HIVE & Vitodens 100 with unvented HW Cylinder

[TMcL]

I've moved into a property which has a Vitodens 100-W connected to an unvented HW cylinder (Vitocell 200). Currently it's controlled by a Nest thermostat but the problem is that there's no separate HW time control & what happens is that if someone has a bath in the morning the boiler gives priority to reheating the cylinder and the heating goes off until the cylinder gets up to temp. It's a big cylinder & I want to time the HW so it gets up to temp in the middle of the night and is off first thing in the morning & use HIVE to do this. I can't find any wiring diagrams for the Vitodens on the web for this setup. Does anyone have any suggestions? I did see in other posts on this forum that some boilers have a low voltage cylinder thermostat - I can't find out what the voltage on the cylinder thermostat is.
TIA

 

[RandomGrinch]

Currently it's controlled by a Nest thermostat but the problem is that there's no separate HW time control

I'm afraid there are quite a few versions of the 100-W, that have different features. Do you have precise a model number / date of manufacture?
Can you post a photo of your system?

Also, what version Nest do you have?
The third gen Nest can control hot water timing - but this may be irrelevant, as it seems some versions of the 100-W can set timers and turn off PDHW.
Also the Nest may be controlling heating via OpenTherm, which can provide better modulation of the boiler (together with an improved efficiency), when compared to an on/off stat like the Hive.

 

[TMcL]

Thanks for this. It's a 100-W B1HA. Installed November 2015 so I guess manufactured 2014/15. The Nest is Display 2.8, Backplate 3.2, Heat Link Amber - 1.5 - I think this means it's 2nd generation (illuminated display diameter 1.75in)? Attached are pics of the system.

 

[RandomGrinch]

I'm afraid I'm not an expert, hopefully others may have different ideas, or a workaround...

However, having looked through the installation manual, I'm afraid this version seems to be one of the least flexible 100-W's.
If I have understood it correctly, to use anything other than the 'low voltage" cylinder thermostat, you need to use the "Cylinder demand terminal box".

Cylinder Demand Terminal Box

7574319 Cylinder Control Box

viessmanndirect.co.uk

This allows for 230V controls to be added, but you would also require a 230V stat to be fitted to your unvented cylinder, this would require a G3 registered RGI to install it - not a DIY job.

On the slightly more positive note, although your existing Nest is a gen 2, the boiler does support OpenTherm, so if you do eventually change your controls, you could benefit from better modulation control, if you wanted to.

 

[TMcL]

Thanks - I was beginning to suspect something like this. I'd wondered about getting a relay fitted into the cylinder temperature circuit but it seems it's not an open/closed thermostat but has a variable resistance from 20k-2K Ohms. The cylinder control box is £115 plus I'd need to find a 230v thermostat which would fit the spare opening in the cylinder. I think we'll just have to avoid baths in the morning on cold days! It does seem bizarre that there is no option to send a 230v HW on/off signal to what is I think quite a sophisticated boiler.
Thanks again.