Hi @Bagpuss I'm getting this as the gist (just heating for now)
You have a boiler with a controller.
Each room you have a valve on the rad or actuator for the UFH.
You also have a thermostat controller in each room
You don't seem to have any repeaters from your message.
When a room calls for heat you want to open it's valve call for heat on the main boiler controller.
I feel like I am not knowledgeable enough to provide an opinion on your whole set of requirements but your TRV element is something I have had first hand experience with.
I tried for some time to get MOES TRVs working (thanks to Kkosev for his driver and help) with the Hubitat Thermostat Controller. In a scenario where the TRVs are providing the reference point for the room temperature then I think it would work well, but in my situation I wanted to use separate temperature devices and introducing this element meant there was a constant conflict between the heating brain in the TRV and what the TC was trying to achieve. In the end I installed valves meant for underfloor heating that have no in built thermostat and are controlled via simple rules and switches linked to the Thermostat State of the TC. This works really well and I graphed my room temps before and after and the variance and over shoot is much lower.
I cannot comment though on how to instigate the wider elements of your needs though but am interested in how you get on and wish you luck.
Hi @BorrisTheCat, yes, that's about it, though:
there is not, yet, a thermostat controller in each room, only in the rooms that have UFH, as these have NeoStats which control actuators. The rooms with TRVs have no physical dedicated thermostat controllers, yet, but we would like that.
There is one repeater; the Zigbee network is physically not large. I had considered that if we used thermostats like the Moes mentioned above, they are mains powered and (I presume) would repeat.
Yes, we want "when a room calls for heat, it opens its valve and turns the boiler on". When no rooms call for heat... turn the boiler off.
(And "just the heating" would be a massive win!)
Thank you. I considered the actuator option, but this required a bit more techie ness, iirc. Kind of building my own controllers. I bought some. They're in a drawer. 
Time, etc. I rather want a more operator friendly solution. "Other people" need to have an interface they can use easily, or at the very least get their heads around. Sadly, the Hubitat interface, even dashboard, is not hitting that target! (And I do understand it's horses for courses.)
Ok so what controls the boiler right now? need to know how it will be called ON. Do you have any "main" valves which would block the flow to a Room or overall zone?
I have two thoughts. The first is to look at the following app and see if you think you can make it work for your situation. It was built for forced air heating, but may work for your situation. I would start by reading the documentation... don't just install it and start clicking. It may not be the right tool for the job.
I'll reserve my second thought for later...
And I'll be honest, my experience with radiators and TRVs is zero, and minimal with UFH.
Thanks @dylan.c , the first couple of pages sound optimistic. It's quite an opus of documentation - I'll grab a glass of wine at some stage and dig deeper!
A Heatmiser Neostat controls the boiler for heating. (And another for the hot water boiler.)
It is currently (incorrectly) running as a thermostat, so to turn it "on" one sets the temperature to 35º. When we get a chance, I think it's possibly to reset it and set it up in programmer mode, so it's simply a switch.
My understanding is that there is a zone valve for the entire UFH setup, and a zone valve for the entire radiator area. Each is controlled by a Neostat also.
The UFH has multiple zones: 6 in total, each with its own Neostat.
The rooms with radiators have only the TRVs on each radiator.
So I think we have enough valves/controllable areas in place... it's just how to control them!
Does the neostat actually monitor the boiler temperature? If yes, this may not be a bad thing. Here in the US, large commercial (hot water) boilers have an operating thermostat that tells the boiler what the normal operating range is (cut in, cut out, and modulating setpoint if capable), a high limit safety that shuts the boiler down in case of thermostat malfunction (ie. it gets stuck ON), and a relief valve to release hot water or steam to prevent the boiler from exploding if both other devices have failed. I don't have any experience with your style of boilers or controls, but I think it prudent to have a "local" thermostat controlling the boiler. That way you still have some type of operating control even if the automation controller (Hubitat) locks up or otherwise fails. An alternative would be to keep your existing thermostat programmed to the highest normal operating range, and wire in a switch in series that allows Hubitat to enable or disable the thermostat's control. Basically you can force the boiler into an OFF state during setback periods.
If the Neostats do not monitor the temperature of the boilers, I would rethink that strategy altogether. It seems unwise to use Hubitat to directly control the boilers based on my experience.
I also recommend you work through this system in small bits. It's too complicated to try and automate everything at once. Start at the foundation (boilers, one at at time, IMO) and work outwards from there.
