Multizone system and OpenTherm, mixing radiators and underfloor heating

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I’m in the process of acquiring a new boiler due to the conversion of my loft and the installation of a mechanical ventilation and heat recovery system. I’ve recently discovered that Google Nest is unable to handle multi-zone systems and reverts to a simple on/off thermostat. My preference would be to have underfloor heating on the ground floor, with the first floor and loft each having their own zone. Given the excellent insulation of the house, I anticipate that the underfloor heating will be used most frequently, with the first floor being heated occasionally and the loft rarely, as heat will naturally rise to this area. By the way, I’m also installing a 300-litre horizontal cylinder due to the limited space in my house.

I’ve been in touch with Baxi and Vaillant in an attempt to find a solution for controlling all three zones from a single location, but it appears that both companies are leaving me to figure this out on my own, offering no assistance. My aim is to use OpenTherm to prevent overshooting and prioritise comfort. I also want to control everything from a single system, for both radiators and the underfloor heating system.

I’m considering the Baxi 800 System 2 boiler, which is OpenTherm compatible and seems to have good internal components. Oddly enough, their own thermostat can only communicate via OpenTherm with their combi models, not the system boiler. It’s puzzling why their latest thermostat can’t communicate with their own system boiler.

My research on various forum threads has led me to Evohome, which I believe is a sub-brand of Honeywell. Others have suggested EPH, but I’m unsure of their quality as their physical appearance suggests a somewhat outdated design. I don’t intend to constantly tinker with the system, but I do want remote control. My plan is to set everything to around 21°C, then leave it to weather compensation and OpenTherm to modulate the boiler to prevent overshooting, thus ensuring comfort.

A thermostat with a pleasant interface or app would be a bonus. I don’t actually need wall-mounted thermostats with controls; I can manage everything via an app and simply have temperature sensors in each zone for control. However, I’m not sure if this is too simplistic a view and may not work well in practice? I don’t need screens and controls if I can set up everything via an app and control the system via Google Home or Home Assistant.

Any advice would be greatly appreciated, chaps!
 
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doing a bit more of research gives me Evohome as the only underfloor heating system that is opentherm compatible. Then comes the question, is it worth going through all the hassle and costs? I would like to use proper opentherm everywhere as I like what it does on paper.

EPH does not have an underfloor heating controller, so I would need to use a 3rd party, not sure how well it would work with a non opentherm 3rd party controller mixed here

Drayton has underfloor heating controller, but I've no idea how well it can work when mixed with opentherm thermostats
 
Why are you limiting yourself to a traditional bulk-zoned system?

Why not control every emiter individually?
 
Why are you limiting yourself to a traditional bulk-zoned system?

Why not control every emiter individually?
I could do as well. I never considered them as I'm not entirely sure how reliable they are.

Do you think it makes sense to go for an opentherm underflor heating system?
 
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After some more digging, it seems the Viessmann 200-W may do what I want controlling underfloor heating and radiators, I'm checking the documentation and have a quick chat with viessmann engineer today on the phone to get some more details, more to follow next week...
 
Hi Alex889
I am in a very similar situation to yours and would love to know how things progressed for you.
Please would you update?
 
I'm going with a Viessmann 200 system boiler, using Esbe valves for each heating circuit controlled directly from the boiler, no thermostats involved. I'll be using weather compensation. It will have a low loss header as I'm going to use 2 mixed circuits.

This way I can control my underfloor heating and my radiators independently from each other. No 3rd party controllers, no underfloor heating controllers, everything done directly from the boiler. I'm planning to leave my underfloor at constant temperature, but as my house is very well insulated, my set back temperature for radiators overnight may turn the radiator circuit down or even close the circuit completely to avoid overheating.
 
Hi Alex889
I am in a very similar situation to yours and would love to know how things progressed for you.
Please would you update?
You might be interested in seeing this video from Urban plumber



 
You might be interested in seeing this video from Urban plumber



That still needs Vaillant thermostat, even if using weather compensation. I would like to avoid any external controls on the wall, Viessmann has the controls directly on the boiler, also the network to manage the boiler remotely. No need to any external kit.

I think his set up would work better in a house with higher thermal loss, mine with less than 4kw would likely overheat. Also the boiler would be cycling. I need to modulate as low as possible, and I may run my radiators at times slightly higher than UFH, and at times slightly lower or even turn them off completely. From what I can see, Viessmann can modulate as low as 2.1kw.
 
I've heard many problems with CCTs on systems like this, I'm not convinced it works well in practice. I would need to learn more about it
 

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