like it.
 
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