Portable generator and gas furnace incompatibility?

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I have a customer scheduled for an inlet/interlock installation. He told me that a friend's HVAC guy can't get his 4-year-old gas furnace to operate on his portable generator, allegedly due to frequency instability.

My customer has one older one and one newer (3 years old) furnace, and opted for a 10kw standard over an 8kw (peak) inverter unit for the capacity., and is now concerned about having the same problem.

Have any of you heard of this, and can tell me what can be done to eliminate, or at least minimize the problem?
 
For your friend with the furnace/generator issue, how old and what type generator?
I would suspect the generator.
 
Does a natural gas furnace typically require a neutral-EGC bond in the system (e.g. by using an ignitor that sparks form L to G)?

Will the installation have a N-EGC bond in it when running on generator? E.g. via a transfer switch that leaves the service N-G bond in the circuit? Versus a portable generator with no N-EGC bond, and the furnace plugged directly into a receptacle on the generator.

Cheers, Wayne
 
wwhitney said:
Does a natural gas furnace typically require a neutral-EGC bond in the system (e.g. by using an ignitor that sparks form L to G)?

Will the installation have a N-EGC bond in it when running on generator? E.g. via a transfer switch that leaves the service N-G bond in the circuit? Versus a portable generator with no N-EGC bond, and the furnace plugged directly into a receptacle on the generator.
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I don't know whether the issue is ignition or control circuit.
 
Could be something in the controls. I setup a customer with a portable a couple of years ago, and none of the home control systems would work correctly on gen power because of the frequency drift. Installed a permanent standby a couple of months ago and all works fine now.


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I believe newer furnaces need a low total harmonic distortion, that typical portable generators do not provide. Perhaps there is a filter or small ups that can be installed on inline with loads, like furnaces, that are troublesome.
 
wwhitney said:
Does a natural gas furnace typically require a neutral-EGC bond in the system (e.g. by using an ignitor that sparks form L to G)?

Will the installation have a N-EGC bond in it when running on generator? E.g. via a transfer switch that leaves the service N-G bond in the circuit? Versus a portable generator with no N-EGC bond, and the furnace plugged directly into a receptacle on the generator.

Cheers, Wayne
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That sounds logical, as some furnace controls return on ground, but you would think the control transformer would be bonded. I know some will lock out if hot and neutral are switched.
 
tank728 said:
I believe newer furnaces need a low total harmonic distortion, that typical portable generators do not provide. Perhaps there is a filter or small ups that can be installed on inline with loads, like furnaces, that are troublesome.
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I think a UPS sized to operate a gas furnace and blower would be a little costly.
 
hillbilly1 said:
That sounds logical, as some furnace controls return on ground, but you would think the control transformer would be bonded. I know some will lock out if hot and neutral are switched.
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I think that's code. Not sure.
 
tank728 said:
I believe newer furnaces need a low total harmonic distortion, that typical portable generators do not provide. Perhaps there is a filter or small ups that can be installed on inline with loads, like furnaces, that are troublesome.
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No guarantee there, either. I have several CyberPower 1500VA UPS's for the home computers and just happened to run across the THD spec recently. THD is 43%! :eek: It's a square wave output unit. You would need to be sure to get a true sine wave unit.
 
A possible option would be to feed the furnace branch circuit through a double throw switch so you could select that it be powered through either 1.) a branch breaker in the main panel, or 2.) a second generator inlet. This second inlet could then be powered by a relatively small 120V inverter generator of, say, 3kW or less (which can be had for less than $1000). That way you could choose to power the furnace from the POCO, the 10kW generator, or the small inverter generator.

This would also have the advantage that you could power the furnace overnight with a quiet, more fuel efficient generator. Inverter generators tend to be relatively quiet, especially in an "ECO" mode where the engine RPM will back down at lower load currents.
Of course it all depends on what the customer wants and whether the furnace has a problem with power from the 10kW generator..
 
synchro said:
A possible option would be to feed the furnace branch circuit through a double throw switch so you could select that it be powered through either 1.) a branch breaker in the main panel, or 2.) a second generator inlet. This second inlet could then be powered by a relatively small 120V inverter generator of, say, 3kW or less (which can be had for less than $1000). That way you could choose to power the furnace from the POCO, the 10kW generator, or the small inverter generator.

This would also have the advantage that you could power the furnace overnight with a quiet, more fuel efficient generator. Inverter generators tend to be relatively quiet, especially in an "ECO" mode where the engine RPM will back down at lower load currents.
Of course it all depends on what the customer wants and whether the furnace has a problem with power from the 10kW generator..
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That is exactly what I'm thinking of doing to power my sump pump when we're asleep. I have an 8 KW generator but it is loud and I would really like to shut it down when I only need to keep my sump pump going.
 
Given how everything is electronic these days -inverter driven compressor in fridge, ECM motor in furnace, LED lights, TVs, etc I would never power a house from a 'construction site' type gas generator. Too risky. Inverter type only, or a larger generator like a diesel tow plant with stable frequency.
 
I had the same situation a number of years ago. Our solution, and it worked flawlessly, was to set the boiler up to be supplied through a double conversion UPS. I don't recall the pricing but I think it was around $300 for the UPS.
 
eagleelec said:
I had the same situation a number of years ago. Our solution, and it worked flawlessly, was to set the boiler up to be supplied through a double conversion UPS. I don't recall the pricing but I think it was around $300 for the UPS.
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Any chance you recall a make or model?
 
I do not recall the make or model, however if you do a search for a double conversion UPS you will find what you are looking for. Tripp Lite is one maker that has them.
 
Update:

Inlet and interlock installed, customer ran house on genny while we cleaned and packed up, both gas furnaces ran fine. :)

He was so happy, he said that he was going to keep the house running on the genny for a couple of hours after we left. :giggle:

As always thank you all for your insight and opinions. (y)
 
Which portable generators provide the best/lowest THD for running a house? I'm working on some nat gas conversions for my own (and eventually customer) generator, but wanted to see id anyone had any recommendations. Currently I have a Pulsar 12kw dual fuel that can power my whole home and 3.5t AC (with soft start).

I've heard Westinghouse gens have decently clean output for this purpose as well. Of course Northstar gens are great but their cost puts them out of reach for most customers.
 
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