When my car did that (blowing out of the tank) it was a blown head gasket. The temp gauge never went high and there wasn't any coolant in my oil either.

 

Hope this is not the case

 

T- stat could haVe been problem, but when you changed it out it sounds like you've got air trapped in the system, especially since you aren't getting heat! You should try to find the highest point in the system & open it up to see if air bleeds out.