Help! CH only works when HW is on

My central heating only comes on when the HW circuit is turned on. With both off, if I turn on the CH circuit via the programmer, the motorised valve turns on and winds open but the boiler does not fire up. If I then turn on the HW circuit via the programmer the boiler fires up and heats HW and CH.
This used to work perfectly until recently. Nothing has (knowingly) been changed since it was working. There was a room thermostat but I bypassed this during renovations by joining both brown wires (L and SL) and insulating the earth and neutral. This was bypassed long before this fault developed.
I have a Potterton Suprima 60 boiler for CH and HW which are individually controlled by the programmer. I have a Myson motorised 2-port valve at the boiler (for CH) and same next to the upstairs cylinder.
The boiler and programmer are about a year old, the valves etc. were installed by the previous owner.
A picture of my boiler can be seen here
http://84.203.164.145/boiler.jpg
Does this sound like an electrical or perhaps a PCB problem? The status light does not change colour when I turn on the CH alone. Another thing that's weird is the reset button doesn't seem to do anything, on my old boiler this used reset the boiler if I pressed it when running.
I'd rather not have to get my plumber out if I can avoid it so any advice would be much appreciated.
Cheers, Chris.
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Your two port valve is stuck.
Andy
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Actually, belay that diagnosis, I didn't read your post carefully enough :0(
Andy
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On Tue, 16 Jan 2007 22:39:05 +0000, Andy wrote:

I think your original diagnosis is correct the 2 port valve is stuck half open, enough that the rads warm up but not enough to close the conacts to fire the boiler.
As for the reset problem it's a Suprima. I'd get buy a recon'ed PCB in in readiness if I were you.
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Ed Sirett - Property maintainer and registered gas fitter.
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Ah, well I've learned something there: I wasn't sure about the way the two-port valve signalled the boiler it was allowed to fire.
cheers
Andy
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With
motorised
status
thing
old
enough
half
to
You should have a lever to manually open the valve. See if this fires the CH when the HW is off.
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Not sure about Myson but on a Honeywell 2 port that does not work. Only the synchron motor can do that and a failure mode I have experienced is that a failing synchron motor opens the valve but does not operate the contacts.
Jim A
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In an earlier contribution to this discussion,

Are you saying that the motor (when working properly) moves the valve *further* than the lever does - and that the lever doesn't move it quite far enough to operate the secondary contacts? Interesting!
To clarify what I wrote earlier about the secondary contacts being duff, requiring the actuator to be replaced - as others have pointed out, there is also the possibility that the valve isn't opening fully but that the contacts are ok and *would* make if the valve did open fully. This could be due either to a problem with the motor or to the mechanical part of the valve being partially seized. So it needs to be bottomed out before rushing out and buying bits unnecessarily.
--
Cheers,
Roger
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wrote:

Can't say *exactly* that. Would have known when I investigated the problem. Can't remember the internals but I remember the valve was operating freely throughout its travel. It may just be the contact travel that is different. May take a look later. The effect was definately as I describe. Got me the first time but not the second time. Circumstantially I have a suspicion the synchron motor "failure" was cause by switching transients on the incoming mains.
Jim A
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On Wed, 17 Jan 2007 16:16:15 +0000, Jim Alexander wrote:

It depends on the make.
The Honeywell ones won't close the contacts with the lever, but if you move the lever quickly enough (but not so hard the gears slip!) the valve can be briefly thrown to the contact point. This can serve of something of a touchstone on the health of the valve: if it can do this, it is not jammed.
The Horstmann ones will close the contact with the lever.
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In an earlier contribution to this discussion,

It's an electrical problem in the CH's 2-port valve's actuator. There is an additional switch inside the actuator which switches the boiler on when the valve is open. The motor is opening the valve ok, but this secondary switch isn't making.
[You have an S-Plan system. See http://content.honeywell.com/uk/homes/systems.htm and scroll down to the S-Plan wiring diagram to get a better understanding of how it's supposed to work].
You may be able to replace just the switch - but it's easier (albeit a bit more expensive) to replace the whole actuator.
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Cheers,
Roger
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On Tue, 16 Jan 2007 23:38:09 +0000, Roger Mills wrote:

Except in my experience the valve is as likely to be mechanically stuck/stiff/jammed as the microswitch defective.
--
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Thanks for all the replies. You won't believe it but it has started working properly again as mysteriously as it stopped! However I suspect it will stop again one of these cold frosty mornings!

the 2-port valve. I did try manually opening the valve but the boiler still would not fire up until I turned on the HW circuit. The valve always closes quickly with a 'whizz' sound but is quite slow opening with more of a quiet creak than a whizz. Not sure if this is normal, sounds like during opening it is tightening a spring loaded coil or something like that.
Is it tricky to replace the motorised valve? I know how to drain the system and basic plumbing. Might be a job for my plumber though. Thanks for all the help which has been very informative.
Cheers, Chris.
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In an earlier contribution to this discussion,

It's not difficult - especially if you've got any manual gate or lever valves which enable you to isolate the motorised valve. Otherwise, it's at least a partial drain down job.
From what you now say, it sounds as if the valve is just a bit stiff rather than seized. You may be able to free it up by removing the actuator and turning the exposed spindle (of the valve itself) back and forth a few times. You should be able to rotate it through 90 degrees with finger and thumb - but may initially need to use pliers if it is stiff. You don't need to drain anything to do that. Don't disturb the electrical connections to the actuator.
To get an idea of what it's supposed to sound like, open the valve manually and operate the actuator while it is off the valve. If you call for heating, the motor will turn in order to think it's opening the valve - and will wind up the return spring. When it gets to the end of its travel, the motor will simply stall and - hopefully - the contacts will close and turn the boiler on (which is why you need to make sure that the valve is physically open before doing this). When you turn the heating off, power will be removed from the motor, and the return spring will move the actuator shaft back to the valve closed position.
When the valve is moving freely, the opening and closing sounds will be the *same* as the sounds made by the detached actuator.
--
Cheers,
Roger
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