- regards UK conservatism re controls - many persons end up in the trades as they are better with their hands than academic pursuits - they get shunted into work with no real education for the why's and wherefores of what they are doing other than that given by their workmates who came up the same way and therefore the vernacular persists while it still provides a profit and nothing ever changes.
Training establishments and trade bodies don't help either, eg. when I last renewed my OFT105E just a few years ago 50% of the module was still concerning single pipe and gravity circulation issues and the totality of heat loss calcs was that ridiculous CE54 worksheet.
The opportunity was missed with mandation of condensing boilers to also mandate controls and techniques for maximising the efficiency predominately by lowering (flow) and return temps which would have left the industry in much stronger position for LT renewable solutions. Boiler plus was another squandered opportunity!
Not a massive fan of the Ecofits as have always regarded aluminium hex as an inferior hex material pressed into service for a cupboard fit solution, also wait until you want to get the burner out for service, that back left nut hard under the primary pipework will have you cursing the designers!
I'm not positive, as Ecofits and I only overlapped a year or so before I ceased dealing with Vaillant (Aluminium, attitude and the farce of Ambisense sent me to Telford!), but as I recall the electronics were totally new and do not offer the flexibility of the ecotec plus to patch into the dual stats without Ebus.
Obviously the VR33 will give you an in via OT to Ebus (but no VF1 connection) but the possibilities are then totally in the hands of your OT controller.
I was never a fan of Nest but I am not aware of a WC facility, but I would expect it to try and load comp which is where the OT conflicts start to raise their head when the boiler is also trying to do the same whilst imposing it's own ramp rates and emissions control.
If you really want to pursue the OT route there are some excellent threads on automated home forums where the guys have been patching Resido Evohome into modulating control of their boilers, and some real world details of where one mans OT implementation differs from the next.
For the control you have outlined I would strongly suggest you fit a VRC700/VR70 system which will give you all you are asking and more in a no nonsense manner with no compatibility issues - the Vaillant VRC400/430/470/700/620/630 and associated modules have always been excellent bits of kit that I couldn't recommend highly enough - Vaillant's ace in the hole as far as I am concerned!! I retired before the Senso range came on the scene so don't know if that has dropped the ball but wouldn't use it myself as despise touch screens.
It might not bother you (and the secondhand situation may already have done for it) but be advised that a VR33 on a UK boiler is instant void on the warranty.
Hope that helps some!