I usually have this "problem" with thermostat housings.
I usually spray them with wd40 and let it sit for an hour or 2. Then 2 firm hits with a hammer and using a long wrench to loosen them. When you feel the head turning but without the rest of the bolt (I guess I'm not the only one who can feel that?) turn it clockwise a little and then you try to loosen it again. If the head should come of then loosen or break the other bolts so you can lift the top of and use pliers or a waterpump wrench to take the rest of the bolts out. Without the tension it's usually quite easy.
No need for heat!
