The thermostats in most modern cars do not fail and if they do then they fail in the open position, ie they will not cause the car to overheat (quite the opposite - it means the car will take much longer to warm up). So it won't be that. One thing that you haven't mentioned is whether or not you are constantly topping the water up. The reason that I ask is that you mention that the heater system is not blowing hot air. This is sometimes an indication that your water level is too low. If you are topping it up, then you must have a leak somewhere. Check all the hoses, including the ones to the heater. If there's no leak and the car is not actually overheating, it may be a fault with the coolant sender unit. It may be worth getting it checked out by a mechanic.