vw passat tdi pd -always cold water in the header tank?help

[scandal]

hi guys
i'm new here,as never seen this site before,although had quite a few passats before.
anyway,i've got a problem and any help would be much appreciated
i've just bought another a vw passat tdi pd 2002 reg.and i've noticed a quite strange thing about it-water in the header tank is always completely cold.it doesnt matter how long you drive it,even after 2 hour journey its cold.everyone would say a water pump gone,as its a common fault on these.but i do get a very good heating inside of the car,it blows hot very quick when you start driving,which in my opinion would be against the water pump failure.i've tried taking the return pipe off,and water is coming out,it splashes really well when accelerated.also the top pipe from the radiator is hot,but the bottom 1 is cold.i've drained completely all the water from system and dont think there are any air traps left.but it didnt help at all.
the car doesnt show any overheating aswell.temperature on the dash goes up to 90 and stays there all the time.
any thoughts or ideas would be greatly appreciated.thankyou very much in advance
regards
scandal