Help! No heating - thermostat wiring! CM927

[benseb]

Hi

I've just replaced an old analogue honeywell thermostat with a honeywell CM927

I followed the wiring picture, but unfortuantely can't get to the boiler end of the wiring to check the colour coding, but followed this thread - exactly the same setup. I have brown/grey/black rather than brown/blue/black:

//www.diynot.com/forums/viewtopic.php?p=1445948

After wiring, there was no power to the thermostat at all. I therefore wondered if the old wiring wasn't quite right. As such, I reversed the grey/black - assuming that one was neutral and one was the switched feed.

This worked and lit up the relay box. However, even when the relay is set on (green light on) the boiler doesn't fire up at all. I have the timer on the boiler set to always on. I've also noticed that the cm927 only has power if the boiler timer is 'always on'.

I'm now wondering if the wiring is still incorrect, but before I start randomly changing things wanted to check if anyone had any ideas?

Getting cold now...!

 

[swbjackson]

You'll have a live from the timeswitch, a switched live out and a neutral. You need to check with a multimeter to find out which is which. You can then wire the stat as per the instructions.

 

[benseb]

Thanks

Am I right in thinking that it's impossible to short anything with this config if wired incorrectly - as all the contacts are 240V so the worst that can happen is that i connect them up wrong and the circuit isn't live

I've just reconnected my old stat and can confirm by going backwards that I did wire it correctly, so i presume it must be a duff box

 

[MJN]

Am I right in thinking that it's impossible to short anything with this config if wired incorrectly [...]

No. If you inadvertently connect live to neutral you will produce a dead short and, at best, blow a fuse.

Mathew

 

[benseb]

Hi

OK, this is still an issue, here's where I'm up to:

OLD STAT:

Untitled

• benseb
• 19 Jan 2012
• 1

So from this:

Brown = Live
Black = Switched Live
Grey = Neutral

To confirm I had the Switched Live/Neutral correct, I wired up the same stat and removed the neutral and checked it still turned the boiler on/off, which it did:

Untitled

• benseb
• 19 Jan 2012
• 1

So I then wired up the new CM927 with the following:

N = Grey
L = Brown
L2 <=>A Link
B = Black

[No photo available]

Which I believe should be correct. This didn't work, no lights on relay, nothing fired up, etc. Essentially dead.

I then accidentally did the following:

N = Black
L = Brown
L2 <=>A Link
B = Grey

(Note this is incorrect in theory):

Untitled

• benseb
• 19 Jan 2012
• 1

Swapping the N and SL - oops. This seemed to work - the box powered up and responded and had the all important green light. However it didn't fire the boiler up (presumanly because it was just switching the live to neutral rather than going back to the boiler.

So I'm stuck. Is this a dodgy unit or have I done something stupid:

 

[picasso]

I reckon your old stat was miswired, try this

black to L

grey to N

brown to B

and link L and A

 

[D_Hailsham]

The wire connected to terminal 2 must be the neutral or the boiler would not have worked when it was disconnected. So 2 goes to N.

As for 1 and 3, 1 should be Live and 3 the Switched Live; but it's not unknown for installers to wire it the other way round (Technically it's wrong - but that's another topic).

If you have a multimeter you can check which is Live. This should go to L and the remaining wire (S/L) goes to B. You also need a link from L to A.

Does the Live supply come from an existing programmer? If so you need to set the CH to permanently on.

 

[MJN]

If you have a multimeter you can check which is Live.

And if you don't have one, get one. Guessing which wire is which is not the way to do this.

Mathew

 

[benseb]

I'll get a multimeter and test it out this evening. Does seem strange...

More soon!

 

[benseb]

Hi

OK - just tested with multimeter.

With the old stat still connnected, but turned down to off:

Brown to Grey: 235v
Brown to Black: 235v
Grey to Black: 0v

So, the only connection with a live when the thermostat is off, is the brown - this must be the Live?

We established earlier that if I remove the grey, the boiler still fires, so Black must be SW/L. Therfore Grey is neutral

So, wiring up the CM927,

N: Grey
L: Brown
L: Link to A
B: Black

Correct? This is what I tried first time to no avail.

 

[benseb]

Also tested with a mains-tester electrical screwdriver:

When stat off, the only one that lights up is Brown
When stat on, all three light up

 

[picasso]

You shouldent have 245v between brown and black, your going to have to trace the wires back to find out whats going where.

 

[benseb]

Surely that's the boiler switch circuit? Live and Switched Live.

I think that's 240v on our boiler (potterton performa 24 eco he) so when connecting the meter, that completes the circuit?

Or am I barking up the wrong tree?

 

[D_Hailsham]

The meter and mains tester results don't make sense.

You will have to find the other end of the stat cable to confirm what each wire does. If you have a large junction box, it will be in there. Post a photo of the box (lid off ) and we should be able to identify the cable.

 

[benseb]

OK, I'll try - boiler has been boxed in and although I can get the top off, I can't pull the bottom part out which is where the cable runs into. Will have to get the tools out...