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Old 11-01-2012, 09:07 PM   #1
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Starting a forced hot air furnace


Hi guys. My parents are in central NJ without power and it's starting to get cold. They have a 3,500watt generator along with an inlet and interlock kit.

I don't know much right now so I have to speak in general, please bear with me. Their house is 2,500sqft and they have 2 natural gas forced hot units (2 zones), the units are the same size but I do not know what size they are. The units also serve as central AC, if that matters. The house was built 5 years ago. The furnaces are Lennox and they have use PVC for intake and exhaust so I guess they are high efficiency?

Both of the furnaces are on the same 20amp circuit. There has never been a problem with the furnaces.

The issue is that their 3,500 watt generator will not turn on the furnace (they are only trying to turn 1 on). With everything else in the house completely off, my father said that he can only feel some cool air bleeding thru the registers and the unit sounds like the motor isn't spinning up.

Apparently the generator isn't big enough to start the unit up, but that seems odd since the units are fine sharing a single 20A circuit yet a generator capable of putting out 30 amps continuous and 35+ amps peak can't start the furnace.

What about a forced hot air furnace would be hard to start? I can't imagine the blower motor is that large. As a test, my father powered up a 12,000BTU air conditioner with the generator without a problem, the condensing unit went on perfectly fine.

Is there some type of coil to ignite the furnace that could be using all that power? What else could it be?

I can't get down to my parents right now to help, my father is getting old and has been dealing with massive flooding and the power outage and my mother is sick so the cold isn't helping. When I get down there, I would like to have some type of knowledge of what the problem could be, so I appreciate any help. Thanks

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Last edited by Hundred; 11-01-2012 at 10:19 PM.
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Old 11-01-2012, 09:40 PM   #2
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Starting a forced hot air furnace


Distance to the furnace? There is such a thing called amperage drop due to electrical componentry being too far away from source of electricity. A few of my electric drills won't run on a 100' electrical cord but have no issues starting and running on a 50' cord and that's plugged into a regular outlet.

Might want to ask about this in the electrical section up above.

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Old 11-01-2012, 09:53 PM   #3
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Starting a forced hot air furnace


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Originally Posted by Doc Holliday View Post
Distance to the furnace? There is such a thing called amperage drop due to electrical componentry being too far away from source of electricity. A few of my electric drills won't run on a 100' electrical cord but have no issues starting and running on a 50' cord and that's plugged into a regular outlet.

Might want to ask about this in the electrical section up above.
I think you mean voltage drop.

There is no issue with voltage drop at any other time, so I don't know why it would come into play here.
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Old 11-01-2012, 09:55 PM   #4
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Starting a forced hot air furnace


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Originally Posted by gregzoll View Post
If it is electric, the heating strips will not even turn on with that genset. Better to get some propane room heaters to warm the place up during the day hours, then when they go to bed, put a couple of layers of blankets on top and wear sweats and socks to stay warm.
No, it's natural gas, as I stated in the first post. I also mentioned twice how there are 2 furnaces on a single 20A circuit, something that doesn't happen with electric furnaces.
Quote:
People need to learn to go old school, back to the days when we did not have generators or technology like computerized hvac systems. Back in the day, when the fire burned down, people would crawl into bed that had been warmed with a heating iron or rock, and had a couple of layers of thick blankets on it, and flannel sheets on the bed.
Thanks, awesome. I'll be sure to tell my parents that you said this. Thanks a lot for the help with the topic
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Old 11-01-2012, 10:00 PM   #5
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Starting a forced hot air furnace


I just read about furnace problems using a generator. Some generators need to have an earth ground for the control boards to work.
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Old 11-01-2012, 10:03 PM   #6
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Starting a forced hot air furnace


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I just read about furnace problems using a generator. Some generators need to have an earth ground for the control boards to work.
The generator is connected to the UFER ground (20'+ of rebar in the concrete footing) thru the main panel.
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Old 11-01-2012, 10:08 PM   #7
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Originally Posted by Hundred View Post
I think you mean voltage drop.

There is no issue with voltage drop at any other time, so I don't know why it would come into play here.
That's why I said to ask in the electrical forum. My bud (master electrician) has a generator hooked through a transfer switch to his home's panel. Everything works that way so I can't say. His is big enough to run a few homes, though.
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Old 11-01-2012, 10:10 PM   #8
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Starting a forced hot air furnace


Quote:
Originally Posted by Doc Holliday View Post
That's why I said to ask in the electrical forum. My bud (master electrician) has a generator hooked through a transfer switch to his home's panel. Everything works that way so I can't say. His is big enough to run a few homes, though.
I don't know if electricians know about the inner workings of HVAC systems, I know that I don't

That's why I asked here.
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Old 11-01-2012, 10:31 PM   #9
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Starting a forced hot air furnace


We can't tell you what's wrong. If it worked with power company electricity but not with the generator then logic would lead us in the direction of power supply. Grounding the generator chassis to a stake driven into earth has solved the problem for some having the same issue. Might or might not work for your folks but for no more work that's involved it's worth a try
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Old 11-01-2012, 10:34 PM   #10
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Starting a forced hot air furnace


Does anyone know in what way a typical Lennox high efficiency furnace ignites? Is there some type of coil that may draw high current?
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Old 11-01-2012, 10:35 PM   #11
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Starting a forced hot air furnace


BTW nothing in a gas furnace has a large enough power draw that the generator couldn't start it.
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Old 11-01-2012, 10:37 PM   #12
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Starting a forced hot air furnace


Quote:
Originally Posted by Marty S. View Post
We can't tell you what's wrong. If it worked with power company electricity but not with the generator then logic would lead us in the direction of power supply. Grounding the generator chassis to a stake driven into earth has solved the problem for some having the same issue. Might or might not work for your folks but for no more work that's involved it's worth a try
As a licensed electrical contractor, I can guarantee that the generator is bonded to the same earth ground as the rest of the house. It's a solid ground, a UFER ground is considered to be much better than a ground rod.
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Old 11-01-2012, 10:44 PM   #13
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Starting a forced hot air furnace


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BTW nothing in a gas furnace has a large enough power draw that the generator couldn't start it.
I've been doing reading on other forums, many HVAC techs are saying that the "dirty power" that generators put out is messing with the control boards in the furnace. That could be it.

I wish I could go ask there, but those forums require you to be a an HVAC contractor so i am stuck on the DIY forum with trolls like gregzoll who know nothing and just want to cause trouble.
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Old 11-01-2012, 10:52 PM   #14
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Starting a forced hot air furnace


Quote:
Originally Posted by Marty S. View Post
BTW nothing in a gas furnace has a large enough power draw that the generator couldn't start it.
Looks like my same problem posted on another forum:

Quote:
Lennox- would not run at all- error lights would indicate that polarity was reversed. Switching wires did not help.
A second (5500 watt) was tried-same error. We tried grounding several ways (water pipe,furnace cabinet etc)
No luck.When electric was restored furnace worked fine.
So it looks like the 5,500w generator I was going to bring to them won't help either
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Old 11-01-2012, 10:56 PM   #15
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Starting a forced hot air furnace


Yep. Those are the places I read about grounding the generator chassis to make the furnace work off of it. Control boards are finicky about power supply. How to clean it up is your area of expertise , I just fix hvac systems for a living.

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