So I use a burner in a (fairly) sealed flower room and natural vent the veg and nursery. My excuse is I can run stupidly hot temperatures without stretch. I am somewhere hot. It behooves me to be in production when my competition is laid out over power bills. I did avg 3lb on 4x8s as well which was pretty good given my halfassedness and learning curve. CO2 is situational. And I sure wouldn't run it in a dwelling. I've had mine hang up and go to 5000ppm before alarm and wifi got to my phone and alarmed. Fire is definitely possible. Death is possible. But I mean what part of agriculture isn't dangerous.
So to review, if you have a nice tight place its a good investment, you can run up to 88 degrees, and have humidity in the 80s and actually have em booming. (I'm at 84 degrees on 75% rh atm, spiking to 86 on 82 as A/C cycles) I will attempt to lower Humidity as I go, but high humidity keeps those stomata wide open and sucking in my Jurassic global warming buffet. at 1500 ppm. My stems are nickle size and larger. They love it. I love it. Given the right circumstances, you'll love it. I call it "growing weed in the bio dome."
Speaking of Bio Dome, I got a buddy sealed tight in a block and concrete room with some exposed dirt floors, he has constant CO2 elevation from the concrete/microbes. Sounds crazy, but read about the original Space ship Earth, biosphere 2, you will read something about concrete and or microbes boosting co2 levels.