I have seen a couple of threads regarding how to wire up the LW920 switch, and less so the LW934 switch. Perhaps it’s due to all of the variations in boilers, but I’ve found no detailed descriptions of installs of these two switches with Vaillant boilers. So assuming all goes well, I plan to use this thread to document what I do with my install.
I’ve got British Gas coming in next week to install a new Vaillant EcoFit 418 (conventional condensing boiler) with Hive. I was going to remove the Hive component on the basis that I planned to install a LW heating solution, but it only adds £10 to the total (long story), so thought I might as well, then I can sell on the Hive kit separately.
The other benefit of allowing BG to install the Hive kit is that I’m hoping I’ll be able to easily replace the Hive receiver with the LW920 and LW934 switches, specifically in respect to how to wire these switches up inside the boiler electronics box. One thing that isn’t clear though is the way to wire up the COM and NC connections.
On the LW920 boiler switch, the LW instruction manual shows that the COM and NC connections are linked (I assume on the inside of the switch). Consequently I only need to worry about connecting up the COM and NO connections to the boiler.
The LW934 switch is slightly different in the fact that it doesn’t have an NC connection. Can I assume that this is because it follows the same approach as for the LW920? I.e. that even if there was an NC connection, it would be linked (internally) to the COM and it would never be used to connect directly to the boiler?
I’ve just installed the LW920 switch for a neighbour and you need connect the room thermostat connections from the wiring centre to the NO and COM connections on the LW920 boiler switch. The diagram you are referring to is showing the NC connection as the default so COM and NC are connected when the switch is “Off”.
For the LW934 electric switch – For Hot Water, you again only need to be concerned with the COM and NO terminals. However, if you have a Y Plan heating set up (with a 3 Port Mid position valve instead of 2 or more 2 way valves) I can’t see a way to use the LW934 so am looking to introduce the inline relay LW821 to control the hot water. This is due to the way the Tank thermostat is electrically linked to the wiring centre and timer in a Y Plan. In both the “call for heat” and “off” situations there is a need for power to be provided to the cylinder thermostat. If “Off” – Then the power feeds the grey wire at the 3 port valve via the tank thermostat which means the 3 port valve just maintains whatever position it was in. So using the LW934 switch won’t do the job as it is just a simple on/off switch.
The LW821 relay does have switched live 1 and switched live 2 and from what I can see reading all of the Lightwave literature, if the relay is switched “On” power is available at Switched live 1 (terminal to the left of COM) and if the relay is switched “Off” power is available at Switched live 2 (terminal to the right of COM).
As I write this it has just dawned on me that a 2nd LW920 boiler switch may actually do what I need for the hot water as the NO and NC terminals will act in the same way as the LW821 relay’s switched terminals in that NC is powered by default when the switch is “Off” – But the relay is cheaper.
Hope this is of some help.
See here for a great description of the Y Plan wiring set up: https://www.flameport.com/electric/central_heating/heating_wiring_Y_plan.cs4
Thanks for your response and the link to the write up of the Y plan and use of Hive etc All of this is really helpful. I’ve also seen another mention on the web about the use of a second LW920 boiler switch to control the hot water, so I’ve ordered 2 boiler switches already just in case!
What I can’t quite understand is why Hive can just be hooked up in its dual channel mode (heating and hot water) and it works in a Y plan system, whereas to replicate this with LightwaveRF using the two different switches (LW920 and LW934), it won’t be as easy? Surely the challenge around the tank thermostat is the same for both Hive and LightwaveRF? I appreciate LightwaveRF configuration is more complex since we are dealing with two different switches, both connected to the boiler.
In fact in the Hive setup (dual channel mode) there is no COM connection. It appears from the Hive diagrams as if there are only NC and NO connections.
Bit more background on my existing set up: I’ve got a 19 year old Glowworm boiler, with the wiring centre in the airing cupboard upstairs, next to the pump and the Y valve. I guess that’s the old style of set up? I assume with the new boiler being installed, all the electrics are within the boiler casing, so the old wiring centre will be removed from the airing cupboard?
Having now watched the Y plan video posted on the link you included, I think I now better understand the need for power (into the cylinder thermostat and the mid position zone valve grey cable), even when the hot water switch is off. So can understand why a simple switch won’t do the job.