Just a quick thought that whatever you do, it might be wise to wire in a parallel, old-fashioned, no batteries or AC-required mechanical thermostat located in the room most likely to get cold first, as a safety backup. We did that in two buildings when our remote Maine home was our vacation house, and we didn't want failure of a Z-wave stat, used to turn up the heat as we drove up, to result in freezing pipes. To be frank, the use of any controller that doesn't allow for the system to come on and open valves at a "failsafe" low temp scares me in any area that experiences serious cold and is unoccupied for more than a few days at a time. I really like the concept of the system you describe, but unless those valves open on their own at a failsafe temp, there are some potentially expensive failure points here,
Thank you for the pointers. The boilers have build in digital thermostats. These are set to about 65ºC for normal use. There is pressure relief, plumbed to the outside. So even if the boilers are left on 24/7 it's not an issue; this is what they're designed to be able to accommodate.
You make a good point about starting small. That's why I am focussing only on heating! But maybe I need to start smaller still...
This is precisely our conundrum. It must be fail safe. Some people have suggested simply putting manual TRVs in - mechanical ones, no electrics, no smart. They very seldom go wrong. They would nearly always work in a frost protection scenario.
But this would mean that people might leave their radiators on by accident, then not visit, and months might go by and we're heating that room wastefully.
That said, given the current grief I'm getting for the lack of controllability by others, it's looking increasingly attractive!
Funny you should mention "potentially expensive failure" here. The reason all this new kit was installed was that we had a flood. 600,000 litres of water from a burst plastic joint in the uninsulated roof space. Genius!
Only two thirds of the building was totally destroyed. This area then received the UFH. The "undamaged" rooms could only be given radiators at best. Previously it was all electric heating. Hence the mix of radiators and UFH!
We are risk averse now. We actually have a remotely monitored leak detection system in place.
I'm wondering if I need to temper my enthusiasm for automation to keep energy bills low, and settle an easy life 
This I am trying to figure out, 1st it looks like this Heatmiser is not local controlled? You need to work out to see if each component works correctly independently 1st before you look at automation.
When you turn on the main boiler to call heat does it start.
when you open the room valve does it open.
then when you call heat on the RAD does that open.
I will need to make a site visit to check some of these things. It's obvious logic to work through, now you mention it, thank you.
I know with complete certainty that the radiators with TRVs have no way to turn the Heatmiser stuff on at present. I have installed the Heatmiser integration, but now played with it, so as not to break the Heatmiser side yet! I knew it was pointless trying that until I had the TRV control working, which remains outstanding as a task.
No but that part is likely easy, when calling heat call heat on boiler else turn off the heat.
Obviously it's more complex than that as you have moving parts but you just need everything working manually through hubitat 1st. Your need a powered zigbee device in each room that has the YRV in otherwise your going to drain the batteries them shouting to get through.
Understood. So the Heatmiser is acting as a simple switch to enable or disable the boiler's built in controls? That seems fine.
I'll sit back for a bit let the others take the lead on helping with the TRV controls.
Ah yes. So let me do some analysis of exactly how things work at present.
You're right on batteries in the TRVs: hammered. I've been using good rechargeable batteries, and getting about three to four months. I have got some buck converters, and plan to use some USB chargers to provide permanent 3V to the TRVs. So many things on the to do list!
Yes, the boiler is simply told on or off, and it heats to the temperature it has been set to.
Thanks to all for input thus far, it's already looking a bit clearer. I'll do some research and checking on current status.
Another important factor is how much "turndown" your system can handle. This is a function of three variables. 1) Does your boiler have any way to vary is output or is it just OFF or 100%? 2) How much thermal mass is in your system? Mostly this is the fluid volume of your piping and storage tank (and if any UFH is embedded in concrete or similar material). 3) How much heat load does each zone represent?
This is important because the heat generated of the boiler needs to go somewhere. For example, if you're only trying to heat one small zone, your boiler can only fire at 100%, and your thermal mass is lacking, the boiler will cycle on and off frequently. Not usually desirable.
It's a good point. The boiler can only be on or off, though one can at least turn the thermostat down so it's heating to 60º instead of 75º.
There is a lot of thermal mass in the UFH areas, relatively little in the areas with radiators. One of the zones of UFH is 80 square meters of floor, concrete screed with 840m of pipe containing 95 litres of water. The other UFH zones are smaller. The whole system was well designed and seems to be well balanced. But there is, of course, significant lag.
It's been used over the weekend, and the big area of UFH needed something like 6 hours of heating overnight on Friday and then... nothing for the rest of the weekend! It remained remarkably stable.
The rooms with radiators don't seem to do too much cycling either, to be fair. I think!
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