Replacing an old thermostat

[bengasman]

...Looking at the old stat's terminal markings is probably the easiest way of working out what's what..

No, it is not. The only way to find out what is what, is measuring like I stated before.

 

[ChrisOxford]

We'll have to agree to disagree on that then, Ben!

Never mind, let's try to help the OP either way.

 

[bengasman]

We'll have to agree to disagree on that then, Ben!

Bearing in mind that I'm a qualified electrician as well as a qualified gasinstaller, you'll forive me for trusting my professional opnion more than your amateur version.
Your advice is not only incorrect, it is potentially dangerous.
By assuming that a green/yellow coded cable is actually a cpc in stead of verifying it, you don't exclude the possibility of extraneous conductive parts become live under fault conditions. As a roomstat like this is usually connected to live 230V, that could in extreme cases lead to electrocution.

Never mind, let's try to help the OP either way.

I already did by so by giving him the only correct advice possible at this stage: Assume nothing and measure everything pertaining to the stat cable.

 

[ChrisOxford]

Fair enough, Ben, I'll leave it to you.

 

[Phatjap1]

Hello people,

I'm determined to beat this, and with your much appreciated help I am definitely going to

I went and bought a cheap electric screwdriver as my skills with a meter are limited it seems.

OK, so I checked the wires once again with my new screwdriver and the findings show that the red is the only live..... No others light up. So this I wired to the common terminal. Then the black one I wired to the N/O terminal and taped up the earth. The fuse blew again. No need to mention my heart sank, but thank the lawd for fuses.

So, is it possible that the live back to the boiler might be the earth wire? I assumed not because it is actually a bare wire with a green/yellow sleeving added. I didn't really want to connect that to the n/o terminal incase it caused irrepairable damage, is that possible?

I'm getting near, I can feel it!

All the best.

ADDITIONAL NOTE

OK, more investigation, I've just found this post on the Screwfix forum:
http://www.screwfix.com/community/message/62188

Which seems just like my scenario: that the earch could have been used for the switched live.? Also the wiring for the old thermostat is the same, the diagram on mine indicates From a diagram inside the cover it shows terminal 2&3 go to Neutral and terminal 1 to live, though I can't remember which colours were connected to where now.

(The difference with the scenario on the Screwfix is that mine is red, black and gren/yellow, not brown, blue, green/yellow. Mine was also an old Honeywell)

Just to add something else, should there always be a live feed, whether the heating system calls for heat or not? Mine seems to be whether the timer is on or not for the heating. The only way to turn it off is to take the fuse out?

More additions!

http://www.screwfix.com/community/thread/57872

This also could point to the earth wire being used as switched live?

**Verdict**
The earth HAS been used for a switched live. I will get some one to swap it around at some point.