Vaillant flow temperature control for HW

[Vikings]

I'm assuming that the demand signal into the boiler will override what the OT control might be doing so - in HW mode - if HW thermostat has reached temperature and CH TPI control say OFF then switching off the demand signal to the boiler (terminal 4 on my Vaillant board) will stop the circulation.

P.S. I found the earlier posting from bruce_miranda and, if I've read it correctly, that seems to confirm that the OT control can operate alongside using input 4 to switch demand on/off if required (providing the link from 7-8 is in place).

[bruce_miranda]

Infact what I would say is that if you were controlling this via the normal Terminal 4. Then DON'T connect the terminal 7-8. It works perfectly as you want it. Because your boiler trigger is still expected on 4. I was using this hybrid setup for a while before I figured out the importance of the documented 7-8 link. But that was required only because I wanted my OT to fire the boiler too, not just vary the target flow temperature.

p.s. The Vaillant doesn't understand OT. So I think the right way to think about this is that the Vaillant eBUS can work alongside normal 230V boiler triggers. And I can see why it's designed that way. Because eBUS is a lot more than just providing boiler triggers.

[Vikings]

I'm intrigued by what you are saying. Can you help me understand a bit more about what changes when you add the 7-8 link?
If we ignore the HW question for now and assume I was just running the CH then I'd have terminal 4 live all the time and using the OT control setpoint command to change the temperature. You mentioned in an earlier post that setting the temperature down to 10 would stop it circulating - does that still work without link 7-8? Is it just that the OT CH enable command is not used (or ignored?) in this scenario?

[bruce_miranda]

OT needs to perform basically two actions, fire the boiler and provide a target flow temperature.
Simply connecting the OT, with the terminals 7-8-9 and 3-4 left unconnected, the boiler wouldn't fire. But the target flow was being set by OT.
If I linked 3-4 the boiler was permanently firing. So what I had previously done was I used the Terminal 4 as a 230V trigger to fire the boiler, while OT just provided the variable flow. This was still while 7-8-9 stayed unconnected. This is what you want.
It was only later that I discovered that if I linked 7-8 and 3-4, my OT was able to both fire the boiler as required and vary the target flow temperature.
Hence why I think you shouldn't link 7-8 and just use a Terminal 4 boiler trigger.

[Vikings]

That sounds perfect. Just to confirm in this arrangement - if I set the target temperature low enough the boiler will still turn off the pump (as you mentioned in an earlier post)?

[bruce_miranda]

When you are controlling the boiler trigger via the Terminal 4, that question becomes irrelevant. The moment there is no 230V on the Terminal 4, the boiler starts the pump overrun timer and then shuts it off.

[Vikings]

That sounds workable for me. Thanks for the help.

[fezster]

Right, revisting this thread, as a user on diynot managed to get SL working with weather compensation and boosting DHW temp using a VR65/VR66:

https://www.diynot.com/diy/threads/v...2#post-4918624

The VRC40x controls the flow temps with weather comp, but the boiler is fired using the Nest SL's for CH. The boiler is also fired using ebus for DHW. He has used the DHW grey/orange as the CYL input on the VR65/VR66 to instruct the controller to boost the flow temps to 80 degrees.

My setup is slightly different as I have a LLH and a secondary circulation pump wired to the orange SL for all 3 of my zone valves (upstairs, downstairs and DHW). The secondary pump only operates when a zone valve is open, so there is no ABV required.

I see 2 solutions to operate my secondary pump. Hoping @bruce_miranda can provide some input here as you have played around a lot with this wiring centre:

1. I leave the DHW orange connected to the boiler SL, but also attach the grey / orange wires from the DHW zone valve to the CYL terminals on the VR65/VR66. They are labelled 230V connections, so I *think* this should be ok. But before I blow up a ~£50 controller, I'd like to check.

EDIT - the technical data says:

Contact voltage for DHW cylinder thermostat: 230V

Which gives me more confidence it should be fine.

2. I use the DHW zone valve contact relays on the VR65/VR66 to power the pump (as well as the SL's from my other 2 CH zone valves). The technical data for the VR65/VR66 shows:

Contact load of output relays (max.): 2A

I have a Grundfos Alpha 2L 15/60, which uses significantly less current than this.

If option (1) is ok, will the ebus fire the boiler as well as modulate flow temps? Is it possible to have it NOT fire the boiler, and leave that to only the SL. I'm thinking of a scenario where the zone valve fails and the boiler continually fires through ebus - not that it matters with a LLH, but would be nice to have it all reliant on the zone valve being open.

[bruce_miranda]

The way I use the VR65 is by using the microswitch (grey and orange) wires in my DHW to close the CYL inputs on the VR65.
My DHW zone valve is switched on by Evohome, but it doesn't need to be. Because 230V is going round that CYL loop, you can effectively use that as a SL for your DHW reheat cycle. You will just need to work out which of the two CYL terminals is Live 230 and which is expecting the loop back from the cylinder safety Thermostat.
The triggering of your boiler can stay the same.

[fezster]

Thanks Bruce. I'm actually thinking of using the grey/orange (230V mains voltage) directly on the VR65 CYL terminals. Even though the orange will backfeed from the other zone valves, only the DHW zone valve will bridge the CYL terminal.

My working assumption is the CYL terminal won't blow up with the mains voltage! I can't make out from your post if you believe this to be safe or not?

The only alternative is for me to use a relay to activate the CYL terminal volt free. But I like to keep things as simple as possible.