I'm in the camp that they are fully feathered out at 4 to 5 weeks and have never provided heat after 5 weeks.
I don't know the conditions in your garage. I'd be real tempted to not give them heat during the day and see what happens if you don't give them any heat at night. Probably not much but then your garage is probably pretty warm compared to the outside temperature. With 25, they will sleep in a pile with or without a heat lamp so they will keep each other warm. They are not piling up because they are cold. They are piling up because they like each other. If they are cold, their distress peeping will tell you.
If they have never been without light, they may be scared of the dark and protest the lack of light, nothing to do with heat. I think it is a good idea to leave them in the dark for an hour or so before you totally take away the light. Otherwise, they may give you the wrong message when you do put them out.
Good luck!