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Joined: Sep 2002
Posts: 37
Not sure if this has been kicked around yet. I have several customers with portable generators that can't run their furnaces. Nearest thing I can figure is that they are not putting out 60 hertz. I also tested several that had an open ground at the GFCI receptacle and L14/30 receptacle.
So 2 questions:
Is there a simple way to get the output hertz up to 60 if the generator is only putting out 57 or 58.

And separate from the above question
Why is there no ground on the L14/30 receptacle of certain generators.

Joined: Jul 2002
Posts: 8,407
Likes: 2
What size generators are these?

The output frequency is a function of the speed of the alternator and it sounds like they might be being overloaded.

Just a thought.

Joined: Jan 2005
Posts: 5,316
Cat Servant
There's no 'easy' answer.

There are $99 generators, and there are $1300 generators - even at the same modest "20 amp / 120v" output range. Ever wonder why the price range?

One of the big areas where they 'save' expense in in the power regulation. The cheap ones are not putting out a very good waveform. This is just fine for lights and motors, but can be pure nightmare for anything electronic - like modern furnace controls.

Other drawbacks to the cheap genny's are high fuel consumption and high noise levels.

And, as Trumnpy has mentioned, overloading the genny will result in the voltage gropping before the machine simply stalls.

Joined: Sep 2002
Posts: 37

1 generator was 7500 watts under half load
1 generator was 3500 Honda with no load
1 generator was 2000 Expensive Honda no load
1 generator was 3500 cheapo maybe 1000 watt load

4 different jobs and none would run the furnaces with consistency.

Also in regards to the grounding or lack there of
the honda 3500 had no ground
1 of the honda 2000 did and 1 did not have a ground

Joined: Sep 2002
Posts: 37

That is what i always tell my customers. We always recommend Honda's and typically between 3500 and 7500 with idle/throttle control.

We really did not have a voltage issue but a frequency issue.

Any thoughts to the lack of ground issue too?

Joined: Jul 2002
Posts: 8,407
Likes: 2
How are these generators being connected to the furnaces?

Forgive me if I'm wrong, but I was under the impression that you aren't allowed to supply permanently isntalled equipment from a portable generator.
At least, that would not fly over here.

Joined: Jul 2004
Posts: 9,663
Likes: 4
Hondas do not bond the neutral in the generator as a general rule. I suppose some do but the ones I have seen, don't.

I am still not sure why either of them would not run a furnace. Did you get power to the thermostat?

Greg Fretwell
Joined: Jul 2007
Posts: 1,335
Crack up the throttle screw ought to boost the freq. I had a portable gen set hooked to an inverter once and the range of the frequency was tight while the generator was all over the place

"Live Awesome!" - Kevin Carosa
Joined: Mar 2005
Posts: 1,803
A furnace should only have about 300-500W running the fans/oil pump at 120v - and with an induction motor, frequency should not be a problem, unless it's so low it causes motor overheating, [which is not immediate anyway]. It may be under or over voltage rather than frequency - many modern electronic burner controllers have voltage protection both sides of the norm and may also baulk at square waves or spikes. Her's the Danfoss spec -

Wood work but can't!
Joined: Mar 2004
Posts: 251
First, do not assume all are caused by the same problem. Many cheaper generators have poor regulation. I have seen many that will show 120v but low hz when you raise the rpm to get 60hz the peak to peak wave form will exceed what the equipment will handle. Second issue is grounding. Newer furnaces use flame detectors (Flame Rods) that much rely on grounding. Power with a floating ground will cause the CPU to detect no flame and shut down. I have a Honda EU200 inverter generator, it will not operate my furnace unless I ground the neutral side (It has no internal ground) and make sure all including the furnace and gas line is all bonded together.

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