I have a Y-plan heating system with a Honeywell mid position valve of indeterminate age. Lately, I've noticed that, on cold mornings when the heating is working hard, the domestic hot water becomes much hotter than normal (I have the cylinder stat set quite low). From fiddling with the thermostats and feeling pipes, It seems that the valve isn't able to completely close off either the CH or the HW. It goes most of the way, but the "closed" leg still has a small flow.
Is this normal, or should I be able to get a total shut-off in both directions?
It should manage a complete shut off for either one and be able to provide some flow to both at the same time. Is the actuator moving the valve all the way, or sticking at part way round?
Try taking the actuator off and moving the valve by hand. It should move with just slightly more than the grip you can get between finger and thumb.
Thanks for the confirmation. I wouldn't have wanted to change it, only to find the new one was just the same.
Difficult to tell. I've peered in at the works with the lid off. It seems to run at a constantish speed and then stop suddenly in one of the three positions, regardless of whether it's the motor or the spring that's doing the driving. I get the impression that it's positively stopping rather than running out of puff.
I think it'll be difficult to get the head off and leave the valve in situ. It's been installed very close to the airing-cupboard door-frame
- there was quite a lot of metal bending involved, just in getting the lid off.
Still, I think I'll just change the whole thing. It's at least ten, probably twenty, years old. If I can be sure the new one will shut off completely, I think it'll be worth it. Aside from the hot water problem, I bet the current one wastes loads of gas heating the radiators in the summer.
That's an interesting point. The rads all have TRVs and there isn't a bypass. I've worried about it in the past, but then the kids broke the head off one of the TRVs, so I stopped worrying.
I guess it could still be a problem, but the extra-hot water thing happens when the house is cold and the TRVs are mainly open. Also, the valve seems unable to shut completely off in either direction, not just on the HW side.
The motor drives against the spring until a micro switch pens, the switch removes power to the motor - at which point the spring ties to return the actuator back, which then closes the switch powers the motor. Basically it should oscillate back and forth.
Usually a couple of screws inserted from below, hold the actuator head on to the valve.
Changing the actuator takes a few minutes and it the most common cause of these problems - you can buy just the actuator for £25 to £40. Changing valve is much more involved. You need to drain down the system to change it and usually they don't fail.
As said, start by taking the head off and manually turning the valve. The range of positions ought to be obvious by looking at the actuator. It should turn smoothly and easily through the complete 360 degrees. It should also control the flow directions. If it passes those tests, then there is nothing wrong with it.
What - oscillate continuously? I didn't think they did that. This one never has. I thought there were three rest positions:-
Motor off, spring pushing the "bung" against one valve seat.
Motor on, stalled against the other valve seat
Mid position, motor held stationary with DC via a diode
This one has four, inserted from inside the cover. Two of them are inaccessible without removing either the valve or the door-frame. Actually, I might just be able to fiddle them out by fabricating some kind of cunning tool but even then, I'm not sure I can move the head far enough away from the valve to clear the spindle. It's a masterpiece of "I'll never see this job again" installation. Also, I'm not sure how easy it'll be to get a new head for this particular valve.
It's a pain, I know, but in this case, just changing the head has a lot of ball-ache about it too, and a significant risk that it won't cure the fault. It's probably about time I gave it a bit of a flush anyway.
The one I fiddled with in a shop just moved through around 30 degrees, and needed some torque applied to close the valve at either end. I don't think I would easily be able to get any meaningful results from trying to test the valve without the head, with only pipe-feeling to test for flow.
Normally, I'm all for doing loads of testing so I can isolate the fault completely and change the absolute minimum of parts to get a system working again. In this case though, I reckon I've gone as far as is practical before just changing the whole lot.
1 "Motor off" actually has a small leakage AC current supplied to the motor to kill any residual magnetism resulting from position 3, but insufficient to drive it. If you just remove the power from a valve in mid-position, it can otherwise stay there until it's touched.
If the primary water has collected some sludge over the years a bit can stick to the ball in the 3 port valve. This will cause the valve to not shut off 100% in either or both of the end positions.
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