(UK) Vaillant + DHW cylinder + Evohome?

[biohead]

Hi all,

I've recently had a new Vaillant Ecotec 424 installed - this is a vented system with DHW cylinder (single heating zone, radiators only). This replaced an ancient system which was woefully too small for the house (15kW).

I (naively) opted to carry over my Hive controller - and starting to see the downfalls now. The biggest one is that the boiler only ever sees a "demand" - it can't differentiate between DHW heating and radiator heating, so I can only set one temperature to heat to.
This is causing the DHW heating to constantly go into anti-cycle, which also stops the pump after so long. Once the temp has dropped low enough everything comes back to life. It just takes a long time to heat the cylinder like this, unless I raise the temps (not an issue in summer as CH not used).
The original installer has kindly offered to drop off a VR66 wiring box and a basic Vaillant programmer from his stock. It's my understanding the VR66 allows the separate control of the DHW and CH systems so will be a permanent fixture. Does anyone know otherwise?

I do like the look of the Evohome, but I'm struggling to see how I could get it to work with the VR66? I know I can get the VR33 Opentherm module (warranty issues aside) - but will that conflict with the VR66 and can the boiler use eBus for that and OT for Evohome?
If I need to remove the VR66, does the traditional y-plan wiring get used (which then puts a single "demand" on the boiler)?
If there is a single demand, is the Evohome is able to tell the boiler what temp to use (so higher for DHW, and lower the CH?)

Sorry for the huge list of questions, but I want to be sure I can get everything working properly before committing. I do like the look of Evohome, and the starter packs seem pretty cheap recently - but it might be wasted money and I'll have to stick with the Vaillant programmer.

[bruce_miranda]

I can answer this because I have done a lot of research into this configuration. I actually have a 438, so same boiler too.
In order to have two flow temperatures for CH and HW, you need to use the ebus wiring centre, an ebus thermostat and then also the HW cylinder sensors etc. Unfortunately its an all or nothing.
The other thing that took me by surprise is that the VR33 has a lower hierarchy priority to the ebus room stat, so is completely ignored. So you cannot have OT managing the variable target flow any more.
Evohome via OT also operates as a single flow system but can atleast determine if it needs to be full or modulates. I can go on forever on this. But you need to make a choice. Its either all Evohome or all Vaillant controls.

[biohead]

... you need to make a choice. Its either all Evohome or all Vaillant controls.

That is excellent info! Also interesting to note you can use the VR66 and VR33 together, albeit maybe of no use if I read that right?
You can't setup the Vaillant controls to control DHW only via the VR66, and have an OT stat controlling the CH via the VR33?

I'd be quite happy to go for all Evohome, as long as it could manage the DHW and CH separately and not present the same issues as I have with Hive (single temperature, causing anti-cycling to kick in). If you do use Evohome, how does the wiring get connected from the valve, DHW stat etc?



Although.... all that said (and do forgive my ramblings here, I'm continually learning about this subject), maybe it's been caused by my DHW water setting being too low (Tank stat 50, flow set to 55). Perhaps if I raise the tank to 55 and flow to 65 that will keep the boiler happy. It's not been cold enough to really try out the heating yet, but at 65C flow there must be enough heat loss through the rads to return below 50 and do this magic condensing business.

[bruce_miranda]

The VR33 uses the PCB eBUS connector where the VR66 uses the marked eBUS connection. But like I said, the moment there is another room thermostat on the eBUS the VR33 is ignored completely.
No other system besides Vaillant's own controls will allow you to have two different temperatures. So in that sense Evohome is not your answer.
You should have your HW at 55C or higher.

[biohead]

So, if I were to settle on a single temperature (say 65C), the Evohome could work??

In that case, I'm trying to image how the wiring would be - what would be the trigger to the 3 way valve, where would the cylinder stat connect to etc?

Are there multiple parts of Evohome that would be needed that take care of all that?

[bruce_miranda]

I think you need to spend some time working out what you need, or better still get someone in.

[mtmcgavock]

This is causing the DHW heating to constantly go into anti-cycle, which also stops the pump after so long. Once the temp has dropped low enough everything comes back to life. It just takes a long time to heat the cylinder like this, unless I raise the temps (not an issue in summer as CH not used).

This shouldn't be happening, even with just a HW demand. There's either something not configured correctly in the settings (Just read you've set the flow at 55oc when cylinder is at 50oc) or there's a blockage or a circulation issue. Has the cylinder been valved down/restricted too much? A 424 will modulate down to around 6kw which should be fine with a normal DHW cylinder coil. Fitted many Vaillant boilers and don't have any issues with just heating HW in summer - anti cycling means there's a bigger issue at play. You need at least a 10oc higher flow temp than your cylinder set point so 65oc.

Also a 3 port valve shouldn't be used with a VR66 as the valve is held open in certain programs - causing it to get rather warm and shorten the life of the valve. Ideally you need two 2 port valves or a diverter rather than a mid position valve.

But yes a VR66 and Vaillant controls will give you the ability to have different flow temps for HW reheat and heating circuits - they've actually just brought out their new Senso range.

[biohead]

Thanks all, I've had a rethink and know what I want a bit more... whether it's possible I'm not sure.
Also, the issue of anti-cycling was a bit of a non-issue. It was an error of me understanding what the symbols were showing - I've now got a better grip of the menu system.

At it's core, I'd like to be able to get the boiler to modulate.
Other than Vaillant, are there any controllers out there which can control 240v valves directly AND talk to the boiler via OpenTherm (or eBus)?

[bruce_miranda]

First of all, what makes you think that your current setup isn't modulating. People often confuse modulation and maximum flow temperatures. Unless you have an eBus monitor or the evohome OT gateway, you cannot even work out what the boiler's current modulation level is. So I am interested to know what you are using?

Vaillants use eBUS and nothing else. In order to set variable flow temperatures you need a control that can talk eBUS. So first off, research that aspect. If you then realise that there aren't too main native eBUS controllers, then you look at OT (via a VR33). There are a few OT controllers, so you have options.

It all depends on what your end goal is. Let the boiler decide how much to modulate, because no matter what you do, it will decide what's good for it and do it anyway. I have yet to see a single control that can control the Vaillant's modulation level.