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I have a portable generator for my standby power connected to a sub panel through a manual transfer switch supplied by a breaker in the main panel. I have separated the neutral and the ground at the sub panel and the transfer switch--they are only bound at the main panel. The problem I'm having is the furnace will not operate on the generator but works fine on normal power.

The furnace model is a GOODMAN GMVC950905DXAB. The generator is a GENERAC XG8000. The thermostat is a Honeywell RTH5160 which uses batter power and only one 15AMP circuit to the furnace, which measures 115.7 volts.

There is a circuit board on the furnace with a 2 digit readout on it that puts out an 88 code that has no definition on the troubleshooting area, but there are two LEDs at the top of the circuit board. One is marked RX and the other is marked status. The RX blinks on generator power but not on normal power. The status LED blinks ounce and then goes out. Normal operation then OP comes on in the readout and the furnace starts to run.

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  • The make and model of the generator and furnace would help us better answer your question. Some generators put out "dirty" power that control boards don't like. Some put out a "modified sine wave" which has a stair step pattern on a scope. – George Anderson Jan 13 at 20:14
  • Some additional information about your generator and how you are connecting to your home's power would be very helpful. – jwh20 Jan 13 at 20:16
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    Welcome! Please clarify what "will not operate" means. That gas won't light and remain lit? The blower motors don't come on? Power-on LED indicator on the control board never even lights up? Etc. – Greg Hill Jan 13 at 20:30
  • yes 115.7 volts enough to operate the furnace I'm new at this site so I'm not sure if I'm to answer a question its a goodman furnace if that's any help – steve Jan 13 at 20:32
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    In the regular panel turn off every breaker except the one for the furnace (and the main breaker). See if the furnace works. If it does, then "not working on generator" may have something to do with quality of power (waveform) or maybe a grounding/neutral issue. But if it does not work, then start turning on breakers one at a time until it does work, and then you'll know what other circuits need to be on the generator to get the furnace to work. – manassehkatz-Moving 2 Codidact Jan 13 at 22:37
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Your portable generator has ratings for starting watts and running watts. When you first turn it on, it starts with a higher number of watts then reduces to the amount of running watts. I am wondering if there is a mismatch in watts needed by the furnace and those provided by the generator. Make and model might help us discern this. 8000 watts might just be the starting level, while the generator provides only 5000 (arbitrary number) watts as running watts, underpowering the furnace.

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    I seriously doubt that they're underpowering the furnace to that degree -- normally, you'd see voltage and frequency sag in that situation – ThreePhaseEel Jan 14 at 12:27
  • That's not what starting/running watts ratings on a generator mean: "running" is the rating for the amount of current that can be continuously drawn from the generator. "Starting" is the amount of current that can be provided for a short time, the absolute maximum the generator can produce, for a short time. This matches the language used on electric motors, which give "starting" and "running" current draws. – reedstrm Feb 19 at 22:51

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