Thermostatic pump overrun temperature?

[ShaunCorbs]

does the head (the part that removes to get at the insides eg synchron motor) has a dimple ?

 

[brian_mk]

does the head (the part that removes to get at the insides eg synchron motor) has a dimple ?

There is no dimple so I'm pretty sure the head cannot be removed without draining the CH system.

I suspect the shaft may be jamming and/or the ball is not seating properly, so the valve body almost certainly needs replacing.

If you look at the earlier photo of the inside of the valve, it would appear that you can remove four screws and replace the top plate, valve ball and drive shaft from a new unit without having to disconnect and replace the main body. Obviously the large O-ring would need to be swapped over from the new unit. Has anyone tried to do this?

As already explained, in my situation there is little free movement available in the pipework. Hence replacing the entire valve would involve draining, disconnecting and shifting the HW cylinder to allow the pipes to be removed from the valve compression fittings. That would involve a lot more work (See the photo below).

 

[ShaunCorbs]

Yea no dimple so 4 screws so not removable sorry to say

you should be able to remove that

loosen all joints

undo the back pipe, push the pipe out of the 3 port spin the body so it’s out of the way

undo left and right nuts

push left pipe out of the valve and at the same time remove from cylinder

if you can don’t remove the cylinder connection

 

[brian_mk]

I've ordered a replacement valve.
In the mean time I re-checked the operation of the suspect valve.

The shaft definitely turns and it returns to the end stop under the force from the springs when no power is applied. In that position, hot water from the boiler should feed the HW cylinder only.
I've tried setting the programmer to HW only but the rads still get hot.
It suggests the problem may be the rubber ball inside the valve. Has anyone taken an old one apart? Does the rubber harden and no longer provide a seal? Does the rubber disintegrate?

 

[ShaunCorbs]

Sometimes they fall off or yes they wear and can let water pass

 

[brian_mk]

The HW cylinder bypass gate valve is cracked open about 1/2 turn.
If the valve is open too far, is it possible for hot water to flow through the bypass valve and through the rads in the opposite direction to the normal flow? i.e could the problem be due to 'Reverse Circulation'?
If that's the case then it could it be that the 3 port valve may not be faulty after all.
I need to prove this before I go to the trouble of replacing the 3 port valve.