Not that difficult, I'm working on a FAQ for this very topic. I've built mine for about 40 bucks that controls fans and co2 based on temp and 8001 controller. Co2 shuts down at lights out, fans only come on to lower temps, and also switch off the CO2 not to waste it. about a 7 degree swing before fans shut down.

Very simple design. you can use a regular thermostat from Lowes (fucking hate HD.. they have shit) (mechanical), 12V relays, 12V transformer from a POS phone you were going to throw out, PC board to hold it together, a light switch from Lowes to control CO2 to a 120V relay, throw in a few lamps to draw down shorts on the relays, a couple of switches (main/CO2 shutoff)... and a FUSE incase you fuck up! Don't want any fires!

Should have the FAQ up with instructions in a couple of days. Mines works great, tested with a ceramic heater in grow room for a few days, cycled just fine. Fun little project that goes well with the home made Krypos controller and the 12 buckets. Just waiting for the tomatoe clones to grow a bit before going into the 'pot's.

Watch for it in the Advanced section.

Negril - Damn rock cliff sheared off at Ricks. Not the same place it was.