Replacing Room Thermostat 3 wires to 2 wires??

[BlockABoots]

I currently have a Honeywell dial type thermostat installed which needs to be replaced, i have bought a Honeywell digital thermostat (DT90E).

The old thermostat has 3 wires....

https://i.*********/M1lOa7r.jpg

but this new digital one only requires 2 wires, live and switching no neutral is needed for what ever reason??.

Now im guessing the following...

Red = Live
Yellow = Switching
Blue= Neutral

is there anyway to make sure this is the case?

 

[ShaunCorbs]

1 live in
3 live out
2 neutral

 

[BlockABoots]

1 live in
3 live out
2 neutral

Ok thanks, so is it safe to just chocolate block off the blue wire then and leave it hanging behind the mounting plate?

Also why does this type of thermostat not need a neutral connection?

 

[ShaunCorbs]

Ok thanks, so is it safe to just chocolate block off the blue wire then and leave it hanging behind the mounting plate?

Also why does this type of thermostat not need a neutral connection?

yes thats fine

it increases the accuracy

 

[townfanjon]

Its an anticipator , its to prevent overshooting after the set temp and turn off slightly before .

 

[BlockABoots]

yes thats fine

it increases the accuracy

Hmm actually looking on the mounting plate of the new thermostat it does actually require neutral and not live out/switching as i was told....

https://i.*********/UTyPeNY.jpg

so it needs live (red) and neutral (blue) ???

 

[snowhead]

so it needs live (red) and neutral (blue) ???

Yes, it's powered from 240v for the display
Some Stats are battery powered.

 

[Matt0167]

also need switch live not just the live and neutral

 

[Ric2013]

The wire should terminate inside an enclosure and be insulated in some way. Realistically, while leaving it hanging will work, it cannot be considered safe. Can you not bring it inside the enclosure and stick a piece of insulated terminal strip (choccy block) around it?

If this were a gas issue, we'd say it had to be done by a registered gas installer. I find it odd how the minute one mentions DIY touching a boiler on this forum the stock answer is get a Gas Safe installer, but no one seems to care about electrics. Legally, electrical wiring of a heating system should be done by a competent person (i.e. a registered electrician), or notified to Building Control.

Rant aside, I think A is live in, B is switched live to boiler, and C becomes live when the room is up to temperature (not sure why you'd need to use C for heating). Assuming your controller is the DT90E model, the instructions to which I have just looked at. Easiest way would be to find the wiring diagram for the old thermostat (probably inside the lid) and identify the cable cores by the terminal they are connected to.

 

[BlockABoots]

Yes, it's powered from 240v for the display
Some Stats are battery powered.

This one does use 2 AA batteries!

 

[BlockABoots]

The wire should terminate inside an enclosure and be insulated in some way. Realistically, while leaving it hanging will work, it cannot be considered safe. Can you not bring it inside the enclosure and stick a piece of insulated terminal strip (choccy block) around it?

Yeah thats what i ment, just adding a chocolate block on the end of it and push it back up so behind the thermostat housing so its not visible or touchable

Rant aside, I think A is live in, B is switched live to boiler, and C becomes live when the room is up to temperature (not sure why you'd need to use C for heating). Assuming your controller is the DT90E model, the instructions to which I have just looked at. Easiest way would be to find the wiring diagram for the old thermostat (probably inside the lid) and identify the cable cores by the terminal they are connected to.

Good call, there is indeed a diagram in the case of the old thermostat....

https://i.*********/KlwYUvE.jpg

though that seems to only relate to just a 2 wire install not the 3 that is currently wired up!??

 

[ShaunCorbs]

3 live out
2 neutral
1 live in

 

[Ric2013]

Good call, there is indeed a diagram in the case of the old thermostat..

https://i.*********/KlwYUvE.jpg

though that seems to only relate to just a 2 wire install not the 3 that is currently wired up!??

I think you're wrong. You'll have to get confirmation from people more qualified than me, but I would say that in that diagram 1 is live, 3 is switched live to boiler, and 2 is neutral.

