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Discussion Starter · #1 ·
I had my licensed electrician install a Generac Guardian 18 KW air cooled standby outside generator powered by LPG and it works great.

Then I had an HVAC contractor install a Lennox signature series furnace and AC with all the bells and whistles, including motorized dampers for 2 zone heating/AC which are run by an internal computer inside the furnace.

The problem is...when the power goes out, the generator kicks on in 15 seconds and lights the house back up, but the heat won't work! The AC works fine but an error code keeps coming up on the furnace when the heat tries to kick in.

Both of my contractor's have been trying to fix this problem for six months and still it won't work. Both of them say the computer in the furnace doesn't like the "dirty" power coming out of the generator.

It's at the point now where they have given up and the HVAC contractor is taking entire responsibility for the whole works, including replacing everything at his cost for a generator/furnace system that WILL work when the generator. Does anyone know of a combination of furnace and generator (air cooled) which work together with no problems? The Lennox and Generac factories haven't been able to come up with a solution either. Thanks, PT
 

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It looks like all those bells and whistles have come back to bite you. The AC computer evidently does not like the quality of power your generator is making.
 

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I had my licensed electrician install a Generac Guardian 18 KW air cooled standby outside generator powered by LPG and it works great.

The AC works fine but an error code keeps coming up on the furnace when the heat tries to kick in.

Both of them say the computer in the furnace doesn't like the "dirty" power coming out of the generator.

The Lennox and Generac factories haven't been able to come up with a solution either. Thanks, PT
You should probably put a scope on the gen. power when the heat tries to start, although this 75A gen. should be able to handle it.
It's a lot easier to filter the power going into the HVAC computer (provided this error code is a "false positive") than it is to changeout the whole setup.

Too bad you don't live closer to DC.
 

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It's great that the HVAC guy is going to take care of you. I bet most of the time there would be a lot of finger pointing between the generator guy and the HVAC guy and no one taking responsibility. It's good to hear about standup contractors. I wish I could help but electronics is definitely not my forte. Since one of the major reasons for backup power is to maintain house conditioning I'm surprised that Lennox and Generac don't already have this figured out.
 

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After reading Yoyizits and vsheetz posts I wonder if they can isolate the computer power from the rest of the furnace and put a computer type battery backup on it??
 

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I assume you have tried running the HVAC through a battery backup with power conditioning?
I was thinking that also. It would be a cheap fix if it worked. Is there 240V or 120V running to the furnace? I'm sure the controls work off of some transformer that doesn't like the gen power.
 

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Once the heat is going it might tolerate generator power but I don't know of an easy way to test this theory. The xfer process itself may set off an error code.

You need the particulars of how your error code is set. The ball is in Lennox's court to provide this (proprietary?) info.

Or, you may just need a single point ground, in the sense that heavy startup current is changing the voltage on the signal ground of the HVAC computer and the computer is interpreting this change as a valid signal.
This not something most people would look for because the schematic is being and has been followed. The fix may be as simple as moving a wire from one terminal to another.
You definitely need a scope for this one. And, I would have expected this on A/C startup, instead.
 

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Discussion Starter · #10 · (Edited)
Thanks for your comments; there are a few items that I forgot to mention. When the heat didn't come on when the generator kicked in, the contractor bought an ACME line conditioner and hooked it up which conditioned the entire furnace but it didn't solve the problem. The tech also checked out the sine wave with an oscilloscope and it was basically uniform. The electrician added multiple grounds as requested by the HVAC contractor, which did nothing.

Then he borrowed an UPS unit to see if it would work and it didn't. Then he bought a Liebert "true" UPS unit for computers, hooked it up and it didn't work either. The tech was so disgusted he left it here next to the furnace while they work on plan C.

I don't think there is any fix for this problem excepting yanking the new furnace and replacing it with a "low tech" unit without all the computer bells and whistles and then it will work. There must be a Lennox or comprable model that will run correctly when running off the generator. I have Alleghany Power, which is just as "dirty" as the power from the generator. What do you think...go ahead and replace the furnace or keep trying out new "theories". They have been trouble shooting this since January to no avail. I want them to fix this now before it drags out too long. Thanks, PT
 

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Is the furnace running when they are doing the switch-over tests? I wonder if the furnace has a lockout that is preventing it from restarting so soon once it loses power? This is a LP furnace, right? You may want to try to ask over at the www.hvac-talk.com forum to see if anyone over there has heard of this.
 

