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.

 

[BlockABoots]

Ok, chocolate blocked off the blue wire, connected red to terminal A and yellow to terminal B, turned the power back on and nothing blew up!. The boiler fires up when i set the temp higher than the current displayed temp and goes off when i set it a few degrees below and pump stops running when i turn the heating off completely.

Only thing i have noticed is even if i turn the heating off the little flame icon on the LCD display of the thermostat stays on so the thermostat is still calling for heat, it only disappears if i set the temp higher than what the current room temp is. Im guess it is normal for the flame icon to still be displayed even though the heating is turned off as it is a battery operated thermostat??

 

[Harvest Fields]

The flame should go off but the display should still be on

 

[Ric2013]

Hmm. I would think it is normal, as the room stat is not intelligent enough to know if the programmer has switched off the heating.

 

[BlockABoots]

Hmm. I would think it is normal, as the room stat is not intelligent enough to know if the programmer has switched off the heating.

Yeah that what i though what with it being battery operated. The main thing is the flame disappears as soon as the target temp is reached, i just didnt know if it communicated with the programmer via the 2 wires so it knew that it wasnt on and thus stop calling for heat....but then i guess if the programmer is switched off then it doesn't matter if its calling for heat

 

[Ric2013]

Well, when the set temperature is reached, the room thermostat itself is the item that is switching off the live to the boiler (pump etc), so in that circumstance it knows what is going on.
Glad you got it going. I was concerned you were going to persist in connecting neutral to terminal C 🙂

 

[Harvest Fields]

If you turn the temp down low enough it should still go off?

 

[BlockABoots]

Well, when the set temperature is reached, the room thermostat itself is the item that is switching off the live to the boiler (pump etc), so in that circumstance it knows what is going on.
Glad you got it going. I was concerned you were going to persist in connecting neutral to terminal C 🙂

Nah, i came across a few other threads on different forms who had the same colour wires as mine and every replied said to connected red to A, yellow to B and terminate the blue

 

[BlockABoots]

If you turn the temp down low enough it should still go off?

Yeah if i lower the target temp below the room temp the flame disappears and the boiler goes off

 

[Harvest Fields]

Yeah if i lower the target temp below the room temp the flame disappears and the boiler goes off

Then what’s the issue?

 

[BlockABoots]

Then what’s the issue?

I was questioning if the digital thermostat should still be displaying the 'calling for heat' icon (flame) when i had turned the heating off or if it should disappear

 

[Harvest Fields]

I’m getting confused! so if you turn the thermostat up it comes on? But if you turn it to 5 degrees the flame stays on?

 

[BlockABoots]

If you turn the temp down low enough it should still go off?

I’m getting confused! so if you turn the thermostat up it comes on? But if you turn it to 5 degrees the flame stays on?

Yeah if i lower the target temp below the room temp the flame disappears and the boiler goes off

 

[Ric2013]

Harvest: it's all good.

 

[Harvest Fields]

So you have stated above that the flame symbol. Which is the symbol that shows the stat is calling for heat or not goes off then there is no issue. Unless you are talking about something else?

 

[BlockABoots]

Thanks for the help guys!

Now i just need to calibrate the target temp so the heat level is just right.....why do they insist on putting room thermostats in hall ways where is probably one of the more coldest areas of a house!??

 

[Harvest Fields]

Thanks for the help guys!

Now i just need to calibrate the target temp so the heat level is just right...why do they insist on putting room thermostats in hall ways where is probably one of the more coldest areas of a house!??

Hold on. So was there an issue or not mate?

 

[BlockABoots]

So you have stated above that the flame symbol. Which is the symbol that shows the stat is calling for heat or not goes off then there is no issue. Unless you are talking about something else?

No you misunderstand.....

When the target temp of the thermostat is set higher than the current room temp the flame is displayed as its calling for heat and the boiler fires up to reach the target temp.

If i set the thermostat target temp lower than the current room temp than the flame disappears and the boiler goes off. This as it stands is all operating as it should do.

