2002 Ford Taurus

What is a release valve?

Radiator drain valve is down near the bottom of the radiator, usually driver's side. Big plastic wingnut type can be turned by hand. Why did you replace the thermostat? Was the car overheating? No heat is usually a blend door issue, clogged core, temp control issue(knob or digital), sometimes, also not enough coolant and resulting air in system.

So the car was running really hot or overheating? You can flush the core yourself by unhooking the inlet and outlet hoses and rigging up a direct garden hose connection to it, this is the recommended way as the design of the hose system sometimes will not allow a complete flushing of the heater core by utilizing the factory setup. There is a bypass hose that can cause an incomplete flush, direct hookup to the core is recommended. Try it first. Are you sure if the blend door is working? Under the dash in the plumbing for the heat and ac distribution system

Might also be known as the a c air temperature control door

Is your car running hot? If yes then I would be concerned with the radiator or the thermostat. If it's not running hot and you don't have heat, flushing the radiator means squat. The heater core could be partially clogged or the a/c electronic blend door actuator may be operating incorrectly, or not at all. I would recommend you get some type of repair manual, Chiltons maybe, something that has the heating/cooling system repair in it. Did you directly flush the heater core as I suggested? You may have to jury-rig the flushing hose and drain hoses, not hard to do. If you have not done this yet, it's what I would do next

I knew I've been forgetting something, in instances of cars with higher mileage, there have been cases where the waterpump impellers have worn away thus not much movement of coolant thru the system. Have you noticed if your car tends to heat up, temp-wise. In traffic(more then normal) and cool down when speed picks up? Obviously in hot weather this would be sort of a normal occurrence.