Thermostat wiring help!

[Slowreader]

Hello!

I need some help installing a Honeywell Lyric T6 (R4H91) in replacement of a 2-wire wall thermostat in my new home. I can't figure out how to get it to work with just the 2 wires.

I've tried the below, which powers on the box but does not trigger the boiler to heat. I feel like I need to connect something to "B" but don't know what. An extension of the "N"?

Here's the existing working unit:

There's a few wiring options
but I can't seem to marry it up with what I have... I have a standard (non-combi) boiler with a separate hot water cylinder.
Thanks!

 

[ianmcd]

your existing thermostat is battery powered the wires in 1&2 go to A&B on the new thermostat, but you wull have to run a seperate cable with a L&N to supply the new thermostat, this supply must come from the same supply as the boiler, you would be better bying another battery powered thermostat

 

[Deleted member 267285]

Alternatively, you could position the receiver by the wiring center and make any alterations there, however, for easiness I agree with @ianmcd

 

[JohnD]

If you can tug that cable out of the wall, you might find it has some additional unused cores. That you could use.

I fear some dummy cut them off.

Or it might be oval conduit with singles pushed down it.

Where does the other end come out?

 

[Slowreader]

Thanks very much. I feared as much! There are only two other unused cores in the wall: Blue and Earth, I am pretty sure they go straight to the fused spur under the boiler (as i blew that 3A fuse with one of my botched wiring attempts!)

 

[JohnD]

I doubt it.

A 3+E cable (old red, yellow, blue, bare) is capable of carrying

permanent live

neutral

switched live

bare earth (if singles, should be green and yellow)

Which is probably exactly what you need.

Your old stat probably only used

live

switched live

 

[gas112]

Edit

 

[Slowreader]

OK that sounds promising... So would the below be something worth trying?

 

[stem]

Possibly. However, if the blue wire isn't connected to anything at the thermostat end, there's a chance that it might not be connected to anything at the other end either.

Ideally check with a multimeter between the red and blue wires for 230V first.

It's unlikely in your case from the brief description you give of your heating system (ie it has a hot water cylinder and is not a combi) that the red would not be a 'live', but some boilers, noticeably combi's only have 24v run to the room thermostat.

Assuming you do have 230V, if there is a separate programmer somewhere else controlling the heating, then the heating would need to be set to be permanently 'on' there to maintain a live on the red wire.

 

[ericmark]

I have a standard (non-combi) boiler with a separate hot water cylinder.

In which case the thermostat likely powers a motorised valve which in turn powers the boiler. As said the two wires go to A and B as shown

with a little luck there is a blue in the cable so link and blue in neutral.

Most instructions say "provide one common electrical supply to the boiler and for the cosponsoring controller."

I have not worked out why with a volt free thermostat this should be required, it seems OK to mix low voltage and extra low voltage in the same thermostat, so see no electrical reason why two independent electrical circuits should not be used, that is clearly what happens with battery powered, and thermostats like Nest have a power supply to give 5 volt to the USB which invariably is not plugged into the same supply as the boiler.

But clearly if independent circuit is used the link should not be fitted.

My central heating now has 10 supplies, one 230 volt and nine 3 volt. When I moved in it had at least three 230 volt supplies two being from different distribution units, with a large sticker saying who installed it, seems even proud of their work.

I would say if possible all central heating should be supplied from one BS 1362 fuse as to if 3 amp or 5 amp depends on how much is supplied. And there should be a means to isolate the low voltage (230 volt) supply. However I can see nothing in BS 7671 which says it must be wired this way, only that manufacturers instructions should be taken into consideration.

The Nest e is about the only thermostat where the base is battery powered. And the Nest e does not connect to TRV heads.

Your the one who should fill in the minor works certificate to say what you have done, and sign it, so your the one to decide how to do it.

I fitted a Nest Gen 3 to my central heating to replace the programmer and re-arranged the wiring so all supplied from one RCBO and FCU. Main reason was so the existing wires only carried low current and extra low voltage (12 volt) from flat under main house to the main house, wires changed colours and no idea where, so don't want low voltage on them.

However the geofencing was switched off earlier this year when the EE mast was damaged in high winds so the system did not detect my phone as being at home. And most the other special features for one reason or another have also been switched off. Non of the so called Smart features are still in use. The linking the TRV heads was first to be switched off. It should have worked, but didn't.

So why bother fitting a Smart thermostat?

 

[stem]

Most instructions say "provide one common electrical supply to the boiler and for the cosponsoring controller." I have not worked out why with a volt free thermostat this should be required, it seems OK to mix low voltage and extra low voltage in the same thermostat, so see no electrical reason why two independent electrical circuits should not be used

I would have thought one reason was fairly obvious. When anyone works on the installation, they turn off the 3A fused connection unit supplying the boiler and expect the boiler and all of it's associated components such as thermostats and programmers to be isolated. If there is a second 230V supply introduced from somewhere else, they run a real risk of getting an electric shock. Any "low voltage" would come from the boiler's control circuits, so would be switched off with the boiler, and even if it wasn't you can't get a shock from 24v.

Can't help wondering how many DIYers that post on here asking for advice to change a thermostat do anything other that switch off the boiler FCU before removing the old thermostat, can't imagine many would go looking for a second supply, or test the circuits with a multimeter. It's hard enough to get most of them to find out where the thermostat wires originate from.

 

[Slowreader]

Possibly. However, if the blue wire isn't connected to anything at the thermostat end, there's a chance that it might not be connected to anything at the other end either.

Ideally check with a multimeter between the red and blue wires for 230V first.

It's unlikely in your case from the brief description you give of your heating system (ie it has a hot water cylinder and is not a combi) that the red would not be a 'live', but some boilers, noticeably combi's only have 24v run to the room thermostat.

Assuming you do have 230V, if there is a separate programmer somewhere else controlling the heating, then the heating would need to be set to be permanently 'on' there to maintain a live on the red wire.

That's exactly right, I leave the heating on the programmer (which is next to the boiler itself) permanently on. I will have a look at the voltage between the blue and the red but also suspect it isn't connected at either end. In which case I'm not sure what to connect it to at the boiler end... It's probably obvious by this point that my level of technical experience with household electrics is minimal so I'm giving serious consideration to going the Nest route or (if also not compatible with my wiring) just switching it out for a less 'smart' option which I can at least add timings and temperatures to and remotely control more easily than my existing stat.

I always use a non contact voltage detector (obsessively!) when doing this stuff so extra circuits would be fine for me personally, though I see the danger!

 

[Deleted member 267285]

I always use a non contact voltage detector (obsessively!) when doing this stuff so extra circuits would be fine for me personally, though I see the danger!

Do you see the danger using non contact voltage detectors?

 

[ericmark]

If you don"t know how to test for dead, you should not play with low voltage.

I Any "low voltage" would come from the boiler's control circuits, so would be switched off with the boiler,

Only if connected to boiler.

Even with extra low voltage it can come from batteries or a USB supply, which in turn is unlikely to come from same circuit as boiler.

 

[stem]

Only if connected to boiler.

Indeed. The boiler, and that's exactly where 24v control circuits do get their power from; as per the example below.

Some thermostats can be powered by batteries, or via USB both of which would be obvious, the wires are not terminated at screw terminals, and are not actually controlling the boiler.

However 12v / 24v is not the issue, the issue is the danger associated with having 230V mains from more than one supply.