 

[Harvest Fields]

Hmm actually looking on the mounting plate of the new thermostat it does actually require neutral and not live out/switching as i was told..

https://i.*********/UTyPeNY.jpg

so it needs live (red) and neutral (blue) ???

Yes, it's powered from 240v for the display
Some Stats are battery powered.

Surely that is not right????? You don’t want to be putting neutral into terminal B!!

 

[Ric2013]

Harvest - is that your way of agreeing with me and Shaun?

See what happens when unqualified people start touching electrics? I've only tried to advise because I've known a few electricians who don't understand heating wiring at all, so the general advice to refer to an electrician would quite possibly not end well

 

[Harvest Fields]

Harvest - is that your way of agreeing with me and Shaun?

See what happens when unqualified people start touching electrics? I've only tried to advise because I've known a few electricians who don't understand heating wiring at all, so the general advice to refer to an electrician would quite possibly not end well

Yes mate. Saw his comments above and saw red. Didn’t read the rest of the post before I commented. Wanted to post before he put neutral in B. But have read all the post now and yes I agree with you both. I wouldn’t have wanted to be in his house when he put the power back on.

 

[Ric2013]

Do you reckon the fuse would have gone bang first - or the new thermostat?
And would the OP have been lucky enough to have a spectacular firework display, or a disappointingly quiet pop, with no further effect other than post 18 being 'Have tried connecting the neutral to terminal B, but it doesn't appear to be working. Any further suggestions?'?

 

[heliotrope]

Some of the information you have been given is very confusing.

Do not connect a neutral wire on your DT90.

Doing so could be dangerous.

Disconnect from the boiler connection or terminate safely at the
thermostat end.

I would suggest that you employ a competent electrician to carry out
this work for you.

 

[Harvest Fields]

Do you reckon the fuse would have gone bang first - or the new thermostat?
And would the OP have been lucky enough to have a spectacular firework display, or a disappointingly quiet pop, with no further effect other than post 18 being 'Have tried connecting the neutral to terminal B, but it doesn't appear to be working. Any further suggestions?'?

Depends what boiler it is.I have been to a couple of jobs where the gas valve has tripped the fuse box but not touched the 3 amp.

 

[BlockABoots]

I think you're wrong. You'll have to get confirmation from people more qualified than me, but I would say that in that diagram 1 is live, 3 is switched live to boiler, and 2 is neutral.

Well going but the correct colour scheme for wiring you would indeed put money on Red being live, blue being neutral and yellow being the odd one out having to be the live switching in this case.....i mean what electrician would wire it up any other way??

So just to confirm i defiantly dont want to be connecting neutral to this new thermostat and why does the thermostat have 3 terminals available?

 

[ShaunCorbs]

A heating off terminal

Red to a
Yellow to c

Blue in choc block

 

[Ric2013]

I would imagine you could use the final terminal (C) to run air conditioning, or if for some reason your heating setup needs needs a switched live to indicate that the room stat is satisfied, you have one.

Definitely not, based on this being the DT90E model. I can't find online instructions that say, specifically DT90 (noE), so I'm assuming E is for English language? Did this notcome with instructions?

For the correct colour scheme for wiring fixed cable in the UK, until about ?2005, live was red, neutral was black, yellow, and blue were also live colours.

The reason there are 3 live colours is to allow for 3 phase systems, and because, (in the case of 3 core and earth cables as used for two-way lights and for thermostats), it would be bloody confusing if there were three red cable cores. Really, if a live colour cable core is used as a neutral it should have been tagged black (tape or a bit of sheathing) to show it is now neutral.

After 2005, we changed to the international colours of live being brown, neutral being blue, and black and grey being the other lives. UK flexible cables colours were harmonised back in the 1970s.

So, confusingly, blue and black, might be either live or neutral, depending on the context.

For this reason, many UK houses now have a mix of new an old colours and the consumer unit usually states:

I'm going to bed.

