whats the min starting temp of a preheat lamp? i have 2x F15T12's running on preheat and 1x on instant start and its min temp is -20 

I would imagine it depending on what lamps you use and what ballasts you use...

Others have said that preheats are very reliable down to the freezing point. Below freezing, things can get a little unreliable.

My experience is that they start no matter what. I have several different preheat installations at my house (house, garage, and barn). I've never had a problem using them in the Winter and I've definitely had temperatures below zero deg. F (well below freezing). The tubes will be dim, but they'll start. Weaker tubes might be the only ones that have trouble. I have several 2' preheat lights in my unheated, uninsulated barn. One unit has a comparatively weak ballast with older tubes. It might blink longer than the other lights, but again, it still starts eventually.