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Discuss Nest - hot water temp in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. Gemma

    Gemma New Member

    Can anyone advise where Nest obtains the hot water temperature reading from? We had a veissman 111 installed recently along with Nest, and the reading on the Nest app isn't correct (eg it displays as 50.5 degrees when the water as tested at the bathroom tap is reading 60 degrees). I'm happy that the actual hot water temp is correct, as set on the boiler, but I'd like to understand why Nest is reading it incorrectly. Nest are saying it's a boiler issue, veissman are saying it's Nest, so I thought I'd do some digging around while my installer tries to get this resolved.
  2. scott_d

    scott_d Plumber GSR Top Contributor!!

    Is it a combi?
    Not sure how nest would be able to read/measure the temp
    Last edited: Jul 13, 2019
  3. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    i would say its nest

    easy way to tell ask your installer how it was wired via open therm or simple on off
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    yes combi
  5. Gemma

    Gemma New Member

    Its a storage combi - it has an inbuilt tank to store hot water, when this is depleted it works as a combi.
    • Like Like x 1
  6. mgw

    mgw New Member

    I wonder what Nest is installed? There is no domestic hot water temperature displayed with my Nest Gen 3, in fact it tells one very little about DHW. There is a history with room heating, and it shows temperature requested and actual temperature in my case in degrees C, but hot water only has a schedule which it seems loathed to keep to.

    With a Combi boiler I would be fitting Nest E as there is no real need for Nest to control domestic hot water.

    As far as I am aware OpenTherm does not send info about DHW to Nest, but that is really the only way Nest could know the temperature, so would love to know more.
  7. Gemma

    Gemma New Member

    Perhaps it's me reading it wrong? I was assuming this is telling me the temp of the water in the tank is currently 50.5. It's a Nest 3rd gen which I bought on offer from Amazon, I have no desire to control the hot water via Nest but I'm just interested in what it's telling me (and making sure the hot water is actually at the right temp to avoid legionella etc)?

  8. Brambles

    Brambles Active Member

    There are two configurations for Nest Gen 3 with respect to setting hot water temperature. Both require the interface to be Open Therm and are dependent upon the boiler manufacturer and model having Open Therm that is fully compliant with Nest. Open Therm is not the universal standard it was intended to be!!

    The two settings ( to control hot water temperature) are Combi and System ( hot water storage tank). Generally Nest will function with a conventional Combi boiler, allowing the hot water temperature to be set from around 40 degrees C upwards. For storage Combi, it is a bit hit and and miss. The temperature received by Nest is only when the boiler is delivering hot water in combi mode - it (Open Therm) does not communicate stored water temperature ( in the Combi store boiler).

    For a system boiler to work with Nest Gen 3 to control hot water temperature, there needs to be a separate stored hot water thermostat input into Open Therm. Open Therm then transmits this data to Nest. Be aware that, in this mode, if you set a high hot water temperature that is above your boiler flow temperature Nest will not recognise this, so does not adjust the boiler flow temperature to suit. Nest operates purely on an on/off basis until the programmed temperature is reached.

    Note in system mode, Nest will not let you set a hot water temperature of less than 55degrees C.

    In simple terms, most people set a hot water temperature once and then leave it - they tend just to want to be able to control the timed functions. This is often more easily (and cheaply) achieved through direct controls at the boiler / tank thermostat. Nest (and Honeywell Evohome) make it overly complicated to display a hot water temperature

    Nest has developed significantly since it’s introduction in the UK, consequently ( particularly with Open Therm control) it is not always a straightforward installation to achieve its full functionality.
    • Informative Informative x 2
  9. YorkshireDave

    YorkshireDave Plumber Top Contributor!!

    Question if I may. What input does Nest use to monitor HW temp? 4-20ma? 0-5v? Whats the input range?
    Reason Im asking, is because once I know I'll be able to make a temp monitor for HW and also use it to cycle my old Mexico boiler - using a lower boiler stat setting... ;);)
  10. Gemma

    Gemma New Member

    Thank you, that's really helpful. I guess I can basically ignore the Nest app reading and be assured that the temp is as it should be as per the boiler setting (and the reading from the tap). My dad was in intensive care a few years back after contracting legionella so I'm a little jittery about these things but I'm reassured enough that the water in the tank is being stored and heated correctly.
  11. Brambles

    Brambles Active Member


    The voltages for Open Therm are 5v to 18v and the current 5ma to 23ma. These “modulate” and are bi directional ( a bit like the signal from a Sky Satellite dish, which is why you cannot just splice in a second Sky box) - the signal from the boiler is then converted into up to ( I think) 128 separate codes or channels. Channel 25 is boiler water temperature and Channel 26
    and Channel 32 report DHW temperature ( ie from 1 or 2 cylinders).

    For what I think you want to achieve - Evohome could do that without using Open Therm - with the hot water kit feeding into the Evohome Controller. Evohome would also give you a wider range of temperature and allow you to modulate it down to + or - 1 degree C.

    Apologies if that is all a bit “techy”, but it is not easy to explain ( well for me anyway)

    If you need any help or ideas as to to how to achieve what you want, please get back to me
  12. Brambles

    Brambles Active Member


    If you have any doubts or concerns, measure the temperature at the hot tap with a simple thermometer.

    Generally if the hot water temp is above 55 degrees, you cannot keep your hand in the flow (well I cannot).
  13. mgw

    mgw New Member

    Interesting what has been said, does that mean with Nest Gen 3 running a boiler analogue i.e. off/on, I can use the OpenTherm input to tell Nest when to heat water?
  14. YorkshireDave

    YorkshireDave Plumber Top Contributor!!

    Thanks for that. Was thinking more about the input needed by the Nest to tell deviation from set point for HW to be honest. Know what that is?
  15. Brambles

    Brambles Active Member

    First point:


    I have never tried to mix “analogue” control with Open Therm. A good idea, but I guess you would need to be a code writer or developer to even start to understand how. There are people on the Automated Home Forum, who can give direction on extracting the raw data from Open Term, but then using it in a control loop is the second challenge. In short, I think in reality the answer is no.


    The Opentherm input for DHW temperature is from the boiler into Open Therm and then out of Open Therm through channel 26.

    Consequently the input criteria is manufacturer specific and will vary from boiler to boiler. Some boilers using Open Therm don’t have an input port for DHW.

    Evohome, can be used to control DHW without interfacing with Open Therm. In general terms it is an easier system than Nest to customise (can be used with any boiler) and tweak the control of DHW /CH - having said that, I don’t believe that Evohome improves efficiency over Nest.
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