But what i did notice that if i turned the heating off from the CP then the flame icon on the thermostat was still being display as it was still calling for heat as the thermostat hadn't reach its target temperature. I was questioning weather the thermostat should still be displaying the flame even though the heating was turned off via the CP, but as Ric2013 pointed out the thermostat isnt advanced enough to know that the CP has switched the heating off.....im guessing if i wanted an intelligent enough thermostat for that i would have to spend more than the £30 i paid for this honeywell one!

 

[Harvest Fields]

Ohhhh. Makes sense now.

 

[Ric2013]

To answer your question on why the stat is generally in the hallway, it is because the hall is generally the last to warm up. If you put a stat in a living room then the living room might warm up first and will be nice and warm, but the rest of the house might still be cold.

I personally dislike this practice, as it assumes that the entire house needs to be heated when, in my opinion, 14°C in a corridor is sufficient for most people, but if the heating has been off and the entire house is at 14°C, then the living rooms will be too cold. This is why I personally prefer multizone systems (or use a wireless stat that you can move around).

In fact, I would argue that a pneumatic time switch that stays on for half an hour after you push it in could be the most energy-efficient stat. When you are cold, you turn it on; when you aren't cold, you simply forget it and the heating goes off again. But then, I am quite alternative in my thinking.

 

[DevsAd]

Blimey, BlockABoots - two pages to get the answer you were after. And you basically worked it out for yourself...

(You were initially confused by that 'neutral' on the diagram seemingly connected to 'B'. But if you look at the diagram again, you'll see that there's a rectangular 'block' on that line coming out of 'B', and that's the boiler, and the 'neutral' is actually connected to the other side of that rectangle! In other words, when your stat closes its contacts inside (when it's calling for heat) the LIVE that's going 'in' 'A' then comes 'out' of 'B' and goes in to one side of the boiler (rectangle). It's then the other side of the boiler that's effectively connected to the neutral. Make sense?)

Anyhoo, that's one issue. The other is the wee 'flame' symbol coming on. The new 'stat is battery operated so works independently of everything else. It hasn't a clue what the rest of the system is doing. It doesn't know if the programmer/timer is on or off. It doesn't know if the boiler is running , or even if it's fallen off the wall.

All it knows - in its isolated, battery-powered world - is whether the temp it is sensing in the air is higher or lower than the temp it's been asked to provide. That's it. If the air is too cold, the wee 'flame' comes on and the two contacts inside close. Once the temp is reached, the flame goes off and the contacts open.

That's it. If your house blew up in a gas explosion - I hope it won't - then amongst the rubble will lie a wee detached DT90 which will still show a flame symbol if it's too cold.

A thought - hadn't you considered a programmable room stat?

 

[Stoney Ground]

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.😛

Roughly translated “I have sausage fingers and keep hitting the wrong keys” 😀

 

[fixitflav]

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..
The thermostat doesn't require neutral. A is live, the contacts are changeover, B normally open (closed when calling for heat), C normally closed, but not used in this case. The N is the neutral of the heating system, remote from the 'stat. The system is represented by the rectangle.

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

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

 

[kozak1968]

It always amazes me how many replies a roomstat gets. Call a professional if in doubt... Second thoughts just call a professional!

 

[Ric2013]

I'm still amazed that on the same forum that whenever anyone mentions gas the stock answer is we won't help because it'd be illegal and dangerous for someone not competent to do the task, I think I'm the only one that has ever mentioned electrical work being a similar matter.

Although, yes, big deal, he's replacing a thermostat, surely electricity, like gas, can be a matter of safety and legality as well?

It is worth noting, however, that since 2013, in England, control wiring for central heating is no longer considered as notifiable work, and so can be legally carried out by amateurs without the need for professional testing (in Wales, central heating control wiring remains notifiable). This does not mean that it need not be tested. Whether notifiable work, as defined in the building regulations, or not, electrical work must be tested to BS7671, which few amateurs will be able to do, through lack of equipment if nothing else.