Multizone load compensation

[Don Daps]

Having a bit of difficulty getting clarity on this. Likely that i'm just not 'getting it'. Also fully understand i am a nightmare for people in the trade, but i want to know what's what before spending more money.

I have plumbed 2 separate zones in my house; upstairs rads, downstairs UFH. Both with their own flow and returns to the boiler, flows on separate 2-port valves.

I haven't actually bought the boiler yet, but plan to go for Viessmann 100-W combi. My plan/idea was to have smart TRVs on the rads upstairs, and a smart thermostat downstairs for the UFH, hoping it could learn the warmup times for my system etc. My budget doesn't stretch to Evohome, so i was thinking Tado.

When the rad circuit is closed and UFH stat calls for heat, I want the boiler to modulate down to as close to the manifold stat temp as possible. I imagine that’s achieved internally by the flow/return deltaT.
Also would like the boiler to modulate the rads to meet the set point because they are oversized. Not sure if that’s needed with TRVs though..

I read though that you can't modulate for systems with multiple zones - is this right? Can Viessmann do this, or is there a 'better' combi brand that can? Any advice on the right way to do this would be awesome.

Thanks

 

[Swwils]

Complex. If I understand you right you would like the boiler to modulate to the correct temp for the UFH when there is no demand from the rad zone.

Here is the lay of the land:

Tado doesn't have heat modulation support for under floor heating presumably because the modulation is mapped to temperatures used for radiators.

Hot water relay on the tado extension can't be used to control a second zone valve at boiler for purposed of heating. Multiple zones for ease, will need hard wired thermostats to do this.

Viessman is by far your best bet since they support by far the most of opentherm - you'll need an installer who knows what they are doing. 99% won't.

Tado by request can override the boiler water temp to the optimal for the UFH.

You'll need to then finish it up with a home automation component... So restricted to savvyness.

The right thing to do would be to bring the rad zone closer to the UFH temp via bigger rads and have a system that's far less complex.

 

[Don Daps]

Thanks for your advice, @Swwils . Taken me a while to reply, because its taken me a while to get the boiler installed. I think you understood my question totally, im just not sure i did.

Re-reading this, i don't think i need any opentherm complexity. I've got massively oversized rads upstairs, the intention was always to run the flow temps as low as possible. Glad you agree thats a good thing.
Your comment about wired thermostats has got me worried though...the house has just been plastered after a full rewire, and i've not got anything in there for a thermostat.

So the way I see it, i need a downstairs thermostat for the UFH, and smart TRVs (that replace the need for an upstairs thermostat). But i think my decision should be driven by how good/smart the UFH thermostat is, rather than what smart TRVs are available.

Is Tado still a good choice? and do you have any more advice to offer on the subject? Thanks!

 

[Swwils]

Its because of a limitation of the tado extension - it only has 2 physical channels - so depending on your setup you need at least an additional wired thermostat for the extra channel (Zone 1, Zone 2, HW).

You can set it up with just 1 wired thermostat, but you have to use the installer menu, in this configuration one thermostat is wired directly to a zone valve, the extension to another (but controlled wirelessly by another thermostat) and then HW is wired controlled by the extension.

When I mentioned "heat modulation" I meant the advanced feature of tado where it will learn the heat output of your rooms in conjunction with the smart TRVs, but as far as I know its only set to expect higher temps for radiators.

If your radiators have been sized for the UFH flow temperature that's great, but this would be rare - and if you think about it; if the system was "perfect" you wouldnt need any TRVs.