How should I set up two heating zones/circuits using OpenTherm

ohnosee

Hi all - long time reader, first time poster...

I am having my boiler replaced and integrating an additional heating circuit (of a basement flat I am amalgamating my house with). The boiler will be OpenTherm compatible (e.g. a Viessmann Vitodens 100-W 32KW system boiler) and the two heating zones and one hot water zone will be controlled by an S plan plus valve system.

My existing heating circuit has tado radiator TRVs and temperature sensors in every room, communicating with the existing boiler wirelessly to an old style extension kit (the one which I believe supports relay and opentherm connections).

Is it possible for me to control the additional heating circuit valve using tado and integrate my extension kit using OpenTherm? I have read elsewhere on the forums that a wired smart thermostat can be used to control the valve (in relay mode) and this is controlled wirelessly by the extension kit.

I am also interested to know how the setup would deal with a demand for different flow temperatures across the two circuits. I've also read on the forum that one user has observed TRVs/Thermostats regulating the hotter flow than required on the receiving end e.g. partially opening the radiator valve.

Thanks in advance for your help!

johnnyp78

You’re going to struggle with using a modulating connection on a multi zone system and having a hot water cylinder too completely rules it out unless you’re willing to replumb for pdhw I’m afraid.

ohnosee

Hey @johnnyp78 thanks for replying to my question.

I'm in conversation with a boiler engineer and plumber at the moment who hasn't raised any concerns. Are you able to explain more about why I'd need to replumb for pdhw?

Newbie question: is this pump delivered hot water or pressure delivered hot water or something else?

I'm currently on a vented system with a 35 year old under counter boiler (65% efficient!). Hot water is stored in a cylinder upstairs and pumped to taps. The cylinder isn't on a thermostat either. So decommissioning the over flow tank, cold water tank and existing hot water solution is part of the job. The flat beneath has an old combi which will be decommissioned and the heating circuit added to the new boiler.

Thanks for helping.

johnnyp78

If you have a hot water cylinder, you either need hot water supplied to it at a high temperature or a means of heating the water once it’s in the cylinder. If you have a boiler connected to a modulating heating system, the max flow temp can sometimes be as low as 30 or 40c. If the valves on an s plan system divert the water flow to the cylinder instead of the radiators, the boiler has no means of knowing it needs to supply the water at a higher temperature.

There’s a good explanation of priority domestic hot water (pdhw) here https://theintergasshop.co.uk/content/189-why-hot-water-priority-is-the-reason-s-and-y-plan-should-be-banned

And here https://lhsplumbing.com/blogs/news/priority-domestic-hot-water-pdhw

ohnosee

@johnnyp78 Thanks for the links! Very useful. Makes sense.

The heating system is undergoing a complete overhaul so I'm not wedded to running on S plan. The existing system isn't - and all that will be left of it is the hot water plumbing and heating circuit for the main house. I'm getting a new cylinder so can also specify it appropriately to work with a modulating heating system.

Do you have any more guidance on what I should ask heating installers to consider as an alternative to S plan?

Thanks again!

johnnyp78

Ask the heating installers about x plan / pdhw. They will need to know what they’re doing though. Don’t use heating zone valves, you are effectively zoning by room if you use Tado trvs anyway. Then you can install Tado on a modulating connection without any problems (hopefully).

There’s a good video here of what’s involved https://youtu.be/-5pH8iERvzU

ohnosee

Hi @johnnyp78!

Thanks or the advice and the link to the YouTube video. I did a bit of googling myself and came across a post mentioning the Viessman 4 pipe setup as a way of achieving different flow temperatures for DHW and CH.

As it turns out I plan to have a Viessman Vitodens system boiler installed as part of the amalgamation (replacing the ancient undercounter boiler) as I know this boiler supports OpenTherm. The image below shows their recommended setup:

The PDF the above is taken from states:

* The DHW circuit operates independently of the CH circuit

* Dual temperature control, which means the CH circuit can be operated on weather compensated system (reduced flow temperature)

* Using the boilers temperature cut-out device and integral diverter valve to protect the cylinder from overheating. 

Do you think this would be an appropriate setup for achieving modulating CH and a cylinder?

johnnyp78

I’m afraid I don’t have enough in-depth knowledge to tell you but yes, looks like it could work. Maybe other more knowledgable people on the site can give their opinion.

bsm

HI @ohnosee , did you solve the issue? I have a similar situation, reading the Vitodens 100-W manual apparently plug 96 (where a thermostat in a relay set-up would connect) can be configured as "External Demand," which I believe could be a 2-port zone valve. Any thoughts?

ohnosee

On the end went with a viesseman vitodens 200-w with priority domestic hot water setup on the 4 pipe kit. The 200 isn't compatible with tado so no longer have boiler control and instead use my radiator thermostats as "limiters" and wireless temperatures as for info. True test will be this winter.