Did that yesterday ... that's why I was firing up the Honda to test.
I have a new plan. If power goes out I am going to shut off the main
breaker and then backfeed the generator output through a 15 amp outlet
that's in the shed. It's on the same branch of the split 240v house
supply as the furnace and a couple of rooms. All my lighting is LED, so
that's a tiny load. The generator will run those plus the furnace
system with no problem and I don't need to have extension cords running
anywhere.
I would trip the 240v breakers if you do that. You can back feed the
other side through the load if you don't and that is not a good thing.
You may not actually have any 240v stuff tho if your big stuff is
fossil.
That goes without saying but maybe I should have mentioned it. Besides
the main, I will shut off *all* the breakers including the 240v AC,
water heater and stove. Then, I'll turn on the 15 amp breaker that
feeds the shed, the 15 amp breaker that feeds the furnace and one or two
others *if* they are on the same leg of the 240 service.