Replace analogue Siemens RAA21 thermostat with Moes Smart Thermostat BHT002 - help appreciated!

[ratulb]

Hello all,

Hope you are having a good day.

I'm looking to replace the analogue Siemens RAA21 thermostats currently installed in my flat with Moes Smart Thermostats. I am hoping someone here can guide me which would be the correct Moes variant I should purchase and also how they should be wired.

Some further detail...

These Siemens thermostats control the underfloor heating system. A number of these are fitted in various rooms/zones. When the setting increases beyond a certain point, it makes a click sound and I can see that the underfloor heating system is then activated. The underfloor heating system uses the Emmeti EWC-2 8 WAY WIRING CENTRE as its connection box.

Below are pictures of how the existing thermostats are wired:

and this is its official wiring guide:

I would like to replace all of them with Moes Smart Thermostats. These come in three variants with different wiring requirements, and I am unclear which variant would be the appropriate one for my scenario and also how it should be wired. Please see below the 3 variants and their wiring guides:

I assumed I would use the 'GA - Water Floor Heating' variant but that didn't work. My hunch is that I instead need the 'GC - Gas Boiler Heating' instead as that involves a dry contact, but was hoping to get some guidance from people far more experienced than myself in this area

Any assistance with this would be massively appreciated!

Thanks

Ratul

 

[cross thread]

Looks like you need a neutral

 

[RandomGrinch]

...or a battery powered stat.

 

[ratulb]

Thanks @RandomGrinch @cross thread

If I am able to obtain a neutral wire, are you able to say which of the 3 variants would be appropriate for me?

 

[RandomGrinch]

My hunch is that I instead need the 'GC - Gas Boiler Heating' instead as that involves a dry contact

Yes, it would be the GC version you would want.

If you can show us a picture of the wires within the wiring centre, we may be able suggest how to connect them up.
However, you are likely to need a 3 core and earth cable run between the wiring centre and each stat, in order to obtain the neutral to power it.
How easy would it be to run a new cable?

 

[ratulb]

Thanks @RandomGrinch

Each of the stats is right beside a regular light switch, and those include a neutral wire. So was thinking I could get the neutral from there...

Would that make sense?

 

[RandomGrinch]

Each of the stats is right beside a regular light switch, and those include a neutral wire. So was thinking I could get the neutral from there...

I'm afraid borrowing a neutral in that way isn't allowed.
The power for the stat should ideally come from the heating circuit alone, that enables a single point of isolation for anyone working on the circuit.
Borrowing a neutral could also cause nuisance trips of the RCD/RCBO within the consumer unit.

 

[ratulb]

That makes sense, thanks for the warning @RandomGrinch

 

[ericmark]

I would assume controlled with a unit like this each thermostat will connect to the wiring centre some thing like this which has first two wires control, second two power the thermostat, not used with your version, and last is an earth? In which case the GC variant. But the thermostat has two different jobs. One open the motorised valve or actuator, and the other to fire the boiler, so some where there needs to be a wiring centre or other device to allow it to do both.

The normal or traditional motorised valve has a micro switch in the valve, where the under floor heating wiring centre has a load of relays inside the wiring centre. Both shown at 230 volt AC digital i.e. On/Off units, however there is nothing to stop an analogue unit from being used, slowly opening and closing, which would be a far better control method.

Oil boilers tend to switch on/off, gas boilers tend to modulate first, so a 28 kW boiler will slowly modulate down to say 6 kW and after that point it can't turn down any more so uses mark/space (on/off) control.

If the boiler can modulate, turning it on/off instead of up/down stops it working efficiently, it will work, but it will not gain the latent heat from the flue gases as designed to do.

In real terms true analogue control is rare, it is just a series of steps, so with 20 heat output areas, you have 20 steps even if each area turns on/off, but with 2 areas the steps would be too great, with radiators we use TRV's which have electrical or wax or bellows which are reasonably analogue. However unless linked to a hub or wall thermostat they have no method to turn boiler on/off, so the boiler would continue cycling even on a warm day, so some where it needs some method to turn the boiler on if days is cool, and off if days is warm.

You need some thing similar with under floor heating, and in the main the wiring centre does that job. And we have no idea what wiring centre you have, with radiators the buzz word is recovery time, or how long it takes to heat the room from eco setting to comfort setting, without over shooting, there have been many attempts to improve this, with TRV heads with algorithms built into them.

I use a simple method, the thermostat raises from eco to comfort over many hours at 0.5ºC raise every two hours, this means the oil boiler so it does not modulate, fires every two hours. Not ideal, but I work on near enough engineering.

So I would say you need to step back, and work out what you are trying to do. The old thermostat

was very clever, it would as it approached the target temperature start to turn off and on, slowly increasing the off time, to stop it over shooting, that was great with on/off boiler, but completely messed up a modulating boiler.

Clearly when not at home you only want a back ground heat to stop freezing and to speed up recovery time, but a simple on/off relay like a sonoff may work better than a programmable thermostat.

I was rather disappointed with geofencing, my thermostat (Nest) has no way to set how far from home it restarts the heating, and it restarts it far too late, so we have to manually go into the app and turn it back on before our return. Yes it have telemetry, but far from smart.

 

[RandomGrinch]

And we have no idea what wiring centre you have,

Try again Eric!...

The underfloor heating system uses the Emmeti EWC-2 8 WAY WIRING CENTRE as its connection box.

 

[ericmark]

Thank you missed that so this is the thermostat so 6 core cable, do like the idea of a "Night Set Back mode" however this

does not seem to relate any way to the diagram. Clearly some botch up. Under floor heating only on the boilers output can have one pump, as boiler set to around 30ºC, but if also running radiators or DHW often two pumps, so circulated water in the UFH is 30ºC but the DHW heated to 65ºC.

So still we have no idea how its been cobbled together, or what @ratulb is trying to do by changing the thermostat.

 

[RandomGrinch]

does not seem to relate any way to the diagram. Clearly some botch up

Definitely not a botch.
If you read note 1, without a timeswtch, TL and C should be linked.
With a mechanical stat, there is no need for TN.
NC is unused and NSB is optional.
Only a line and switched line is necessary.

The only botch is the unsleeved blue.

 

[ratulb]

Many thanks again @RandomGrinch @ericmark

Running new cables with the neutral I'm told is not going to be easy (not to mention likely costly).

So it seems I need to go down a battery-operated smart thermostat? My preference is one that runs on the ZigBee protocol.

Any recommendations? Don't think Moes do such a thermostat.

(I should add that my reason for wanting to switch to smart thermostats is so that I can set heating schedules, preferably through an app, for each heating zone independently of each other. That's not something I can do at the moment).

 

[RandomGrinch]

So it seems I need to go down a battery-operated smart thermostat? My preference is one that runs on the ZigBee protocol.

Any recommendations? Don't think Moes do such a thermostat.

Please don't take this as a recommendation!
All I'm saying is there are some available

https://www.amazon.co.uk/Thermostat-Temperature-Controller-Powered-Electric/dp/B0CG3H948R

 

[ratulb]

Hi all,

For anyone also interested in this thread in the future, I ended up getting this from eBay:

It uses 3 AA batteries hence no neutral necessary. Installation was straighforward. Brown went into COM and the other into NO (Normally open). Setup straightforward as was pairing with my Moes/Tuya Zigbee hub. Smart Life app interface is likely not as slick as other options but this setup is a hell of a lot cheaper. So we have here a satisfied bunny!

So I will now order myself some more of the same, but this time from AliExpress.

Thanks to all for your help on this. Very much appreciated!