Swapping out an old Honeywell thermostat for a new one

[stevebh]

What seemed like a good idea at the time has turned into a real problem!
We have an old Honeywell room thermostat - not sure of the actual model but this is a picture:

It was working ok - we are just about to decorate the room and decided it would be good to replace it with a more modern looking one.
I disconnected it from the circuit and made a note of the wiring. The colours seemed a bit wrong but it worked and so assumed it must be right.

Note wires are only connected to 1, 4 and 3 and looking from the back we can see the colours:

So Blue is on 1; Yellow on 4 and Red on 3

Looking on the diagram inside the cover, we can see that 1 is Live; 4 is the switched Neutral and 3 is the Neutral.

This is where I think the colours are wrong as I would have expected Red to be Live but it works so must be correct.

I intend to replace this with a Drayton Combi-stat RTS8, the wiring for this is:

So, I connected the Red wire to N; Yellow to 3 and Blue to L. Turned on the power and when turning the dial right up to 30 nothing happens - the boiler doesn't fire.

I put a meter on L and N and it showed 240V but when putting it on L and 3 is showed nothing. To test the unit, I disconnected and measured the resistance between L and 3 when turned to 0 degrees - it showed no connection but when turned up to 30 it showed 0 ohms, so I believe the unit works.

I did also connect the wires to a ring mains tester and it said the L & N were reversed, so I swapped the Red and Blue wires round but all that did was blow the main heating circuit fuse!

I don't see what is wrong - wish I hadn't started!

 

[stevebh]

Just to add a bit more info. I have just attached the common probe of my multimeter to earth and then in turn connected each wire to the positive probe. The red wire returns a voltage - 244V while the others (yellow and blue) return no voltage. This tells me that the Red wire is indeed the Live but it was wired to the N on the Honeywell and as I said when I reversed blue and red on the Drayton (so red was connected to L) it tripped out the heating circuit very much confused

 

[RandomGrinch]

Those original wiring colours are definitely confusing, and the schematics aren't much better!

I'm afraid your assumptions are incorrect.

Looking on the diagram inside the cover, we can see that 1 is Live; 4 is the switched Neutral and 3 is the Neutral.

Your original stat is connecting together terminals 1 (Blue) and 3 (Red), when it is calling for heat.

1 Should be Live (currently Blue)
3 Should be Switched Live (N/O - currently Red)
4 is an optional switched Live (N/C - yellow)

(In practice, with this setup, it doesn't matter which way the live and switched lives are connected)

You don't appear to have a Neutral at all; that would have been connected to terminal 2.

However, it would be worth investigating where the other end of this cable is connected, to give a definitive answer (if these are all switched or live wires, I wouldnt have expected a trip).

For now, you can try and connect Red to L, Blue to 3 on your new stat and keep the yellow safely isolated in a separate terminal block.
Then see what happens.

Edit:
Actually use Blue to 2, as per Drayton's two-wire spec.

 

[stevebh]

Thanks. I have wired Red to L and Blue to 3 (before I saw your edit) and this is working.

 

[fixitflav]

Looking on the diagram inside the cover, we can see that 1 is Live; 4 is the switched Neutral and 3 is the Neutral.

I'd have thought 1 is Live, 2 is Neutral (for the booster heater), 3 is Switched Live (to load). 4 is Live when the stat not calling, but isn't used here.

In Figs 5 and 6, I assume the thin line round the outside represents the outline of the box, not a wire, or it would be a short from L to N. Confusing or what?

 

[RandomGrinch]

I'd have thought 1 is Live, 2 is Neutral (for the booster heater), 3 is Switched Live (to load). 4 is Live when the stat not calling,

Yes, correct, but in this case, 2 (Neutral) isn't being used..

In Figs 5 and 6, I assume the thin line round the outside represents the outline of the box, not a wire, or it would be a short from L to N

Yes. But what is more confusing (and what I've seen plenty of times before on here), is the schematic showing a nameless 'box'...

Many, as with the OP can see the N and don't realise that the box represents the boiler/load, with neutral connected to one side and switched live to the other.
And so we get the mistaken ...

we can see that 1 is Live; 4 is the switched Neutral and 3 is the Neutral.

 

[fixitflav]

Yes, correct, but in this case, 2 (Neutral) isn't being used..

Yes, I hadn't noticed that. So the booster wasn't being used, giving in theory greater hysteresis.
Presumably the system needed a live output when the stat not calling. If so, could be problem with the new stat, as it doesn't have changeover contacts.

Many, as with the OP can see the N and don't realise that the box represents the boiler/load, with neutral connected to one side and switched live to the other.

That's right. Calling it LOAD in Figs 5 and 6 helps, better to show a part dotted wire, to indicate it's remote.