Wiring a thermostat to the system

[k_d_ch]

I have some questions regarding wiring a Honewell CMT927 thermostat to the electric supply and the boiler. The boiler is plugged into a spur socket at the moment and as I understand, since no extra spur from a spur is allowed, the existing spur will have to be modified in order to connect the thermostat to it.
My first question is whether I understand it correctly that since the thermostat needs its own fuse (the wiring diagram supplied with the thermostat says "<5A", does it mean the only option is a 3A fuse?), and the fuse installed into the boiler plug at the moment is 13A, I need to change the existing socket for a 2-gang one where the boiler and the thermostat can be plugged in independently: the boiler as it is with its 13A fuse, and the thermostat with it own. Or perhaps there is a simpler solution?
Also, am I allowed to carry out this job as a DIY, non-registered, electrician?
I would greatly appreciate any advice.

 

[malcolmsanford]

Isn't this a wireless thermostat ?

http://www.plumbnation.co.uk/site/honeywell-cmt927-wireless-7-day-programmable-room-thermostat/honeywell-cmt927-technical-specification.pdf


Your quite right you should not have a socket spurred of an already unfused socket.

Yes you can carry out this work as long as your competent, unfortunately though by your description of the work you may need to notify it

New central heating control wiring installations are notifiable even where work in kitchens and bathrooms is avoided.

 

[k_d_ch]

Sorry I wasn't precise: I meant installing the thermostat relay. The boiler is actually in the bathroom, does it mean I can't do it myself, even if I notified it?

 

[ACEELEC]

The Honeywell CM927 is a wireless stat that we instal a lot as they have lots of extra features on them like Party (Boost) / holiday mode and you can easily set high / low temperatures.
Anyway they are normally connected via the boiler feed or from the existing spur.
They require a permanent Live, Neutral (and Earth) which is normally picked up from the Boiler L / N / E and then they require a connection from the correct terminals on the boiler through terminals 1 and 2 in the base relay.
We generally use a bit of 5core flex , 4 core if feeding a relay or 2 bits of 3core flex.
The competence part applies so only attempt if you are competent / confident
Connecting from the Boiler (appears to be plugged in) is definately more advisable than altering final connection (installing double socket), In my Honest Opinion that would appear to be the Building Regs bit

 

[HandySparks]

You don't need a second supply (and shouldn't use one). The whole central heating system should be supplied off the one socket or FCU. I doubt whether the existing 13A fuse is correct; you would need to check the installation manual for the boiler but I would expect 3A or 5A would be more likely. The thermostat should be supplied from the boiler or via a new junction box.

It sounds like you need to get in an electrician who knows what they're doing.

 

[Doomed]

I spend alot of time wiring up boilers.

1) Is the boiler, and therefore the spur in a cupboard in the bathroom, if not then more details from you are needed.

2) I really doubt that the 13A fuse is the correct fuse for your installation, 3A is alot more common in a domestic environment (infact I can't remember the last time I saw a domestic boiler with a fuse greater then 3A)

3) Using 5 core flex pick up the permanent feed from the boiler (L/N/E) and use the other cores for switched live and feed (if needed)

4) If any of the above needs further explanation STOP and seek professional help

 

[k_d_ch]

Thanks, Doomed.

1) The spur, along with the pipes, is in a cupboard underneath the boiler, the boiler itself however is not enclosed by a cupboard. The installation instructions for the relay say that is has to be at least 30cm away from any metal objects, which suggests the space outside the mentioned cupboard, midway (about 50cm to each) between the boiler and the shower cabin, underneath a window. Would this be acceptable?
2) You are right, I have opened the plug and the fuse inside is 3A. Is says 13A on the plug itself though!
3) I think I understand how the 5 core flex should be wired here. (Please see the attachment.) L/N will go to L/N at the boiler, there is no E terminal on the relay so the E core will not be used, and the remaining two cores will wire A to T1 and B to T2 (I guess it does not matter which of the two cores wires which pair).
What I don't understand is why there are two L terminals on the relay and why on the diagram L is not wired to the existing boiler L wire, the same way the N wire does.

4) As far as you know, can this be done by a competent/confident DIYer who notifies Building Control Dept?

Many thanks for your advice!

 

[andekoch]

There are 2 L terminals on the programmer (relay) because the programmer should feed live to the boiler and not boiler to programmer. T1 needs to be the feed to the roomstat and T2 needs to be the return from the stat, like the diag says, connect to the correct terminals on the boiler or you will have perminsng live on the incorrect stat terminals in the programmer.

Most plug tops say 13A because that is the rating of the plug top not necessarily the fuse inside it.

 

[k_d_ch]

Thanks andekoch. This has probably to do with some basics of AC as opposed to DC, but I always thought that once you wire three terminals of the same ``polarity'' together (L in this case) the result is the same no matter what exact configuration is.
Does what you are saying though imply that:
1) The existing L wire that goes from the boiler to the socket at the moment has to be re-routed to one of the L terminals of the programmer;
2) The other L terminal of the programmer has to be wired to the L terminal in the boiler socket;
3) The two L terminals of the programmer need to be wired to each other by an additional wire.
This is what the diagram seems to suggest at least.
Thanks again.

 

[andekoch]

Correct. Boiler control logic is fsu (in your case plug) then programmer then stat then pump then boiler. In the case of a combi the pump snd boiler are the same unit and in thr cast of you wireless stat the programmer and stat are one unit so; plug to wireless control unit to boiler.

Not quite sure what you are on about re ac and dc though. The 'order' I have mentioned is current best practice control logic. Technically the live from the plug, controller and boiler could be in any order and simply in one Terminal but this is messy and not best practice especially for future fault finding.

 

[k_d_ch]

I see what you are saying regarding fault finding, this is a valid reason indeed. In terms of how messy it is, I actually thought that one 5 core cable going from the programmer to the boiler would suffice, but from what we have come to it seems that single core wires will have to be all over the place: extending the existing L from the boiler for it to reach the programmer, individual wires to go from the socket to the programmer and from one L to the other L on the programmer, not to mention wiring the neutral.

 

[k_d_ch]

Correct. The 'order' I have mentioned is current best practice control logic. Technically the live from the plug, controller and boiler could be in any order and simply in one Terminal but this is messy and not best practice especially for future fault finding.

I have understood why it has to be the way it is on the diagram: because there is a switch in the programmer that controls the boiler. The switch itself isn't shown on the diagram for some reason.

 

[k_d_ch]

T1 needs to be the feed to the roomstat and T2 needs to be the return from the stat, like the diag says, connect to the correct terminals on the boiler or you will have perminsng live on the incorrect stat terminals in the programmer.

Could I ask you how I distinguish T1 from T2 when wiring? The boiler diagram does not seem to indicate which one is which, they seem to be identical. Thanks again.
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