 

[Harvest Fields]

A heating off terminal

Red to a
Yellow to c

Blue in choc block

Are you sure that yellow shouldn’t be in B shaun?

 

[Ric2013]

Shaun, are you sure:

Based on this, I think yellow to B, not C, but I may have the wrong instructions...

http://www.honeywelluk.com/Documents/Installation-Guide/pdf/1072.pdf

 

[Harvest Fields]

Shaun, are you sure:

Based on this, I think yellow to B, not C, but I may have the wrong instructions...

http://www.honeywelluk.com/Documents/Installation-Guide/pdf/1072.pdf

So this time your agreeing with me then mate and I thought you was going to bed.

 

[Ric2013]

I always agree with you Harvest, except when I don't.

I was going to bed, but I thought the OP might do it wrong and have heating that only runs in the summer if Shaun is mistaken. Shaun is probably now checking and will be back soon?

Shaun and mine and your posts crossed, because I need to write an essay instead of 5 words

 

[BlockABoots]

A heating off terminal

Red to a
Yellow to c

Blue in choc block

This is is what i mean the the manual and the diagram on the inside of the shell are completely misleading as they clearly show that Neutral is to be wired up which i thought was wrong in the first place but then had second thoughts when i read the manual.....

https://i.*********/laFpKnN.jpg

Not to worry guys im not going this right now just want to be sure before i replace the thermostat

 

[Ric2013]

I think you should get an electrician in.

 

[Harvest Fields]

I always agree with you Harvest, except when I don't.

I was going to bed, but I thought the OP might do it wrong and have heating that only runs in the summer if Shaun is mistaken. Shaun is probably now checking and will be back soon?

Shaun and mine and your posts crossed, because I need to write an essay instead of 5 words

I have he same problem mate. Sometimes I am midpost, then I get distracted by the kids or the missus and by the time I actually post someone has beaten me to it.

 

[Harvest Fields]

I think you should get an electrician in.

Me too mate

 

[Harvest Fields]

This is is what i mean the the manual and the diagram on the inside of the shell are completely misleading as they clearly show that Neutral is to be wired up which i thought was wrong in the first place but then had second thoughts when i read the manual...

https://i.*********/laFpKnN.jpg

Not to worry guys im not going this right now just want to be sure before i replace the thermostat

They are not mis leading mate. They are perfectly clear and do not take this the wrong way but you are not understanding them at all. The new timer unit and the instructions say nothing about a neutral. Do yourself a massive favour and get a heating engineer or Sparks In to fit it for you. It’s a 30 minute job max with coffee, biscuits and a chat for a pro.

 

[ShaunCorbs]

This is is what i mean the the manual and the diagram on the inside of the shell are completely misleading as they clearly show that Neutral is to be wired up which i thought was wrong in the first place but then had second thoughts when i read the manual...

https://i.*********/laFpKnN.jpg

Not to worry guys im not going this right now just want to be sure before i replace the thermostat

Yep your right, it's the neutral writing on it that through me

Red to a
Yellow to b
Blue in a choc block

Sorry for any confusion

 

[Harvest Fields]

Yep your right, it's the neutral writing on it that through me

Red to a
Yellow to b
Blue in a choc block

Sorry for any confusion

What chance do we have if even the pro’s are making these kind of mistakes shaun?

 

[ShaunCorbs]

What chance do we have if even the pro’s are making these kind of mistakes shaun?

be fine, just would only turn the heat on what ever the stat is set at eg desirable temp

 

[oz-plumber]

Re: The old thermostat, did that have a light come on when the heating was on?
If so, that's what the neutral is for.

As for the DT90, all that thermostat does is switch the active.
When the thermostat calls for heat, it sends the 240v supply back to the boiler to switch the boiler on.
Connections in the DT90 - A & B ( C is used for cooling )

All you have to do is find is the wiring connections in the boiler thermostat terminals and use those wires. If there's a neural there, cap the neutral.