heating?

thanks for your info I have shut down the flow side to the upper rads and it does force the heat down to the next level but if i shut down the mid level rads the only one that gets hot is the small one in the toilet, the one in the hallway and the kitchen remain cold. If I crack open the upper rads a small amount the mid level ones turn warm and do not remain hot. Not sure if there is a bypass valve have you any tips as to were it would be without seeing the setup ?