Simple CH/HW Question.

Hello All
Can't find it in the FAQ, and google's turning up too much useless junk, so:
Central Heating won't work unless Hot Water also turned on.
Most likely fault is: " " <- Fill this bit in please. :)
It did used to work ok, but now selecting CH means the boiler doesn't fire even when Thermostat is set anywhere UNLESS hot water is also activated, then all works ok.
I thought at first the thermostat was broken, but the heating appears to keep going even when HW tankstat is satisfied - though I could be wrong and it's just residual heat that's giving that impression.
Conventional system, small bung, oil fired boiler, 3-way port.
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Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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A look at the wiring diagram for a Y-plan system in http://content.honeywell.com/uk/homes/systems.htm shows that with hot water on (and the cyl stat unsatisfied) the boiler and pump are powered directly WHEREAS when only the CH is switched on, the only path to the boiler and pump goes via the actuator.
This system does also require a HW OFF live feed from the programmer and a change-over switch in the cylinder stat - so another cause could be a wiring fault in that area.
HTH, Roger
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Hello Roger

Aha, that's a new one on me.

Thanks Roger. If this is the fault, is it an easy part to source and replace, or better to replace the whole 3-port valve?
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Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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wrote:

Hi Simon,
You can usually change just the actuator bit, by removing it from the top of the mechanical bit of the valve (couple of screws), without affecting the water system. Make a careful note of where each of the wires connects before disconnecting them.
When fitting the replacement, make sure that the D-shaped hole in the actuator is lined up with the D of the shaft - and connect the wires as per the old one.
There are some in this NG who advocate changing individual components within the actuator (motor, microswitches, etc.) rather than the whole thing. I'm sure it can be done, but I reckon it's a bit fiddly!
Roger
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I'd be looking at the port valve. Turn the heating on and see if the port valve moves at all. It might be possible to move the port valve with the manual over ride lever, which can also tell you if the valve is moving by going loose when the heating is turned on. If it doesn't go loose, then it is more likely the micro-switch inside that's knackered and needs replaced.
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Hello BigWallop

Hmm, something does indeed seem to be odd with the valve upon playing with it.
A - HW Mid B - CH
When just one is calling, it moves to A or B correctly, and when at A and CH starts to call, it goes to Mid. However, when it's at B and HW calls, it *doesn't* to Mid, preferring to stay on B.
This means the CH never activates the boiler so the rads don't get warm UNLESS HW is also calling and the position is at Mid. I'm guessing the microswitch confirms to the boiler that the port is in the correct position to start sending hot water through?
I'm guessing a new motor unit is in order? Not done one before - I'm guessing it's a fairly straight forward "Unscrew unit and replace wires like for like"?
It's a Landis and Staefa MAV322, 3 yrs past its warrantee expiry.
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Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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wrote:

Turn all the power off to the system and remove the tin cover of the valve head and take a good look inside. If you have a continuity tester, then try placing it over the micro-switch terminals and operating different part of the system to where all the wires are connected too. It will let understand how and where the things are that make the whole system work.
The micro-switches are more commonly the culprits in this type of fault and by chance they work out very cheap to replace, so if you can find out which one is at fault and replace it, you're back in working order.
http://www.directindustry.co.uk/images_di/photo-p/17682.jpg
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Check that the microswitch on the 3 way is working, and that the valve is moving far enough to operate it. If the microswitch has failed, it's often possible to replace it on its own, depending on make.
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*Why can't women put on mascara with their mouth closed?

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Hello Dave

That's three votes for the valve being of the borked persuasion.
The manual side arm isn't actually doing anything - there's no resistence when I move it and it's not moving the indicator at the front.
Replacement would be the top bit of the valve, containing the motor and sensors? I guess I can do this without turning the water off, hopefully there's a sealed tap underneath that gubbins.
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Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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Hi Simon,
It sounds like the motor is fine. I just replaced the synchron motor on my system and the motor was about 16 (though I now see you can get them for about 9 online). For the whole unit my local DIY shop was looking for 40+. Replacing the motor alone was simple but I don't think thats your problem. It does sound like one of the microswitches and while small,fiddly etc will only be a few quid + time copared to A fair chunk of dosh and a fairly simple rewire.
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It will depend on make, but most micro switches are under a quid - even from Maplin. The more expensive ones tend to be for signal or low voltage use so have rare metal contacts.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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oh, its a dry job, so no need to worry on that front!
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