In message
,
MikeH writes
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

The V4073a in my airing cupboard is on its third motor, and the
insulation card inside is cracking at the bends and the resistor's gone
black in the middle and you can't read the value anymore, but it's 28
years old, and I remember working out how it worked years ago, it struck
me as an ingenious design,although DC injection is a common method of
emergency stopping machines in industry. I think your Idea of an
inadequate DC holding current is correct, but I can't remember the
circuit in detail, and since you can buy both spare motors and
powerheads it doesn't seem likely that the springs are different.

If I turn the power to the system off with the heating demand satisfied
and the DHW programmed "off" the valve runs back.

If you take the aluminium cover off the syncron motor you can turn the
rotor by hand.

--
Neil J. Harris