Honeywell mid-position diverter valve woes

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

Reply to
MikeH
Loading thread data ...

In message , MikeH writes

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.

Reply to
Neil J. Harris

The FAQ has this to say about de/magnetisation

"The 270K resistor supplies a small AC current to de-magnetise the motor from the effects of the rectified DC that is used to hold it in the mid-position. Without this, there is some risk that the return spring will not be able to overcome the residual magnetic stiction to return it to the end position."

formatting link
If I turn the power to the system off with the heating demand satisfied

Phil

Reply to
Phil Addison

Thanks for the replies.

I eventually bought a new spare ball and plate assembly so that I could return the complete new valve back to Screwfix for a refund. I then bought a replacement powerhead from another supplier. It all works OK now.

I did ask the question on another board and had a reply from someone who'd had at least two new Honeywell valves behave that way.

To be fair, although it hunted a lot initially, the one from Screwfix was eventually showing signs of beginning to settle down - it only hunted for a couple of minutes each time it moved to the mid position. But at a cost of over =A360 it seems reasonable to expect it to work correctly straight away.

It is a fairly ingenious design, though. Maximum functionality from a minimum of components.

Reply to
MikeH

Some would say that it's too clever by half - and prone to numerous failure modes.

Reply to
Roger Mills

That's true.

I've always assumed that, when replacing this valve, it's better to take the simple option and use an identical replacement - if only to avoid having to hack the pipework to make a different brand fit. Is this a sensible approach, or are there good alternatives to the Honeywell valve that are cheaper and/or more reliable?

In these days when we are supposed to be concerned about wasting a couple of watts by leaving a TV on standby, these valves seem truly profligate in the way in which they waste power ;-)

The Honeywell unit spends much of it's time being held in its last selected position when there is no call for heating or hot water, whereas a more power-conserving design would completely isolate the unit and allow it to spring back to its resting position. Instead it stays parked where it was and gets quite hot - presumably consuming quite a bit more than the average telly on standby.

I should really tidy my system up and convert it from its original Y- plan to a proper S-plan. Since having a small UFH circuit added a little while ago its really a bodged Y-plan at the moment - and the pipework round the valve is a right mess.

But if I don't get round to doing a proper clean-up, what valve would you use instead of the Honeywell next time it fails?

Thanks,

Mike

Reply to
MikeH

I've got a Danfoss 3-port valve in my system - which works ok most of the time. When it fails, it's usually the actuator rather than the wet part of the valve - and that can easily be replaced without needing to do any plumbing. During the 18 or so years for which I had had my existing CH system, the complete valve has been replaced once - when the 'wet' part developed a leak - and the actuator has been replaced about 3 times. I doubt whether Honeywell would have done any better than that.

With regards to power wastage, even if you convert to S-Plan, the valve motors will run stalled all the time when a valve is open - unless you go for much more elaborate valves which are motored open *and* closed.

Reply to
Roger Mills

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.