So if I use a second LW920 boiler switch for the hot water, the two connections I need to link to the boiler are NO and NC. Does that mean that the COM (on the boiler switch) needs to be connected to the L on the boiler switch, since it needs power? Or is that connection already made inside the boiler switch? Sorry – probably a stupid question.
Not a stupid question at all. Yes, you will need to connect the COM to the Live so either a bridge to the incoming L or direct to the L in the wiring centre. This is the same for the CH thermostat’s COM connection.
As for how Hive does it, sorry but no idea.
Just to add, after looking at the HIVE installation instruction, it does have dual channel control via the HIVE receiver and it will attach to a standard timer/programmer backplate so it is controlling Heating on and off as in a traditional (dare I say analogue) configuration. With LightwaveRF the parts are all separate so I must assume that 2 boiler switches with one connecting to a room thermostat for CH and the other controlled directly from the Lightwave Link Plus for Hot Water, you will achieve the same result. I make the distinction between the Gen 1 Lightwave Link and the Gen 2 Lightwave Link Plus as the Gen 1 version will not directly control the boiler switch (only through a Thermostat) and Gen 2 does – I have personally seen this in action on the Gen 2 system.
I make the distinction between the Gen 1 Lightwave Link and the Gen 2 Lightwave Link Plus as the Gen 1 version will not directly control the boiler switch (only through a Thermostat) and Gen 2 does – I have personally seen this in action on the Gen 2 system.
That’s interesting. I was about to send my Link Plus back since I couldn’t see any point in keeping hold of it. If keeping it means I don’t need a second (pointless) thermostat to allow me to control the HW boiler switch, it’s worth holding on to, and keeps me up to date to boot.
As before, very much appreciate your comments.
Definitely keep the Link Plus for future proofing. I have the Gen 1 version so if I add a second Boiler Switch for Hot Water control I’m going to have to upgrade to the new Gen 2 one. I’ve just installed the Gen 2 along with the Boiler switch and Thermostat (only for CH at this stage) for a friend and it was as straightforward to set up as the original so if you have any questions about it once you start the installation and you think my experience may help just let me know.
Good news (well for me anyway) – British Gas installed my new boiler yesterday and did a fabulous job. When the sparky arrived, he was happy to wire up the two LW920 boiler switches for me instead of the Hive controller. We had quite a long and protracted discussion about how the standard Hive wiring would need to be adapted for the two LW switches, but came round in the end – background info provided by this forum helped me pretend I knew something about relay switches and the like. British Gas even took the Hive controller away and organized a refund!
I need to work out the best way to programme it all now. Will write up some notes about the install in due course with pictures etc.
This is a great result and I’m glad it has all installed nicely. I do like a clean looking plumbing installation (sad I know!).
As for programming, I was a little confused with the Gen 2 set up as, with Gen 1, there was a specific “Heating” section to the app. With Gen 2 it is all done through “Automation” and then “Timers”. This is a little more clunky to set up but it is far more flexible. Good luck with the programming and I look forward to your write up with pics.
You’ve inspired me to get my hot water Lightwave managed tooso have just bought my 2nd Boiler Switch. I was going to buy the inline relay to switch on and off an outside non-dimmable light but Lightwave has a Gen 2 version of the relay in development so will wait until that is released.
Now Lightwave need to develop a temperature probe or smart cylinder stat so we can see the temperature of the hot water in the cylinder.
I am a little confused between the LW921 (Thermostat) programs and the timer schedules in the Link Plus app. Are they completely independent? And potentially conflicting?
Also, with me having TRVs in some of the rooms in the house, if I set up a schedule for that, will they “call for heat” if the boiler is not on?
The timer schedule in the App supersedes the schedules set in the Thermostat. To make sure I cleared the Thermostat back to factory settings (remove the batteries) and just set the time once the batteries were reinstalled. At this point I did not set anything else on the Thermostat. Make sure the Thermostat is linked to the Boiler Switch and the Thermostat is linked to the App (via the Light Wave Link Plus). Then I set up the Timers on the App and it has been working fine doing it this way.
As for the TRVs, I’m not sure how this works with Gen 2. With Gen 1 you set up the “Call for Heat” in the Web Manager. (There is no Web Manager yet for Gen 2) There you set a parent device – the Thermostat – and the children devices – The TRV heads. If the TRVs needed heat from the boiler the request is sent to the Thermostat which in turn contacts the boiler switch.
I think on this one, a call to Lightwave RF tech support is the best way to understand this under Gen 2 control. Their documentation for Gen 2 is lacking.
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