These valves can usually be adjusted manually by a lever on the side, which you push against a fairly strong spring. However, the good news is that the motor on top (the power head) can be replaced without draining the system down, so it isn't such a nightmare.
John
July 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.