I'm going to have a NG unit heater installed in my garage sometime before winter. All of the units come with explicit instructions to have them installed by a licensed professional. My question is, why is a 60K btu heater more dangerous than if I turn on the 4 15K btu burners on my NG stove? Aren't the combustion products the same?

FWIW I'm not questioning the danger of CO from the heater, but more curious what the difference between the two is.