Honeywell V4073A mid-position diverter valve woes

[Heedyheed]

Apologies for the rather long post....

My Y-plan central heating/hot-water system uses a Honeywell V4073A mid- position diverter valve. I've previously 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 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 whiuch then in turn caused the valve to move towards the mid position again. 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 resultant constant jiggling of the motor cog on the rack would quickly wear out both components. When it's doing this 'hunting' thing at the mid position, the angle of movement of the valve actuator spindle is tiny, but it's enough to cause the microswitch to change state.

I found a couple of documents on the web 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 but 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 when power is removed. I should stress that although the old valve doesn't spring back when power is removed, it does work in all modes - hot water, heating, or both.

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? Also, I suppose, the springs are eventually going to weaken a little on an old unit.

Do you think that's why my old valve doesn't return under spring force when the power is switched off - a combination of weakening springs and some permanent magnetisation of the motor?

Initially I thought the fault with the new valve might be because I had mis-wired it, but I've since checked the wiring over and over again. I know it is right. Besides, it matches my records of how the earlier valves were wired.

So, my question is, is it commonplace for these valves 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

 

[ChrisR]

Have had 3 do that, two were Chinese copies of Honeywell valves, and the third genuine.
They were all int cheap rented flats though & the tenants weren't likely to complain!

 

[MANDATE]

I only have experience of a potterton valve, which is now using its 3rd actuator head.
I would say you are right about the cause of the problem. It is supplied with both the full voltage and the 'modified voltage' and when the microswitch is triggered the full voltage is dropped leaving the modified voltage to hold the quadrant against the pressure of the return spring.
It should not do that, so it is a faulty unit.
A valve should return to the HW only position when the power is switched off. By switched off, I mean no power to the programmer at all.
If you mean the CH is not on and the HW is not on, then that is not the same thing. If HW is not on then you have power from the HW OFF terminal which feeds the grey of the valve and keeps the valve in the CH only position until the demand changes.
The mid position is only temporary till one side is satisfied.
I would say HW usually gets satisfied and stays satisfied for longer periods than CH.
When CH is satisfied the room stat turns of the power to the white wire and in turn turns off the power from the valves orange wire and the boiler stops. The valve does not move, cause its still energised by the grey.
Likewise if you turn off the CH at the programmer (HW already off or satisfied) the boiler stops, but the valve stays there till it's turned on again. And that could be all night or any length of time.

 

[Heedyheed]

Thanks for the responses.

I've left it with the old powerhead for the moment. Funnily enough, it has now started returning to the HW-only position when the power is turned off (by that, I mean totally isolated from the mains - not just turned 'off' at the controller or stats). Maybe it had got a bit stiff through lack of use because the ball and plate assembly had been seized for a while. Anyway, the old head now works exactly as it should.

I wanted to figure out why the new unit was hunting, so I put a mains plug on it so that the thing was energised as if both CH and HW stats were calling for heat. It worked just fine. It went straight to the mid-position with no hunting at all. I powered it off and on loads of times and it worked perfectly every time.

So I put the new powerhead back onto the valve body, but (rather than wiring it back into the CH system) left it connected to the mains plug. Again, it worked just fine.

Finally, I took the mains plug off and wired the unit back into the heating properly.

Guess what... It hunted again, but after a minute or so settled down and seemed to behave normally.

The ONLY difference in the two environments is that in one case it's powered directly via a plug, while in the other it is fed via the CH controls. In the former case it gets a very slightly higher voltage than it does in the latter (due to cable losses), but the difference is only about a volt or so.

Maybe that's enough to make a difference to what is obviously a unit operating on the edge of its limits.

I'm going to send it back to the supplier and get a refund.

Mike

 

[MANDATE]

Interesting! Works one way but not the other!
The difference being the number of terminal connections made.
1 at programmer, next in terminal box, next at roomstat (in) , next at room stat (out), next at terminal box.
Could one of these connections make the difference, or the contacts in the room stat. I'm thinking the voltage may drop if there is a resistance due to poor contacts.
Perhaps the test should have used the same power source (ie programmer) but by passing the room stat by putting a link across in the terminal box.
To do that you have to know the wiring of the terminal box.

