central heating wiring

I would like to upgrade my central heating from gravity hot water to fully pumped. I have bought a cylinder thermostat and it says to connect it with 2 core flex.
Do I run a long length of flex all the way from the cylinder upstairs to the boiler, pump, and valves, downstairs or should I run a short length of flex from the cylinder thermostat to a junction box in the airing cupboard and then use twin and earth for the long run to the boiler? Or doesn't it really matter? Do you use whatever is to hand?
On the subject of using whatever is to hand, I am also replacing the old clockwork time switch with a digital programmer. I know I can buy multicore wire to connect to this but if I have 2 or 3 core cable already, can I just use a few lengths of these?
I have something in the garage. I cant remember if it is 2 or 3 core. If it is 3 core, can I use this for the thermostat and just snip off the earth or is that considered bad practice? Should earths always be connected to something?
If it is 2 core, is it alright to use that to run from the wiring centre to the programmer since the programmer does not need an earth? Or is it good practice to always have an earth (there is a brass terminal in the back plate to connect one to if required).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Flex all the way would be fine. Some would use TW&E as it's cheaper if they already have some. I generally use high temperature flex to a cylinder stat - butyl or similar - which is more expensive than ordinary flex so perhaps another reason to use TW&E if it's a long run. And use a flex outlet box to secure the flex at the 'wall' end so is a convenient place to change to a different cable.

Yes - but the earth wire *must* only be an earth. Colour code the others at both ends to aid identification. But for the cost of a few feet of multicore...

If there is no earth connection at the stat, don't snip off the earth but tape it back to the sleeve. Might be needed one day. Connect it at the other end.

Providing an earth even if not needed future proofs things in case it is later.
--
*Sherlock Holmes never said "Elementary, my dear Watson" *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 07 Feb 2010 11:20:26 +0000 (GMT), "Dave Plowman (News)"
Thanks. I wasn't sure whether the convention was to use twin and earth for the fixed part but I think there is an advantage to using flex all the way because as you say, at the other end it will have to go into a wiring centre and won't that involve some loops and curls which twin and earth does not like?
Also the pump, boiler, and valves all have flex so is it better to have all flex connections in the wiring centre or can you mix and match flex and solid twin and earth?

Yes, I knew that, sorry I should have said.

True but I was thinking of doing it on Sunday when the shops are closed and I have some 2 core at home.

If I used a multicore cable I would be happy to have an earth for future proofing. I was thinking of using three lengths of 3 core cable: using the earth wire as an earth and taping the blue wires with a brown flag or sleeve. Only that way I would have three earth wires at the programmer and three wires at the other end in the wiring centre. It seemed a bit redundant having three earths going the same place and I am sure the wiring centre will be full enough; that's why I was wondering if I could omit the earths. As it happens it is 2 core cable I have bought (ready for the stat). Could I use this if necessary? I will probably wait until Monday and get the multicore now but it would be nice to know for reference!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 07 Feb 2010 10:19:14 +0000, Robert wrote:

Most cylinder stats are single pole change over (indeed some wiring "plans" require it to be so) and earth. As pulling cables in is the hard part I'd use 3 core and earth to a cable outlet near the cylinder and then heat resistant 3 core and earth to the stat.
The back plate of most (all?) programmers doesn't really have enough terminals or space for connecting up all the required cables neatly and sensibly. Use a proper wiring centre, something like the Danfoss WC4B, this has enough terminals to terminate all the wires form all the cables and then have short "jumpers" between terminals to connect up the system as required.
To be honest the questions you are asking are rather basic and I wonder if you would be better off getting an electrician (with experience in heating controls) to do the work.
--
Cheers
Dave.




Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 07 Feb 2010 11:43:09 +0000 (GMT), "Dave Liquorice"

They do

The Honeywell L641A is double insulated and has no provision for an earth
http://www.screwfix.com/prods/40051/Plumbing/Central-Heating-Controls/Honeywell-L641A-Cylinder-Stat
Is double insulation something unique to Honeywell stats?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.