If you emptied the system, then refilled, you could have air
trapped at cylinder. You will see two pipes verticly in line
with each other. one about three inches from bottom the other about eight inches above that . carefully loosen top nut off,
a few threads NOT ALL THE WAY or you will get a bath, waggle pipe see if air comes out, if it does bleed till water appears, tighten back up should solve it other thing could be zone valve stuck if fitted.
good luck
gordon
gordon
September 2007