I believe I have a faulty zone valve or thermostat. No heat to the baseboards serviced by this zone valve. With the thermostat "on", the manual lever on the zone valve is floppy. It appears that the valve is stuck "open", yet there is no heat going to the zone. I manually opened a valve that is working correctly and noted the position of the arm on the valve stem. When I look at the valve in question, this arm looks like it's gone past the normal open position. The copper pipe into and out of the valve is hot but the return pipe is cool, no heat. Now, I shut off the thermostate by setting it to "off" and the arm on the valve goes back to it's normal position and the manual lever works as it should. I used the manual lever to "open" the valve yet there is still no heat. Any suggestions on what might be causing the problem? Thank you in advance!

Neither the stat nor the valve are the problem unless the valve isn't closing the end switch to turn on the circulator.

What type of thermostat do you have?
How many zones?
Other zones working properly?
Is the zone in question possibly a second floor?
What is the pressure on the boiler?