Sticky zone valve

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Long story short, I traced down to this zone valve problem. It is stuck open, which signals both circulation pump and boiler to run. The weird thing is when I took off the head (the actuator and spindle part), I can't even manually rotate (clockwise) the valve back to close. If I rotate it anti-clockwise a bit then it is free to go any direction. So that means the actuator is fine, it is able to rotate the valve to open position, but once the pump is running it is no longer be able to rotate back to close. I don't quite understand the structure inside the valve, I thought it is just simply a valve with a rotating shaft. But it seems the water flow can make a difference?

The model is Danfoss HPA2, any help appreciated.

John
 
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Nothing to do with water flow. Either replace the internals of the valve, kits are available, or replace the whole valve for something more reliable. Either way, a bit of a drain down is required.
 
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