Two zone setup with Opentherm possible?

Benham

I recently had wet UFH installed by a gas engineer, but we had an issue getting the Tado to integrate. It feels like it should be straightforward but I’m struggling to get an answer.

- The combi boiler is in the corner of the same room as the UFH. In the same location is the extension kit (wired to boiler with opentherm), 2x zone valves (1 for UFH, one for CH) and the UFH pump.

- The zone valves and pump are all connected to a wiring centre which just needs a switched live input from a thermostat to activate. (terminals are COM, L,N,E and S/L).

- I have one wired thermostat in the UFH room which I want to control UFH with.

- I have a second wired thermostat (not currently connected) which could be connected in the same area (as a communication device only) if required to connect to the wiring centre and trigger the CH zone valve to open.

- All radiators in the house (excluding the bypass) have Tado TRVs.

I would have thought I could use the wired thermostats to connect to the two zones on the wiring centre to send a switched live signal to the respective zones. These would both communicate wirelessly with the extension kit connected to the boiler. If any TRV in the house calls for heat, it would trigger the boiler to start and also open the zone valve via the connected ‘slave’ wired thermostat?

In an ideal world I would have the extension kit controlling the CH zone valve and the boiler but apparently it can only do one connection (i.e. opentherm or relay) at a time?

Surely this is a fairly common setup?

Thanks

policywonk

Suggestions.

1. Rewire the UFH side, to add a dedicated zone valve for UFH, driven by a dedicated UFH oriented wiring centre for the UFH, to which you add the Smart Tado wired stat. This would enable the following:
   1. Instruct the UFH wiring centre to wake up the UFH pump.
   2. Open the actuators on the relevant UFH loops.
   3. Power the zone valve for the UFH (the instruction coming from the UFH wiring centre)
   4. When there is a problem with the UFH you'll be able to diagnose it more swiftly and logically.
2. An option: if your system is more than 5 years old, ask the heating engineer to reverse pressure flush the central heating system. This removes any muck in the system which have a habit of causing smart TRV pins to stick.
3. Crucial: fit an automatic bypass valve for the central heating side and remove the zone valve for CH. Make sure that the radiators are fully balanced.
   1. Thus your original CH zone will be pressured only when there is a call for heat somewhere - it could be the UFH side or the CH side.
   2. However: the bypass valve will refuse that heat to move into the CH side unless one of the TRVs open, at which point the pressure change will force the bypass valve to open.
   3. Hence,without using electricity, the bypass valve serves as a zone valve. The nice thing is they dont breakdown as much as motorised valves!
4. Decommission the original wiring centre. Reduce the wiring into a simple accessible state.
5. Follow the boiler installation guide and refit all the wiring loops needed to get it to listen to you in Opentherm mode. Get a wireless receiver to cook for you in Opentherm.

Benham

Thanks a lot for this.

To answer your points…

1. This is the setup I have currently - The zone valve (dedicated) and wiring centre are both setup for UFH (wiring centre is a UFH5 by EPH controls). This all seems to be working quite nicely and tidily now I have organised the wiring (was only a mess for testing).
2. Good shout, I have a drain valve on the flow by the boiler so can do this. All the rads bar 1 are only a year or so old but worth doing anyway.
3. This is a good idea as it will basically do what I want without the need for Tado involved to control that second zone valve which seems to be impossible with the V3 Tados. What happens with my bypass radiators? Is the valve set to be above the pressure point of the two bypass radiators?
4. This is a new wiring centre for UFH so not applicable in this case - also now much tidier!
5. Boiler is connected to extension kit via opentherm and working.

This would basically hopefully do exactly what I want it to do if the automatic bypass can be set to not trigger due to the two bypass radiators without TRVs.

Thanks a lot for your help.

policywonk

I know this is going to confuse me, but got to ask. Is there any way you would consider having the bypass valve handling the entire CH zone loop? It means your bypass rads have to be TRV'd but not necessarily Smart TRV'd. Really important to have the zone valve handle all of them and it gets triggered when all the rads are off.

If you can do that, it will ensure that the primary pump's pressure is only ever serving an active call for heat, and more importantly, a specific call for heat. Your boiler has thermal sensors on the outbound and inbound pipes, it keeps a log on the relationship between temperature differences, pressure differences and the actual heat curve needed. A similar log is kept on the Tado side - but more capable of mapping to time of day, external temps. As the stats build up, distinct differential curves are formed which become the basis for energy saving whilst ensuring that all rads get heat at the speed they need.

I have zone valves on my UFH, my upper floor CH and my HW tank. The Tados control all heating zones and do it really well. But my boiler is dumb, on-off, so even though I've put in a pressure differential controlling pump, I cant bring modulation into the level that you can.

Impressed with that EPH wiring centre and blending valve by the way. Neatly structured.

Dont use Smart TRV heads in the bathrooms. High humidity causes problems with anything smart.