Does this zone valve need replacing?

[Owl]

I've got a heating zone that is getting hot when the zone thermostat is not calling for heat, but when another zone is correctly calling for heat and thus creating hot water in the system. The zone valve is a Honeywell V4034H (normally closed), new about 15 years ago. I have taken off the motor part, and found that the valve spindle can be easily turned - through about 40 degrees - with fingers.

I think this suggests the valve may be faulty in that it is not closing properly. Would I be doing the right thing in draining down the system and replacing the whole valve? Or is there anything easier to try first?

[The Natural Philosopher]

Test by turning it off with motor part removed: If tat fixes it all you need is a new motor bit. A lot easier than a drain down.

[Owl]

Thank you for your comment.

If I manually turn the spindle to the closed position, with the motor part removed, the valve still lets water through.

I understand that the internal valve mechanism is a rubber ball on the end of a pivoting arm. I don't know if the motor part has a spring part which holds the ball firmly against its seat, when in the "off" position. If it did, then trying the zone valve without the motor part in position would not be a good test. How else is a firm seating obtained?

Honeywell sell a kit which enables the ball to be replaced. The kit costs about ten pounds, a new valve about fifty pounds. But then if it was the seat that was faulty, and not the ball, then using this kit would be a waste of time. And I do not see any way of knowing which is the faulty part, ball or seat, without taking the lot to pieces. It looks like it's got to be a drain-down and a new valve.