Hello and welcome to the forums. While NIBE systems are nothing like as common for contributors to these forums as say Daikin or Vaillant, there are several people with NIBE installations who have a very good understanding of how they work and I’m sure they will be happy to contribute here. As you noted, there are some other threads which relate to similar symptoms - I’m thinking in particular of this one and this one.
We now understand that the BT25 temperature sensor reading is one of the critical inputs to the NIBE “Degree Minutes” control algorithm, so it is important to have it provide a representative reading and you are right to be concerned it might not be correct. Since the BT25 sensor is fitted by the installer (rather than at the factory, like BT12) there is scope for it to be wired incorrectly or located in an unsuitable place - though as you’ve had the NIBE supplier back you’d hope they would have confirmed the wiring was correct.
Please would you share some further details about your installation:
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You mention you have a 2-pipe ‘buffer’ which sounds like it is acting as a ‘volumiser’ tank.
- Does that tank have a label - is it a NIBE UKV, for example?
- Are you able to tell if that tank is located on the ‘flow’ or the ‘return’ side of the heating circuit?
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Are you able to identify where your BT25 sensor is physically located, and post a photo?
- Where there is a volumiser tank in the system, on the ‘flow’ side, it is common for BT25 to be located at the top of that tank, under a cover plate
- Is that where your BT25 is located, or is it fixed to the pipework elsewhere?
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You mention that BT25 is your “flow line sensor to UFH”
- Is the UFH your only source of heating or do you also have radiators?
- Is there just one UFH manifold or several?
- Is there a separate circulation pump on the UFH manifold(s)?
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The NIBE control algorithm calculates a Target Flow Temperature, S1, and then keeps track of how the Measured Flow Temperature, BT25, compares with that Target - including during defrosts and DHW cycles
- Would you be able to post a graph showing S1 together with BT12, BT25 and BT3?
A few general photos showing the layout of your pipework would also be helpful.
David