There used to be a growfaq on this. Nic eof you to share.

1. Make sure you don't over do it, because you can burn your plants with too high a concentration of co2.
2. You want humidity at 40%, and no more than 505 in flower. n Higher in Veg.
3. Do not try to use a burnable gas. Although we use co2 generators/heaters in a commercial greenhouse; I really do not think this is a good idea for a grow room unless you have a very big room or greenhouse setup.