We use those sunflower heaters in the fish houses in MN all the time and they don't burn up the oxygen as fast as you think.
It's not the lack of oxygen I am worried about, it is the accumulation of either CO2 which could lead to hypercapnia or CO2 poisoning, of CO which would lead to CO poisoning.
You can have a good supply of oxygen, but if the CO2 doesn't have somewhere to go, like in a garage sealed to keep the heat in, it could have harmful consequences.
Safer just to to some loading stages indoors and the seating/crimping in the garage.
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