my friends who grow in low temps run their lights at night, then run a small space heater with a fan in the day (night time for the plants, 10x15 foot room), and they run CO2 generators as well. The CO2 generators can greatly increase yield (if used correctly), and because they are propane or natural gas burners they also generate significent heat. They also recycle the air in room so that there is very little fresh and cold air coming in (hence the CO2 use to compensate).

I always have to deal with trying to make everything cooler. It always amazes me that there are people out there who actually have to heat their reservoirs. I always found it easier to heat stuff then cool it.

One guy I know had such a problem with cold temps that he reversed his fan that was hooked up to his lights, so the fan actually exhausted out the hoods right into the grow room instead of intaking through the hoods and exhausting out of it.