How big is your room, if I may ask.
It feels so weird that just a small candle can ge the CO2 on such a level.
Did you also notice some higher temperature because of the candle?
I know it is small, but I read somewhere that even a small candle can raise the temperature in a car in winter if it got stuck. That's why I ask.
My basement grow room is 7x9x6.5'h so 410cft. I've been waffling about building an addition to it that will be 8.5x6x7' and used for flowering only. I have a 6' Light Rail that with a 1000W SHPS should be enough to light up the whole space. Only 8.5x4' will be actual grow space so I have 2' of room to walk thru. I'll make a curtain from panda film to act as a reflective wall.
Temperature is not an issue for me down there. Could be tho once I have the new space working with a 1000W and another 1000 or a couple of 400s going in the main room. I don't think so tho.
I got similar results for CO2 in the spare bedroom which is 10x10x8'h.
The burner is sitting in a metal pail half full of sand so if the jar should break, hasn't in 5 years, the flames won't amount to much and it's always sitting in the open with nothing above it to catch fire so I'm not concerned. A gallon of methyl hydrate, aka methanol aka wood alcohol, is $10 and lasts for one grow. I don't use it until I flip to flower and it really helps during the stretch when the plants are doing their growth spurt. During that time I reset my temp/rh controller to allow up to 90F and 80% RH so the exhaust fan may not come on for hours. They also get a good feed of nutes with a half dose of Big Bud a week before flipping to 'prime the pump' so to say. Usually have ping-pong ball sized buds by the end of the stretch.
Eventually I'll get a tank of CO2 and it'll all be automatic. And safer.