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Gas furnace plugged into 120V solar/wind generated power bypasses the thermostat control and starts immediately. Why? Answered

The sine wave generated by my 12v to 120v inverter scopes even better than the grid current.  All other appliances function normally on off-grid power.


Are you still sending power to the thermostat? Are you taking everything off the grid? If so then don't for get that the thermostat still needs to be powered also and not all hvac units power up the thermostat.

The solar system is intended for emergency power only. Normally the furnace is plugged into a grid circuit. During a power failure I just unplug it from the grid and plug it into the solar circuit. The thermostat automatically receives its power from which ever circuit the furnace is currently plugged into.

Thank you very much for your reply.

This is really interesting I learned a lot. Does it matter what brand of furnace you use? Would a coleman gas furnace work better?

Does the furnace need constant stand-by power, is this a case of it tripping-on simply because it's been powered-up?


Thanks for the reply. The furnace apparently does not require constant stand-by power because when the thermostat is not calling for heat I can unplug the furnace from the grid and then replug it back to the grid without it starting.

Do I understand that you unplug it from the grid, plug it into the 120v inverter and it's that (plug-in) action that causes the thing to start?


Thanks for your reply. No, it's not the plugging in from one to the other that causes the problem because the furnace shuts off again when I re-plug it into the grid.

. This just keeps getting curiouser and curiouser. So all you do is unplug from the grid and plug into the inverter? No other re-wiring?

. Are you sure the furnace isn't 220V?

. Does it stay on or does it start cycling normally after a while?

. Have you double-checked the output wiring of the inverter? Maybe the furnace won't work with the hot and neutral swapped.

. PS: I can't imagine the system being all that sensitive to dirty AC (especially if you have a mechanical thermostat), but I guess it's possible.


Thanks Rick. I know what you're saying. My son, who has 20 years experience as an electronics tech with the Canadian Air Force, has checked it out several times and it's a complete mystery to him also.

You seem to know what you are doing with electricity, so I can only assume that you have checked the thermostat itself. It's the only thing I can think of If everything else is as you say. It's broken, stuck, set wrong, wired incorrectly.

.  +1.
.  My first guess is that you have accidentally bypassed the 'stat. Second guess is that the 'stat is bad.

Thanks for the reply. Since the thermostat is wired directly from the furnace wiring it is automatically powered by which ever power source it is plugged into.

Thanks for the reply. The thermostat works properly when grid powered so I can only conclude that it is not broken.