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Which temperature sensor for a Steibel buffer and Ecodan?

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by ASHP-BOBBA 1 month ago

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downfield

(@downfield)

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Topic starter 19/09/2025 9:28 pm

My installer configured our Ecodan 14kW system with a 100 litre Steibel 4-port buffer (surprise) and I have recently had this converted into a volumiser in order for it to work better when in cooling mode (see other thread on rapid cycling when cooling with the old set-up).

Anyway, the buffer has "dry pocket" near the bottom for a temp sensor that uses a 14mm dia thread.

I would like to install a sensor and connect to the Ecodan controller so that it shows up on the temp graphs in iOS MELcloud app as "Mixing cylinder". This line on the graph is currently unused.

I asked Steibel tech support for recommendations but they just said that they didn't supply a sensor and the type needed would depend on the heat pump.

Two questions:

1 what sensor to use that will fit and have the correct wiring

2 where to connect it in the control box.

There are lots of temp sensors with 14mm diameter threads on Google but they mostly seem to be for car engines not heat pumps...

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Mars

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20/09/2025 7:54 am

@ashp-bobba might be able to help as an Ecodan expert.

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ASHP-BOBBA

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22/09/2025 6:34 am

@downfield I have never added a buffer sensor as we never have used a buffer, I know on many brands you can add them as well as a flow sensor. I would need to look it up for you or you could speak to Mitsubishi and they would tell you which port would work for what you want.

1st we would need to know which FTC you have 5,6 or7?

On the FTC6 it is most likely DIP switch SW2-6 is labelled “Mixing tank WITHOUT / Mixing tank WITH” indicating the controller has a feature for a mixing or buffer tank.

Mitsubishi typically uses 10 kΩ NTC thermistors at 25 °C with a specific resistance/temperature curve.

This post was modified 1 month ago by Mars

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downfield

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Topic starter 23/09/2025 9:40 am

@ashp-bobba many thanks for the information.

I have found the manual which has the numbers PAC-IF71B-E -IF2B-E and IF3B-E on the front - not sure what that means.

Anyway there is a table of thermistor inputs (attached) which mentions the Mixing tank on the last row and gives a part number PAC- TH012HT(L)-E for the thermistor.

So if I can get that will I be able to plug it in at the FTC end?

What do I need for the 14mm dry pocket at the volumiser end? will it just screw in?

thanks

Julian

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ASHP-BOBBA

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23/09/2025 9:51 pm

@downfield Hi, if it has a 14mm dry pocket you should just be able to insert heat transfer paste and push the probe into it to get a close reading, if you dont have paste or want to use it we recommend inserting the probe at the same time as some armaflex insulation to make sure the probe is tight up against the wall of the pocket. I think the probes for mitsi are around 8mm so it should fit.

All of this can be checked with Mitsi and its worth checking as there are often multiple choices, also you can ask them how to set up the reading and it is likely you will need to turn the power off, change a dip switch or 2 and power back on for it to read.

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DavidAlgarve

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24/09/2025 7:11 am

@ashp-bobba Re:

"On the FTC6 it is most likely DIP switch SW2-6 is labelled “Mixing tank WITHOUT / Mixing tank WITH” indicating the controller has a feature for a mixing or buffer tank."

I wonder if you could expand on this point, please? I have recently had my buffer tank converted to volumiser, but with a contractor who had very little knowledge of Mitsu equipment and who could get even less support from them.

Are you saying that without a buffer SW2-6 should be Off i.e. "Mixing tank WITHOUT" ?

This post was modified 1 month ago by DavidAlgarve

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ASHP-BOBBA

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24/09/2025 9:26 pm

@davidalgarve As advised above, its worth speaking to Mitsi as I do not know having never used this function and not fitting buffers. I do not know what will happen with the control logic on the FTC5/6 if it can see an additional probe, it may be that is simply reads it only or that the reading feeds the info back and actually will make the machine try to run in a way that it takes this value into account, considering in most cases the last thing you want is a buffer or mixed load on your system its may not be desirable for the unit to read this probe.

Its just worth speaking to them, and TBH, in my experience they are helpful them techies.