It seems the valve is defective. You may not have a temperature difference right at the valve even if it is not opening 100% but you will have an amount of water or GPM lose that you cant tell without gages and thermometers. Because it is not turning the circulator or boiler on tells me it is not opening fully to make the end switches. Having something stuck in the piping after years and years is highly unlikely.
Replace the valve
Hope this helps
This type of valve doesn't have any end switches, just 2 wires to open the gate. Thanks for your advice anyway because you were right, I just found swaping coils that my bad valve coil was partually shorted and never lifted the gate fully.
Unfortunately "Electroplastic Inc" doesn't exist and to get new coil will be a big problem but I'll be able to make it myself as a quick repair before switching to new AC valves and controll box. Thanks again it was helpfull.
"Atlantic Kenmark" - motor rewinding shop onXXXXX North Alington, don't have ph.# XXXXX me, they charged $150 because they had to manufacture new plastic core for it.
The new one works O'K having 25 Ohms instead of 28 so they can make little more turns because there is a space for it inside the metal capsule.
Try( (XXX) XXX-XXXX as fare as I remember. Took them 2 days.