Replacing an old thermostat

[Phatjap1]

Hi people
Sorry if this is covered elsewhere but my eyes are stinging from researching this and I promise I would post it in the right forum if I found it..

I want to replace my old analogue thermostat wit this

http://cgi.ebay.co.uk/ws/eBayISAPI....75318&ssPageName=STRK:MEWAX:IT#ht_2104wt_1139

The old one has black, red and earth to it. Is is possible to replace it with this one?

If so, what would the correct sequence be?

Thanks in advance.
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[ChrisOxford]

In theory, this should work OK as it says it will switch mains voltage.

HOWEVER... Your old stat sounds like it has live in (red), switched live out (black) and earth, BUT NO POSSIBLY NO NEUTRAL, which this digital stat MIGHT WELL need, so you need to double check this. A neon mains testing screwdriver can help here, but BE CAREFUL as mains voltage can kill if you don't know what you're doing. However, as it takes batteries, you might not need a neutral, but the info on eBay doesn't actually tell you.

Are you sure there aren't four wires? And I have seen installations where someone's used the green or green/yellow wire for switched live, to save buying the right cable (brown, blue and black, with or without earth).

You may not need the earth wire as the new one will probably be all-plastic and "double insulated", although it will probably have a terminal where you can "park" the earth wire to prevent it touching anything it shouldn't.

Ask an electrician if you get stuck, unless you can work out how it's actually wired from scratch.

Also, if I were you, I would go for something from a well-known name. Look on the Screwfix webiste.

Chris.

 

[bengasman]

1. The product looks like cheap shyte; get something better e.g. Honeywell or Siemens.
2. The color of the wires to your current stat mean absolutely nothing. If you don't know how to measure/test, don't do the job yourself as you will be dealing with 230V live.

 

[ChrisOxford]

1. The product looks like cheap shyte;

You put it better than I did

 

[bengasman]

1. The product looks like cheap shyte;

You put it better than I did

I've a way with words; subtle, but still gets the message across.

 

[Phatjap1]

Thanks for the replies, and comments!

It's not 240 volt, I've checked with a volt meter. Nope, definitely 3 wires.

This is what the seller says:

"You only need Live In plus Switched Live Out. No Neutral is required"

Only trouble is, I don't know know how to identify the the live in or switched live out.



Many thanks.

 

[ChrisOxford]

Red = live in,

Black = switched live out.

If it doesn't work properly, swap them round.

Insulate the earth wire, unless there's a terminal specifically for it.

It's not 240 volt, I've checked with a volt meter. Nope, definitely 3 wires.

All stats I've come across have been mains voltage. There could be 2, 3, or 4 wires.

How exactly did you check? Where did you connect the meter probes?

Really, you need to set the meter to AC (not DC), and try measuring between the earth wire and red first of all, with the central heating switched on. YOU MUST NOT TOUCH ANY WIRE OR METAL PART OF PROBE WITH YOUR FINGERS/BODY/CLOTHES THOUGH.

Good luck

 

[bengasman]

Red = live in,

Black = switched live out.

1. That is an assumption based on assuming the presence of a flawed logic.
2. It would still be the wrong color coding.
3. The only correct answer is: measure and find out what is what.

 

[ChrisOxford]

True, Ben.

But red for live and black for switched live was the convention (for light switches and 2-wire stats) until the harmonised codes were introduced fairly recently, though technically, the black should have had a red sleeve or red insulating tape on it to indicate that it's not neutral.

BTW, what colour's the "earth", PhatJap?

Can't disagree that measuring is the best way, but not the easiest or safest thing to do for someone who's not au fait with mains wiring, especially as there's (probably) no neutral for reference.

PS Added an extra para to my post above while you were posting yours.

 

[bengasman]

True, Ben.

But red for live and black for switched live was the convention ...

No it was not. It was a common thing to do for people who were not qualified to work on electricity and not bothered to do things by the book. A lot of people doing something, does not make it a convention, unless you'd call things like illegal gaswork, benefit-fraud, or drink-driving a convention.

 

[ChrisOxford]

You're the expert, Ben, so I'm not going to disagree. What I meant to say, perhaps, is that 90% of old, professional installations that I've seen that have 2-wire stats have been coded like that. Probably penny pinching on behalf of the installer so that they could use cheap 1.0mm T&E.

 

[Phatjap1]

Hello
I checked by turning on the CH, then touched the black probe to earth and red to the black, then the red. There was a reading tough but I expected it to say 240.

I then connected the red to common, the black to N/O. t blew the 3 amp fuse though in the fused switch before the boiler. Hmm.

 

[ChrisOxford]

Hmm indeed

There was a reading tough but I expected it to say 240.

What reading did you get? Was the meter set to measure AC volts?

I then connected the red to common, the black to N/O. t blew the 3 amp fuse though in the fused switch before the boiler.

Whatever you do, don't put a 13A fuse in, replace it with 3A.

The fact that the fuse blew does suggest that red and black might be live and neutral respectively. So where's the switched live then?! Once again, what colour is the wire that you describe as "earth"?

Another line of attack... what are the markings on the old thermostat's terminals? Let us know what colour was connected to what terminal.

 

[bengasman]

...Let us know what colour was connected to what terminal.

The colors are comletely irrelevant; I've seen plenty of cpc's used as live conductor on a stat.

 

[ChrisOxford]

Agreed on the last part, so have I.

I have a feeling we're looking at L, switched L and N here.

Looking at the old stat's terminal markings is probably the easiest way of working out what's what. Even if the colour coding's up the spout, the terminal markings can't be (unless it's been mis-wired, but then it probably wouldn't have functioned).