Your thermostat opens ONLY when the coolant passing it reaches nearly the rated opening temperature, commonly 88 degrees Celsius.
When it is shut, more or less NO water passes through the radiator. The whole circulation goes down the by-pass system instead, and round and round it goes. For this reason it is usually a good idea to drill a few small holes in the rim of the thermostat to make a little flow go through the radiator all the time. Others may argue of course.
As the thermostat opens, it does so progressively, and to start with it may allow only a little (very hot!) water into the radiator, but at a slow trickle of a speed. This will immediately displace the same trickle of cool water out of the other end of the radiator where it will mix with the by-pass coolant, cooling it down a bit, and then the cooler mix will go back to the block.
Thus the block should not get hit with a sudden rush of COLD coolant risking thermal shocks. The coolant going back the engine is cooler than when it comes OUT, but it should still be damned hot.
Until the engine gets well up to heat AND possibly driven quite hard, there won't be a lot of heat to dissipate, (this is called efficiency) and the flow through the radiator may be so slow that the return hose will be quite cool to the touch.
Start worrying when the return hose from the radiator is as hot as the input hose.
In round numbers, the energy in the fuel is used in three ways:
1. to drive the car
2. as heat down the exhaust pipe
3. as heat lost to the air by the radiator and the cylinder block.
(Trivial amounts are used to make electricity and work the air-con if you have it, but ALL of that is still lost as heat to the air eventually.)
Never forget, the engine itself acts like a radiator, and a lot of heat is dissipated from it. Thus on a very cold winter's day the thermostat may NOT OPEN at all, and there there is a serious risk of blast-freezing the radiator core, especially in engines with belt-driven fans. If this happens, the radiator becomes choked inside with mushy icy water, or hard ice even, which will not flow, and then even if the thermostat opens there won't be a flow, and the engine may overheat and even boil up. A few little holes in the thermostat rim lets a small but enough flow of warm water to pass into the radiator which will greatly reduce the risk of it blast-freezing.
CharlesY
