Replacing 3 wire with 2 wire room thermostat
Discussion
Hi,
I was going to replace my 3 wire room thermostat with a 2 wire digital one Honeywell DT90E, a quick google tells me this should be easy and I should take the red wire from terminal L in my existing thermostat and put it in terminal A on the new thermostat, the blue in N I can ignore make safe, and the yellow wire (Call) should go into terminal B. My issue is though as you can see from my picture what I thought was the Call wire in my existing thermostat is green/yellow (earth colour) and there is nothing in the earth itself (bottom right). Is it possible that when they fitted my old thermostat they just used a earth coloured wire for the call ? Confused.....
I was going to replace my 3 wire room thermostat with a 2 wire digital one Honeywell DT90E, a quick google tells me this should be easy and I should take the red wire from terminal L in my existing thermostat and put it in terminal A on the new thermostat, the blue in N I can ignore make safe, and the yellow wire (Call) should go into terminal B. My issue is though as you can see from my picture what I thought was the Call wire in my existing thermostat is green/yellow (earth colour) and there is nothing in the earth itself (bottom right). Is it possible that when they fitted my old thermostat they just used a earth coloured wire for the call ? Confused.....
They have used a 3 core flex, as this looks like it is in the wall it should be ideally replaced with a twin and earth. If this is not practical then find the other end at the wiring centre and connect the green/yellow to the earth, then use the brown and blue as feed and call, sleeve the blue at both ends with brown sleeving or brown phase tape
Cerbhd said:
They have used a 3 core flex, as this looks like it is in the wall it should be ideally replaced with a twin and earth. If this is not practical then find the other end at the wiring centre and connect the green/yellow to the earth, then use the brown and blue as feed and call, sleeve the blue at both ends with brown sleeving or brown phase tape
Ok thanks I might get someone in then to have a look as I will probably screw it up 
I know its not ideal / best practice but could I just uses the existing wiring as is, and keep using the yellow/green wire as the Call one as I assume that's what its doing at the moment ?
cheers
Not what i would spec if i was installing but not problem regs wise using flex for fixed equipment as long as it does not exceed current carrying capacity and is suitable for conditions. As flex insulation tends to be softer/more flexible 
it should really be contained in conduit/trunking to provide mechanical protection. Common sense applies, if your junction box where that cable connects is on the other side of that wall, relax, if its a 20m run under your carpet probably not ideal cable selection!
As above rewire as two wire using N as call (bit of brown sleeving required) and ensure that earth/cpc is terminated at earth both at the junction box and in the stat.
it should really be contained in conduit/trunking to provide mechanical protection. Common sense applies, if your junction box where that cable connects is on the other side of that wall, relax, if its a 20m run under your carpet probably not ideal cable selection! As above rewire as two wire using N as call (bit of brown sleeving required) and ensure that earth/cpc is terminated at earth both at the junction box and in the stat.
Edited by Mpov on Friday 7th October 00:53
Mpov said:
Not what i would spec if i was installing but not problem regs wise using flex for fixed equipment as long as it does not exceed current carrying capacity and is suitable for conditions. As flex insulation tends to be softer/more flexible it should really be contained in conduit/trunking to provide mechanical protection. Common sense applies, if your junction box where that cable connects is on the other side of that wall, relax, if its a 20m run under your carpet probably not ideal cable selection!
As above rewire as two wire using N as call (bit of brown sleeving required) and ensure that earth/cpc is terminated at earth both at the junction box and in the stat.
Thanks to all who replied, I'm going to have ago myself after getting quotes of around £200 for the job :-0, I found the wiring centre as mentioned in the airing cupboard this morning will post a pic later. I will swap the wires over as suggested and sleeve, when you say "earth/cpc is terminated at earth both at the junction box and in the stat", do you mean just put each end in in those plastic block things with the screws in ? I don't see a earth point on the new thermostat Honeywell DT90E this model use batteries as well. it should really be contained in conduit/trunking to provide mechanical protection. Common sense applies, if your junction box where that cable connects is on the other side of that wall, relax, if its a 20m run under your carpet probably not ideal cable selection! As above rewire as two wire using N as call (bit of brown sleeving required) and ensure that earth/cpc is terminated at earth both at the junction box and in the stat.
Edited by Mpov on Friday 7th October 00:53
ta
The stat will not have an earth as it is double insulated, you should see several green/yellow cables connected together in the wiring centre, could be connector block as you mentioned, connect the green/ yellow if the flex to these and make safe with insulation tape at stat if there is nowhere to park the cable. This ensures the cable is earthed. £200 wow, do you live in Mayfair or Knightsbridge??
Cerbhd said:
The stat will not have an earth as it is double insulated, you should see several green/yellow cables connected together in the wiring centre, could be connector block as you mentioned, connect the green/ yellow if the flex to these and make safe with insulation tape at stat if there is nowhere to park the cable. This ensures the cable is earthed. £200 wow, do you live in Mayfair or Knightsbridge??
Thanks, yeah just under £200 it was but it did say supply and fit, they seem to ignore that fact I said I would supply, guy couldn't even fit me in till mid Nov. No not Mayfair or Knightsbridge but north west London just inside of the M25. It always seem to be the same around my neck of the woods get jobs done and be expensive it one of the main reasons I do a lot of stuff myself, but for jobs I could screw up and that would create a lot of head ache if I did(worried about having no heating / hot water) I would get someone else in. Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff