My grow is in a garage and low temps were bit of a problem, I had the Enviros coming on during the day and off at night.............DUH!
I now have the Enviros on at night which keeps the temps up fine during this period and off during the day. I use a 3kw heater, one of those ones with a built in timer from Argos and have this coming on and off, 15 minutes on and 30 minutes off, during the day which is doing the trick. The radiator is not a solid panel one, but has slots in it and is situated by one of the passive intakes so it draws warm air in through the radiator.
Im my GR piccies you'll see I tried to secure an oil filled rad to the rear wall, however I could only fit it in on it's side and for some reason only the top of the rad got hot. With the extractor at the top pulling this air straight out it made no differnce to GR temps, so it's been removed.
Reckon it'll depend a lot on how big your GR is though and how your garage is situated, you've just got to get started and fine tune your own environment as there's so many variables.
Details of my GR and grow in my sig, good luck mate.