 

[Heedyheed]

Yeah, I wondered if it might be a connection problem, but I checked them and they are all OK. If I'd had a variac to hand I would have checked the valve operation at a range of voltages within normal mains tolerance to see if a slightly lower voltage would be enough to make a difference.

Fortunately I know the wiring of the system like the back of my hand. A few years ago I had a new boiler installed in the loft but subsequently discovered that the installers had done nothing about frost protection. So I had to figure out the wiring at the connection box in order that I could add a frost stat and pipe stat.

I checked the voltage difference between the diverter valve orange wire and the incoming supply at the connection block. It was only 0.11 volts, indicating that there is no substantial volt drop via the controller relays, cylinder stat or connection block.

I just think that there are a whole load of factors that will determine how the valve will behave in its mid position, i.e. spring strength, residual permanent magnetism in the motor, the supply voltage, tolerances on the resistors and diode that produce the 'holding' voltage, the stiffness of the valve mechanism, the natural hysteresis in the mechanical operation of the mid-position sensing microswitch, etc. It's surprising that they work at all.

If it had cost a couple of quid I wouldn't be bothered. After all, it does eventually settle and stop hunting after a minute or two. However, at a cost of over £60 I expect the darn thing to be spot-on. It's going back to Screwfix tomorrow for a refund.

Thanks for everyone's help.

Mike

 

[Londoner30]

My last powerhead on my Honeywell three port valve has failed and luckily I had a brand new spare from a previous valve change.

I fitted the new powerhead and everything was fine for a day and then the gear box started slipping.

On investigation the brass cog activated by the Synchro motor was not aligned properly and when the cog moved about an inch of travel the brass cog would slip off the top of the brass cog on the motor.

Some fine adjustments with the biggest slot head later I had resolved the allignment issue however somewhere along the lines I have obviously damaged something else.

The problem I have now is that when the lever which activates the two micro switches hits the micro switches I think the power to the synchro motor is turned off and the two internal springs then pull the lever back from the micro switches which then reactivates the power to the motor which pushes the lever back onto the micro switches and deactivates the power etc.

Luckily this doesnt turn the boiler or pump off so heating isnt affected however what I do get is a constant ticking from the powerhead which I am sure will not last the years expected.

Can someone tell me what mechanism keeps the lever on the micro switches until heating is no longer required.

Should the motor remain powered keeping the lever pressed against these switches until central heating is no longer required and if so can anyone explain why somehow the micro switches being activated is turning off the power to the motor.

I know I can resolve this by getting a new powerhead however there is a strong will from me to hold onto the £60 I would need to spend on a new one.

Hope this makes sense.

 

[matt1e]

Some fine adjustments with the biggest slot head later I had resolved the allignment issue however somewhere along the lines I have obviously damaged something else.

The problem I have now is that when the lever which activates the two micro switches hits the micro switches I think the power to the synchro motor is turned off and the two internal springs then pull the lever back from the micro switches which then reactivates the power to the motor which pushes the lever back onto the micro switches and deactivates the power etc.

first off the micro switches don't operate simultaneously, one operates slightly before the other (I mention this because you have been adjusting things)

Can someone tell me what mechanism keeps the lever on the micro switches until heating is no longer required.

Should the motor remain powered keeping the lever pressed against these switches until central heating is no longer required and if so can anyone explain why somehow the micro switches being activated is turning off the power to the motor.

yes it remains powered but there is a DC component on the waveform that stalls the motor and holds it against the sping
what is wrong with your actuator is in order of probability is

1) the microswitches are out of alignment
sw1 MUST changeover before sw2 0r else you will get exactly the symptoms you are getting
2)sw2 is faulty
3) the PCB is faulty or one of its components (diode or series resistor)

here is a schematic to help you, but my money is on "1"

Matt

 

[Londoner30]

Just spent this evening messing about with switch lever positions. Nothing works. No matter what sequence the switches are activated in the power to the motor is turned off.

Obviously an electrical component issue now. I will continue to put up with the clicking and await its untimely demise then try and use bits from the old to fix the new or its going to be another £60.

Thanks for your help and explanation of how it SHOULD work.