simple explanation - wiring two pumps into boiler

Having done a google, I have come across a technique to wire two pumps into a gravity system to produce a fully pumped system as per http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selmA065024%240%246440%24ed9e5944%40reading.news.pipex.net&rnum=8 http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selmA065024%240%246440%24ed9e5944%40reading.news.pipex.net
I am confused of how I can acheive this with my setup.
I have got a danfoss RX2 that provides the inputs for both heating and room stats (there is no programmer as the room stat is programable)
The connection box RX2 has a zone (zone 2) output that switched live, there is then two other outputs that is switched live (zone 1 on) or (zone 1 off)
The boiler has a live imput that fires the boiler and pump. There is then another input (mains Swl) that is the switched live from the external gravity hot water circuit.
If the boiler is run as fully pumped, it will need to have pump overrun selected - this will need to overrun the heating pump only.
As far as I can make out I need to wire the RX2 outputs from that stat to the pump, and then wire two relays across the pumps (1 per pump) so that when the stat fires the pump, the relay is activated and the "main L" input to the boiler fires". As they are connected via relays the boiler input going live will not fire the other pump.
The problem is the boiler overrun - I cant just wire the hot water pump to the boiler, and ever time the boiler fires the HW pump would run. How can pump overrun be implimented?
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wrote:

Hi,
Scuse the dummy question, but why do you need overrun?
cheers, Pete.
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To disperse the heat from the heat exchanger into the system
--
geoff

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I see. If it's to prevent kettling then a 'delay off' relay would do, some cheap ones available here:
<http://www.engsurplus.btinternet.co.uk/control.automation.htm
More information available from:
<www.rswww.com>
cheers, Pete.
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wrote:

Just wire the pump through a pipe stat - cold side of stat to boiler call, hot side to permanent live, common to pump
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Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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wrote:

Hi,
Sounds good, I wasn't aware they were available with a changeover contact.
cheers, Pete.
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wrote:

All the ones I have installed have had this. IIRC the older MV's needed DHW on and DHW off feeds
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Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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In an earlier contribution to this discussion,

Most 3-port mid-position valves still do!
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Set Square
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I've always avoided them like the plague - reading the FAQ explanation of how they work just makes my brain hurt. With 2 x 2-port valves it's always obvious what's happening.
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Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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In an earlier contribution to this discussion,

I agree with you and - given a clean sheet - would do the same, since they *are* a little complex and they *do* represent a significant single point of failure in heating systems. However, I inherited a Y-plan system when I bought this house and it works well enough for me not to want the hassle of re-jigging everything. I've had to replace the actuator a couple of times, but I've got quite good at recognising the symptoms when it goes wrong - and (hopefully) at advising others about the delightful things when they enquire in this NG.
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