DO NOT USE INDOORS
Hat Tip to
Who is John Gall @ American Preppers Network
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" .... highly recommend that you not run the generator in a garage.
The generator will put out carbon monoxide (CO). You cannot see it, you cannot smell it, and it will kill you. I have lived in hurricane or typhoon country just about my whole life and have see it many times, first when I was an autopsy tech and now as an ER nurse. Pretty corpses.
If the garage is attached to the house, the answer should be no, period. You mentioned ventilation, but you also mentioned ice storms, etc. What if the ventilation becomes occluded with ice, snow, dirt, leaves, etc? What if a strong wind is blowing right at the vents? The CO will follow the path of least resistance and will probably end up in your bedroom. In fact, the CDC's recommendation (and ordinances in some areas) is that if you have a garage attached to the house you should have a CO detector inside of the house. If the garage is separated from the house, then maybe. Maybe you could safely run the generator in there, but beware that the entire structure could become filled with CO and that is what you will be walking into to service the generator.
Personally, I like one away from the house and other structures, or in a shed with a large door that I can open and allow it to air out. My mother has hers in a plastic Rubbermaid like box, with sides that can be pulled away when you run it so that they don't melt. I keep ours in the garage, but just for storage. After the storm passes I haul it into the backyard and build a shed around it with cider blocks and a plywood roof. I drove a big spike into the ground and chain the generator to it to discourage a 5 fingered discount."
MORE:
"The back porch might work, but you still have to be careful. Even having it in the open air is not always enough. I will admit that most of the people that I have cared for with carbon monoxide poisoning secondary to generator use had the generator going in the somewhere in the house (to include the garage, or my favorite: in the room next to the kids bedroom--fortunately all of those kids got better). However, I have read of families being poisoned by having their generator outside but too near their air conditioner. Through the night the a/c intake pulled in the fumes. Same with the intake of other ventilation systems. The CDC had an issue of the MMWR all but dedicated to carbon monoxide a few years ago, and they reported on a number of water skiers who sustained carbon monoxide poisoning while being pulled behind a gas powered boat! Generators and other internal combustion engines are great, but they can kill. Gotta use them with severe caution."