Set a Minimum Central Heating Flow Temperature (OpenTherm)


The main purpose of this post is to try and gain an understanding of whether setting a minimum central heating flow temperature is a feature that other users of this forum would value and like to be developed by Tado as part of their algorithms. Are there any other users out there that have abandoned using the OpenTherm protocol due to low demand boiler start up cycling? 

For me, I would like to be able to define a minimum flow temperature in the same way that we can set a maximum flow temperature within the professionals Tado app although I’m also open to the idea of demand curves.

A couple of weeks ago - I installed a Tado v3+ smart thermostat in my home using the OpenTherm protocol but unfortunately, it appears that my boiler is often left repeatedly cycling the ignition sequence with three minutes of pump overrun. This happens at times of low demand usually in the evening when the house is already up to temperature – Tado appears to be requesting a 25c central heating flow temperature for 0.2c top ups.  

At the moment I am manually intervening to stop the cycling by increasing the thermostat temperature from 20.5c to 23c in 20 minute blocks. At 23c, Tado increases the flow temperature setpoint to above 32c. Above this setpoint [32c+], the boiler seems to run quite well.

For the remainder of winter, I will change the setup from OpenTherm to Relay (+ outside weather compensation) but ideally, I would prefer to continue with the OpenTherm configuration if I can set a minimum flow temperature to stop the cycling.  

I would be grateful if other users who would like to see this feature developed could comment below as it’s not currently something that’s on Tado’s development radar and I’m unsure if it’s just me that’s been affected by this when trying the OpenTherm configuration.

PS my case is already logged with Tado support as they have made alterations to my PID but unfortunately to no success with the cycling issue.  

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  • Valaki1234
    edited October 2023
    Usually you can configure both the minimum and maximum flow temperatures in your boiler.

    About the cycling: for long time I thought my boiler cycles because too low flow temp is requested, then I realized it was caused by the SRTs. Too many were closed at the same time and for heating e.g 1 single room the boiler needed to cycle as the flow and return temps were really close. So I removed all SRTs and the boiler cycles significantly less.
  • _C_
    edited October 2023

    When configured for OpenTherm all of the controls on my boiler are disabled as it becomes a slave.

    All of the TRVs are open to max and the system is balanced - I prefer to heat the whole house rather than individual rooms. The towel rails and hallway don't have TRVs.

  • On my Baxi boiler I can confirm that those settings are not overwritten, I also use OT.
    Having MIN setting only makes sense if the boiler uses it so it it would be strange if it is overwritten when connected via OT as in simple relay mode it is useless (maybe in there are other digital connection methods ?).

    Good strategy what you follow, I also do the same. Why do you assume that the root cause of the cycling is Tado asking for too low temperature ? Do you capture OT communication ?
  • _C_
    edited October 2023

    Which model of Baxi boiler do you have? I have the Potterton Promax Ultra 40 which is technically a Baxi.

    Like most boilers, I can set a flow temperature using the +/- buttons below the display when it is in relay configuration and the IFOS is disconnected. There doesn’t appear to be an option to define a min central heating flow temperature.

    I can see the central heating flow setpoint on the information display after pressing the information button until it reaches information point number 9.   It’s useful as I’ve not had to place a gateway between the boiler and Tado. It’s also useful to watch Tado decide on what setpoint to use before the boiler fires up.

    edited (added below):

    It’s exactly the same type of issue if you deliberately select a really low curve on warm day when the IFOS weather compensation sensor is plugged in.

  • Valaki1234
    edited October 2023
    I have a Baxi Luna Duo-Tec E bought in 2022.

    Using the plus-minus on the panel only set the temperature to be used in relay mode.
    For OT you need to turn on the ‘advanced’ menu which will enable many additional parameters.

    In my case I needed to enter into the parameter settings menu then navigate to parameter 22 and set its value to 22. This enabled many additional parameters, the ones I use:
    - 16: to set the MAX temperature used in OT mode. Not the same what you can set via the +/- buttons on the main panel
    - 67: set to 2 to use OT standard protocol
    - 83: to set the MIN temperature used in OT mode
  • I’ll have a look for the advanced menu and parameters.  I know that a minimum central heating output can be defined as a percentage but that’s not what I’m looking for.  I’ve never seen a parameter for a minimum flow temperature like there is for a maximum reachable flow temperature but I’ve never actually looked for it until now.

    I think my main objective of this thread is to help make OpenTherm low voltage controls more accessible to people who happen to have high powered combi boilers that struggle to maintain flow temperatures as low as 25c - whether that’s during the ignition start up sequence and / or due to the modulation ratio that their boiler offers.  

    I didn’t choose the boiler that I happen to have, it was spec’d by a large UK housebuilder and most likely based on its ability to deliver instantaneous hot water.

  • Hi, just letting you know that I found your post while searching for the exact same thing for my NEST stat! I used to have Tado at my previous property connected to a Baxi 600. I'm not sure if it was because that home was not so well insulated or what but I never noticed a short cycling issue there. In my new home where I've just had an Ideal Logic Max Heat 2 fitted with NEST I am experiencing the short cycling and have come to the same conclusion that there needs to be an option to set a minimum flow temperature request. Great minds.

  • _C_
    edited December 2023

    Yes, Tado should implement this.

    I was able to resolve my cycling issues in the end by setting a minimum central heating flow temperature of 45c with help from Valaki1234 who convinced me that there is a minimum central heating setting (P83) – I think our boilers have similar controls.  I also used Google to translate the advanced Baxi manuals from the Czech Republic (I’m English / UK based).

    Opentherm is working perfectly now and the boiler is not on all day like some people describe their Opentherm experience – it heats the house up in the morning at 60c (my Max setpoint) if the house setback drops over 1c through the night then for the rest of the day it comes on now and again at 45c for top ups.

    If Tado are reading this then I think they should have a look at what Home Page - EPH Controls are doing with their Opentherm products as they appear to have a minimum central heating setpoint (see pic below).

    I’m really happy with how the boiler, Tado and Opentherm are working together now. 

    My settings changes to my boiler currently are :

    Range rated DHW and CH to suit my property

    P18 - Delay prior to new ignition in CH mode = 10mins (was 3mins)

    P22 change to 22 to display advanced menu:

    P83 - Minimum boiler temperature in heating mode = 45c (was 25c)

  • C3ham
    edited January 16
    I have the exact same issue when tado requests low flow temps my boiler can’t modulate low enough(6.8kw is the lowest it goes) so cycles every few minutes would be amazing if tado could add a low flow temp setting and all my tvrs are set to max so it’s not because rads are shutting down