Help with a Y Plan where some wires are missing

OK, This is a new boiler and the electrician has gone missing so I have to sort the job out.
The set up consists of a boiler in the kitchen with the programmer, pump and wiring junction box in the airing cupboard some distance away. The boiler should power the pump and allow pump overrun. However, that cable is missing and the boiler and the pump come on together as they are connected in the wiring centre. This is causing the boiler to banging when it turns off.
This is not a new electrical installation. The electrician has used the existing cables from the old system when the boiler was changed to try to make the system work. Adding a new cable would mean ripping up the bathroom floor tiles that are on top of chipboard flooring, ripping up chipboard flooring or taking ceilings down.
I think I have a solution that will save this damage. All I need is a pipe stat and a 230V relay (hopefully).
My solution is to split the boiler call and pump connections in the wiring centre so that the boiler is still called for by the programmer/room stat and cylinder stat and then use a SPDT 230V relay that is activated by the boiler call wire and connect the NO terminal to boiler call wire, the NC terminal to a pipe stat (fed from a permanent live in the wiring centre and mounted to the pipework just below the mid position valve) and then connect the common terminal of the relay to the pump.
Does anyone know if it will work?
Any 230V SPDT relay recommendations are welcome.
Thanks
Adam
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In an earlier contribution to this discussion,

Presumably, what you're setting out to achieve is to continue to run the pump while the flow temperature is above a certain value, even when the boiler demand has been removed?
If so, you may not even need a relay - as long as your pipestat has changeover contacts. In this case, connect the stat's NC contact to the boiler demand wire, the NO to permanent live, and COM to the pump. The pump will then run when the boiler demand is live - whatever the pipe temperature - and will also run when the boiler demand is removed as long as the pipe temperature is above the stat's set point. Ideally the stat should be much nearer the boiler - but that would require more cables to be installed, which you wish to avoid. What you suggest *may* work - but there is always the danger that the residual heat after the demand is removed from the boiler could produce local overheating without making your pipe hot enough to trigger the stat.
An alternative - which is perhaps a bit less scientific, but is guaranteed to work is to use a timer. If you feed the pump through the sort of timer designed to make bathroom fans over-run after turning the light off, you can make the pump continue to run for a set period after the boiler demand is removed - regardless of boiler or pipe temperature. About 4 or 5 minutes should be enough to ensure that any residual heat is carried away. I do this even though my pump is controlled by the boiler because, without it, the boiler's pump-control logic can sometimes get caught out and still cause the boiler's over-heat stat to trip.
--
Cheers,
Roger
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wrote:

I will look for a timer, or use a timer and relay.
Adam
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In an earlier contribution to this discussion,

This is what I use with my pump: http://www.tlc-direct.co.uk/Products/BGDT20.html
The delay is adjusted by turning the little screw. It's slightly fiddly, since it's only about 5 degrees of turn per minute of delay, but it seems fairly consistent, once set. You can calibrate it before installing it by lashing it up on the bench to switch a lightbulb off.
Why do you think you need a relay? Are you concerned that by forcing the pump on, you will also force the boiler on? With either a changeover pipe stat (as suggested earlier) - or with this timer - that isn't an issue. There are no circumstances where the boiler demand is connected to permanent live when it isn't already being fed by switched live.
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Cheers,
Roger
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Until you suggested a pipe stat with changover terminals in your first reply, then yes, I was concerned about seperating the boiler and pump supply. I had never given a changeover stat a thought. Brilliant idea Roger. The relay with the timer was in case I could not get a timer that was powerful enough for the pump (I had not looked for any timers this morning)

I have ordered something similar to this
<http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R '73520#header>
for 15 from my wholesalers. I can put it in a din rail enclosure next to the wiring centre.
I will save your link for future reference. It would have being the easiest option:-)
Thanks
Adam
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In an earlier contribution to this discussion,

<http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R '73520#header>
If it's the same spec as above, I would question whether 2 minutes is long enough. I started with my timer at 2 minutes, and still got the occasional boiler trip - so upped it to somewhere between 4 and 5 minutes.
YMMV of course!
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Cheers,
Roger
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http://www.tlc-direct.co.uk/Main_Index/Distribution_and_Switchgear_Index/Relays/index.html
These would do what you want and have the advantage of having a screw terminal base. They're a universal design and can be bought more cheaply elsewhere - but usually from firms with a minimum order value.
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*Reality is a crutch for people who can't handle drugs.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

http://www.tlc-direct.co.uk/Main_Index/Distribution_and_Switchgear_Index/Relays/index.html
Cheers Dave
I have decided to go the timer route (see my reply to Roger) but those relays look like just the sort of thing I need for a different project. I will shop around, but only as far as RS or Maplins as I only need one.
I had even considered a wireless switch of some sort. Anything to save ripping the tiled floor up.
Adam
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