I have a iron radiator on the second floor that the hot water won't flow through. I have opened the valve and hot water does flow with the valve open. No air in system. How to I open the return valve so the water can flow through. I loosened a nut and backed out the screw as far as I could but the water still doesn't seem to be flowing through. What am I doing wrong?

TED.. I am unclear... iron radiator (check)... forced water system (check).. water does not flow through (check)... open valve water flows..?

Do you have two shutoffs associated with this radiator? Is the coin or key vent working? Does water flow from the vent when the vent is open?? Get back to me with answers to questions.. and any more details you can and we will see what we can do to help.