HVAC system heat won't run with standby generator running...

PTighe

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

vsheetz

I assume you have tried running the HVAC through a battery backup with power conditioning?

J. V.

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.

Yoyizit

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.

jogr

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.

jogr

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??

jerryh3

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.

Yoyizit

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.

MALCO.New.York

Certainly need a Conditioner before you go and FRY that PC Module!!

PTighe

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

jerryh3

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.

PTighe

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

Billy_Bob

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/

PTighe

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.

jerryh3

Yep. Sounds like they tried everything. Again, try the HVAC forum. They may have a suggestion or two.