Phear: I've found that mine work anywhere I'm physically comfortable. I use mine indoors and I usually keep the house at around 70 degrees year around. It seldom drops below 65 degrees in the winter or goes above about 75 in the summer. I've yet to have a problem related to temperature. There was one young man in one of the forums who covered his planting deck with aluminium foil and then complained that his reservoir temperature soared into the eighties. I haven't done that as I use the plugs Aerogarden sells to close of the extra holes so I've had no temperature problems. I hope this helps.
