Not a silly question at all, very astute actually! I had a big fish tank (a 15' diameter swimming pool) so I just placed it on top of the fish tank inside the greenhouse. With it inside, I never had to worry about it freezing. If I had to do it over again, I would make sure I painted all pex pipe that was exposed black as some of my feeder lines were not painted and I had some trouble with algae growing in the exposed section and shrinking the inside diameter and thus decreasing flow. I could flush periodically, but a little more time and effort in the building stage would have saved me some headache. I had rushed it together due to time constraints and cut corners that later I regretted.
If you put it outside the greenhouse you will have to be concerned with it freezing or have to put it an an elevation where it will drain back to tank. If you can find a way to position it in the greenhouse , it will make it much simpler to use. I do realize that greenhouse space is at a premium, so that is a judgement call the user will have to make.