Chrysler sebring 05 overheating

I have replaced the thermostat as the tech told me it has been removed by the first owner. When driving it seems to overheat really fast as well as loud boiling in the filler box (overflow). Radiator fan or fans are working well. Radiator has been flushed and water flows well from end to end with horses disconnected.

Turn the HEAT on Max.
1) Are you getting Heat?

Any shortage of coolant can give a false indication on guage, thermostat wont open, causing to over heat again!

Air pockets in the coolant system won't circulate the coolant.

Thank you for your answer. When I put the Heater on, it Heat very well. The technician who changed the Thermostat said the radiator is the problem but I don’t Believe it because water flows well from end to end and the air from the fans flows well too. On my V2.7 engine, the thermostat is under the engine. Does that means the antifreeze will flow from the bottom to the top of the radiator? If this is the case, I think Thermostat stuck closed. Because I took out the hose at the bottom of the radiator. The one coming straight from the thermostat and engine overheated and no water came out from the horse.

Finally I have got my car fixed. Technician who changed the thermostat mounted it in reverse. Side of the Thermostat that should detect overheating water in the engine was facing the radiator. And the guy told me I have to change the radiator and requested $400 for it. He took me $50 to flush the radiator to see if that works. Finally I took the car to another garage because I was pretty sure the radiator wasn’t the issue because I took Radiator hoses down and water was flowing perfectly from end to end.
All the new technician did was to mount the Thermostat correctly and no more heating.
If you leave in Humble, TX area, never bring your car to CMC Auto Care on 1960 Bypass Rd. Any time I go there I see customer complaining about having to bring their car back over and over for same issue.