Apologies for the rather long post....
My Y-plan central heating/hot-water system uses a Honeywell V4073A mid- position diverter valve. I've had to replace this unit once or twice in the 35+ years that I've lived here and on previous occasions I've had no problems.
I recently found that the valve had stuck in its mid position so, whenever the boiler fired up, I was getting hot water and heating.
I ordered a new V4073A from Screwfix and fitted the ball plate and powerhead assembly from the new valve onto the old body. I didn't want to replace the complete valve because the pipework is a tangled mess, so undoing the unions and getting the valve body out would be a nightmare.
As it turns out it was the ball and plate assembly that had seized. but I replaced everything except the body.
On powering it all up I found that the new powerhead was 'hunting' when required to move to the mid position. This resulted in a regular 'click-click' every second or so as the mid-position sensing microswitch was activated, but the valve position then moved back slightly - enough to de-activate the microswitch. This cycle continued for as long as the valve was required to be in the mid position.
I'm pretty sure my other 4073A diverters have not done this. I suspect that the constant jiggling of the motor cog on the rack would quickly wear out both components.
I found a couple of documents that describe how the valve works. My understanding is that in the mid-position the valve is fed with a voltage (via a diode and a couple of resistors) that has a DC component. This stalls the motor in the mid position and provides it with enough energy to balance the effect of the springs trying to return it to the normal 'central heating port closed' position.
From what's happening on this new valve, it appears that either the spring force is too strong or the stalling current is too weak to balance the valve in the mid-position and hold it stationary. The result is that it keeps moving slightly back from the mid-position causing the microswitch to apply full power to the motor to restore it to mid-position, and so on.
I put the old powerhead back on the valve and it works fine. It moves to the mid position and there is no hunting at all. The only thing that is different to the new one is that it doesn't spring back to the 'central heating port closed' position when the power is turned off, whereas when I tried the powerhead from the new valve it always returns (under the force of the springs) to the 'central heating port closed' position when power is removed.
From what I've read about how these valves work, it appears that the DC holding current is designed to be low in order to avoid permanently magnetising the components of the motor. It struck me that an old unit may inevitably become slightly magnetised - with the result that it would become harder for the springs to pull it back to the normal powered off position. Presumably the same effect might reduce the likelihood that the valve will 'hunt' in the mid position.
Initially I thought I might have mis-wired it, but I've since checked that the wiring is correct over and over again. I know it is right.
So, my question is, is it commonplace for these valve to hunt in the mid position when new, and do they eventually settle down to work normally?
Or have Screwfix sent me a faulty unit?
Thanks for any advice you can offer,
Mike