At the moment am using a small 800w Bio Green gas burner outside of the room pulled in through ducting. It is very much DIY.
I have a 10" fan set at 25% pulling out just to keep the smell going through a filter. The problem is I have to switch it on manually every day. I also have another passive 8" intake through a long stretch of ducting just in case it gets too hot in the summer, but this is not often in North UK and I run the room at 29 degrees.
It runs steady at around 1100 ppm and costs around £50 per grow 8 plants, running it for 7 weeks on a 9-week strain. I increase the fan speed during week 7 until it goes down to 700 ppm, a bit of a waste but I only have on or off. The increased heat and humidity suit me as I grow in cold outbuildings and I like the humidity at 75% mid-grow.
One of the reasons I decided to run Co2 was because of the cold. To get anything like normal Co2 levels I had to have the fans on full intake and outtake which made the room freezing even with a 2kw heater going all the time with HPS lamps. I struggled to keep temps up to 23 degrees and it worked out very expensive for electricity. I now run Migro Array LED.
I generally pull between 75 and 90 oz dry per grow 8 plants from seed but looking for 100oz when I get a decent mother.