Vaillant system with 2 boilers

[MJHamilton]

Hello there - long time stalker and found this forum to be a very useful resource - first post so please be gentle

Some may find this interesting


Just moved to a large grade 2 property which has a rather big heating installation with 2 boilers and a DHW tank that is taller than me. It appeared to work for the first week we were here but then seemed to lose comms with the boilers and get confused, power cycling the boilers seemed to bring them back to life again (it ran as though it thought they were there but they did nothing) - I noted the settings and did a circuit digram (also corrected some distressed wiring) and then factory reset the controller - all worked again for a few weeks until the same happened again (power cycled the boilers are all good again).

Here is a the system - all Vaillant
- VRC700 controller
- VR70 wiring board
- ecotec plus 728 (appears to be triggered by DHW)
- ecotec plus 730 (appears to be triggered by CH)
- VRF 693 outside temp sensor wired to the 728
- There is also a larger Grunfoss pump which is triggered whenever the DHW/CH valve opens
- CH on on the VR70 R2 output has zone valve to rads (only one zone for entire house / 15 rads)
- DHW is on the VR70 R3 output has zone valve to DHW tank
- there is a DHW + 2 x temp sensors installed on S1, S5 & S6
- Boiler 728 is daisy chained to VR70 eBus
- Boiler 730 is daisy chained to Boiler 728 eBus

I added a VR 930f gateway so the whole lot works on myVaillant app now
- myValliant apps only see the 728 DHW boiler and not the 730 (same as the VRC700 which only sees 1 heat generator)
- occasionally the VRF 693 outside temp sensor throws an error and then comes back (this may be related)
- I noted inside the 730 boiler (which is more modern there is a bus id wheel set to 1)
- I have Zone 1 disabled as it is not used
- I have Zone 2 connected to the VR700 for the temp of the CH

So to summarise the issues
- when it all works it works great
- occasionally there is a hiccup and the 2 x boilers seem to drop or lockup on the eBus (power cycling both seems to resolve) - the system tries to run the main pump with the valves open but the boilers do not fire up when they should - it ends up pulling the stored heat in the DHW tank into the CH system so eventually it all goes cold)
- on the VRC700 and myVaillant app only the 1st boiler shows up on the device list even though both work (728 DHW & 730 CH)
- I replaced the VR70 but no change in behaviour
- It was like this when we moved in so I do not think it was ever configured 100% and the previous owner didn't realise (do not know who installed or configured it)

Is there a white paper for the correct setup and configuration of 2 boilers on a Vaillant eBus system? do I need to adjust the eBus ID for the second boiler? I am puzzled as to how this is working at all - when it does work the system uses each boiler for a specific purpose and yet I see no configuration for this.

Any help or advice would be great, also if there is a Vaillant specialist in the Crawley area I am happy to pay for an inspection/tune up

 

[Mister Banks]

Are you sure the 728 is for DHW? I ask because the outside temp sensor which you say is connected to the 728 should be controlling the flow temp of the CH boiler.

 

[MJHamilton]

yes sir... when the DHW demand comes on the 728 fires up -- this is the older of the 2 boiler and closest to the house on the boiler room external wall so I can see the exhaust when it does - also when sitting idle the 728 shows a tap icon and the 730 a rad icon

What I cannot understand is how the second boiler is seen and how the system is calling one for each type of demand - it's a puzzle - I assume there is a small something that was never configured 100% which is the root of the issue

the previous owner didn't really know how it worked hence not realising there was an issue - he thought they worked as a cluster and both would fires up together depending on total demand

apparently the 730 was installed to replace a boiler already there so I think this is where the issue started

 

[MJHamilton]

Here is what the VRC700 and Gateway see - this is the 728, the 730 doesn’t show up at all even though it works

Only heat generator 1 - my brain tells me it should also see heat generator 2

I am wondering if I need to adjust the eBus ID wheel on the 730 to give it a unique ID on the eBus -- maybe it is the same as the 728 and there is a conflict which kicks off sometimes (although I cannot see a way to change this ID on the 728 so this may be a red herring)

 

[MJHamilton]

So again the system has been working perfectly for the last few days - heating and water using both boilers as demand dictates


I have noted that I haven’t seen the error with the outside temp sensor during this time and am wondering if this is tripping the bus signal when it occurs causing my issue

For the sake of £45 I may just replace this for belt and braces and see

I still cannot work out
1. Why only one boiler (first in the eBus physical wiring) shows up on the VRC700 and myVAILLANT app
2. What determines the boiler assignment to CH and DHW when called

I sent a message to Vaillant asking if there is a white paper for best practice when having 2 eBus boilers connected to a system

 

[MJHamilton]

Ok figured it out

1. It seems to cascade boilers the extra ones need a VR32 eBus cascade card installed so you can change the id from 0 to something unique
2. Whilst checking the second boiler I found said card installed
3. Whoever installed the boiler left the id on 1 which means it was the same ID as the VR70 - so this was a direct conflict
4. Changed to 2 and power cycled the entire system

I now see the boiler on the network as Heat Generator 2 and extra menus have appeared to control and manage their firing order etc - have set them to swap over each day and come on one or both depending on the demand

It was basically an eBus device conflict, boilers much have been trying their best with some ghost configurations but under certain conditions the conflict would simple crash the VR70

Happy days, I guess whoever installed the boiler last year didn’t really know what they were doing and left because it “appeared to work” and the previous house owner didn’t realise it was just about running on only one unit