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Discussion Starter · #12 ·
I'm surprised that Lennox and Generac don't already have this figured out.
The electrician who installed the generator is an authorized Generac installer and went to their school and is certified to install these generators. He has called them several times and they don't have a clue; the HVAC guys did the same with Lennox, nothing. PT
 

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Discussion Starter · #14 ·
Is the furnace running when they are doing the switch-over tests? I wonder if the furnace has a lockout that is preventing it from restarting so soon once it loses power? This is a LP furnace, right? You may want to try to ask over at the www.hvac-talk.com forum to see if anyone over there has heard of this.
These guys have tried it every which way there is; it is an LP furnace with a 500 gallon underground tank supply to the house and dedicated only for the furnace. Once you switch the furnace off it off, it takes about 10 minutes for all this computer coding to "recycle", then it comes back on. They got a reading of about 13" of head on their manometer at the furnace, so it has plenty of gas going into the furnace. I give up.
 

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These guys have tried it every which way there is; it is an LP furnace with a 500 gallon underground tank supply to the house and dedicated only for the furnace. Once you switch the furnace off it off, it takes about 10 minutes for all this computer coding to "recycle", then it comes back on. They got a reading of about 13" of head on their manometer at the furnace, so it has plenty of gas going into the furnace. I give up.
Yep. Sounds like they tried everything. Again, try the HVAC forum. They may have a suggestion or two.
 

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Try switching out the Lennox control board that gives you the error code. It may be hypersensitive.

Otherwise you need the board schematic (you'll probably have to sign a non-disclosure agreement) and one of these
http://www.pctestinstruments.com/

These guys are stumped, but their troubleshooting method was by replacement. Since everything else was tried, I'd say it was a grounding problem.
I'd do the minimum grounding required for it work and debug this config. if necessary, and then cautiously build back up to an NEC ground system.

I'd love to tackle this one. . .maybe twice in my life I faced a problem like this.
 

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Discussion Starter · #17 ·
You need an "online" true sine wave UPS. This makes electricity ALL the time whereas other UPS only make electricity when power is removed.

FYI here is what dirty power looks like...
http://www.jkovach.net/projects/powerquality/
I think that the Liebert UPS that is sitting my basement now is the same as what you mentioned above. The tech installing it got sine wave that looked like this also. PT

 

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I think that the Liebert UPS that is sitting my basement now is the same as what you mentioned above. The tech installing it got sine wave that looked like this also. PT

You may need a storage scope because almost certainly you are looking for a transient. That's what logic analyzers do; capture the transient that caused the problem. In your case you may need one that runs on batteries so it is totally isolated from the AC line.
 

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"Dirty power" sounds like BS to me -- especially because they checked it with a scope and tried a Liebert double-conversion online UPS.

I don't know why they wouldn't try a new controller board before ripping out the entire system -- did they?

My only other thought is that there is something attached to the control computer (circ. pump, temp. sensor package, damper motor, etc) that is getting power from the mains panel and is not getting switched to generator along with everything else. That's unlikely, and even more unlikely that 2 contractors missed it, but possible.
 

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Discussion Starter · #20 ·
"Dirty power" sounds like BS to me -- especially because they checked it with a scope and tried a Liebert double-conversion online UPS.

I don't know why they wouldn't try a new controller board before ripping out the entire system -- did they?

My only other thought is that there is something attached to the control computer (circ. pump, temp. sensor package, damper motor, etc) that is getting power from the mains panel and is not getting switched to generator along with everything else. That's unlikely, and even more unlikely that 2 contractors missed it, but possible.
Yes, they did put new computer controller board on the wall, the one with the flashing function lights, that didn't fix it either. You guys are starting to talk over my head on this! It would help if I was an EE.
I'm going to call them tomorrow and see what they're going to do, since they are picking up the tab. BTW, this HVAC company is very large with about 50 employees, not one guy and a helper.

I appreciate everyones comments but my original question was...did anyone here know of a combination of generator/HVAC system that they KNOW works during a power outage? PT
 
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