Honeywell T6360B to CM901 replacement with wiring centre

[catnipsta]

I want to replace my faulty mechanical room thermostat (Honeywell T6360B) with a programmable thermostat (Honeywell CM901) to a system with a wiring centre.

I understand that the existing 3-core & earth wiring for the T6360B is for Live-Live Switch-Neutral & earth and is connected to the 4 "thermostat" terminals in the SC.

From forum topics and installation diagrams, only the Live and Live Switch wires are to be used with the CM901, and for the Neutral & earth to be made "safe" using connector blocks - and that the existing programmer (for CH & HW) should be set with CH constant-on.

Sadly, it fails to function properly. When I looked into the wiring, I disconnected the existing "Neutral" wire from the terminal at the wiring centre, and found that there was still a current coming from it, even though the other end was made safe.

Could this Neutral wire be connected somewhere between the room thermostat and the wiring centre?

A botched job by the original installer perhaps?

PS. I have never known the room thermostat to have ever worked since moving into my apartment 1 year ago. Perhaps the original "faulty" room thermostat was not faulty after all, just wrongly wired?

 

[D_Hailsham]

What boiler do you have? Without this info the correct advice can't be given.

You are right in thinking that the CM901 does not require a neutral or earth.

 

[simond]

If you had a T6360 before then the CM901 (or 907 = 7day) will work just dandy. If you have a combi you may (depending on type) need to put a permanent phase supply on the stat, which is why Honeywell give you a loop-in live terminal.

I suspect you have made an assumption in the wiring centre which is incorrect, thus the return feed on the neutral.

There is no grounds for describing intercepting the thermostat cable as a botch; it may not suit you, but providing it is safe and not labelled otherwise, the original installer probably knew exactly what he was doing. There is nothing in the wiring regs to say that heating controls have to be wired radially.

 

[catnipsta]

I have tested the CM901 by connecting a new cable to the wiring centre, and it works fine.

As for the original room termostat cable, even when all 4 wires had been disconnected from the wiring centre, one of the wires at the room thermostat end was still Live upon testing with a mains tester.

Since the Live feed should come from the system controller (CH on), I can't see why there should be a Live from elsewhere.

 

[D_Hailsham]

even when all 4 wires had been disconnected from the wiring centre, one of the wires at the room thermostat end was still Live upon testing with a mains tester.

Since the Live feed should come from the system controller (CH on), I can't see why there should be a Live from elsewhere.

You are correct in saying it should come from the controller, but it obviously does not.

If the wire is live at the thermostat end, it should also be live at the wiring centre end. If it is not, then it is not the same wire.

You need to find the other end of the live wire. The easiest way is to disconnect, in turn, all wires of the same colour at the wiring centre and boiler until you find the culprit.

Is the whole system controlled by one master switch, which turns of the power to the boiler and the wiring centre? It should be.

Reading your first post, I see that the room thermostat has never worked. It obviously has been wired wrongly.

Please post the make/model of your boiler. If it is not a combi, how many motorized valves do you have?

 

[catnipsta]

Hi D_Hailsham

As I have successfuly tested the CM901 with a 2-core cable to the wiring centre, I have decided not use the original cabling and will make it safe behind a face plate and also at the wiring centre end.

I will be running a new 2-core cable between the wiring centre and the CM901.

My system is a S-Plan config.....Honeywell has a good page on the various system configurations.

http://content.honeywell.com/uk/homes/systems.htm

 

[D_Hailsham]

I have decided not use the original cabling and will make it safe behind a face plate and also at the wiring centre end.

But the old wire will still be live, is this wise?

My system is a S-Plan config.....Honeywell has a good page on the various system configurations.

That saves me pointing you to the correct page

Presumably you are keeping the old two channel programmer to provide control over the hot water times. Many people suggest setting the CH to continuous so the CM901 controls the times. I think it is much better to remove the CH wires between the old programmer and the wiring centre, so there is no confusion in the future.

 

[catnipsta]

It would be ideal and safer to remove the old cabling, however the walls plasterboards adhered to solid wall, and there is no indication of any junctions on the other side of that wall - I assume the cabling runs embedded vertically up or down the wall.

Finding where it is still connected to would be task.

I shall leave a warning note behind the face plate for future reference.

 

[fuzzytrifle]

I want to replace my faulty mechanical room thermostat (Honeywell T6360B) with a programmable thermostat (Honeywell CM901) to a system with a wiring centre.

I understand that the existing 3-core & earth wiring for the T6360B is for Live-Live Switch-Neutral & earth and is connected to the 4 "thermostat" terminals in the SC.

Hi, I came across your post on Google - I've just got hold of a CM901 which I've bought to replace a T6360B. It's connected to a master wiring centre on the underfloor heating manifold by a 3 core and earth cable.

None of the wiring diagrams shown in the installation booklet are similar to my system - I know that I no longer need to use the neutral wire, but am unsure which terminal on the CM901 is 'live' and which is 'switched live'.

Is it, A to switched live and B to Live?

Many thanks

 

[D_Hailsham]

Is it, A to switched live and B to Live?

There are only two options and you have chosen the wrong one!

Not that it matters, as it's only important when you are controlling an air conditioning system when A is common, B is heating and C is cooling.

 

[fuzzytrifle]

Really glad that I asked - thank you

 

[D_Hailsham]

There are only two options and you have chosen the wrong one!

Really glad that I asked - thank you

But, as I said, in your case it doesn't really matter. However, it does make fault finding easier in the future if it is connected the "correct" way round.