You're welcome. I have my box fan on a timer and it runs mostly at night. When you do run your heater also run the fan and set it up so it's like mine - fan a bit higher than the heat:
My heater is a forced fan milkhouse heater and is on a portable thermostat and is set to go on when the temp gets below 40 degrees. You can get the thermostat here:
http://www.kkontrols.com/poth8...
You might also consider using passive solar heating. It's simply 2ltr soda bottles painted black, filled with water. I'll sit my extra tropicals on top of them and it will keep the soil temperature about 5 degrees warmer than everything else. They collect heat in the daytime and release it at night.
As to Spider Mites, what you want to look for is miniature spider webs surrounding leaves. When they start to show up it's an indication that there is not enough humidity. I usually start seeing them in January so when the humidity starts to get low, get to misting everything daily.
GH's are a LOT of work to run and maintain but a must for people like me that love the tropical plants!