How does a 2-port morotised valve work - exactly

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I have seen plenty on the web, including PDFs of wiring diagrams and alike, for how a 2-port motorised valve works, but it doesn't make sense

I have a pumped gravitiy C-Plan system. This means that I have pumped boiler heated water to my rads controlled by a room stat and the boiler heats the domestic hot water by simply circulating hot boiler water through the HW cylinder

Now, I want to control my DHW as it is getting too hot. So hot in fact that I think it has destroyed a fairly new show pump I installed a while back, but that's another story.

So I have me a 2-port valve (Salus) and a tank stat to go on the cylinder. I also have the Centaur 27 Hortsmann controller

No matter which wiring set up I look at there doesn't appear to be a way of enabling the room stat to turn on both the boiler and the pump.

Take the Hortsmann setup, which seems a good starting point and look at diagram 4:
(I am not permiited to send links yet, so google 'horstmann centaurplus c27' and take the installers guide link that ends - Central-Heating/C21-InstallerGuide.pdf

Not only does the boiler not get called by the heating room stat, it also appears to me that when the DHW is satisfied the grey Live will then be switched to the pump via the white wire and so the pump will run regardless of what the room stat is doing (grey is common on the Salus and appears to be Normally Closed on the white while the valve is closed)

So, can someone please explain what the M-Valve is really doing. I can't see why the boiler cannot run if there is demand from the heating but not the DHW
 
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Just a thought: am I being too black and white here?

I'm taking the view that the valve is either open or shut - fully. Perhaps the valve doesn't fully close when the DHW stat is satisfied. Maybe the grey live wire continues to feed the valve motor to just restrict the flow of water and the orange wire to the boiler remains on. Only when the valve is fully shut does the contact to the white wire and the pump get made, and not the boiler. That would make more sense because the pump would be used to circulate and therefore cool what would be very hot water - the boiler cannot be called while it is doing that.

Is that it?
 
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From memory the zone valve on a "C Plan" has to be a 6 wire one with a change-over valve in it. The last time I did this was about 20 years ago and I used a Honeywell 28mm valve.
Zone valves are open or shut (no halfway)

3 port ( divertor valves) are mainly either way or mid position these days.

Look here for wiring

http://wiki.diyfaq.org.uk/index.php?title=Central_Heating_Controls_and_Zoning#Gravity_DHW:_C-Plan

Regardless of the picture DO NOT fit the valve on the flow to the cylinder or you may then be fitting a valve in the expansion route to the F&E tank fit the valve on the domestic return.
 
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Thanks Pete

The diagram is correct for the Honeywell version but I'm sure the Salus has the grey and orange wires different - grey is common rather than orange and grey to white is NC rather than orange to white

I'll be going for the return pipe from the DHW cheers
 

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