Ecodan Thermistor Specifications


I want to install a therminstor in my Ecodan connecting it to CN20 connector.

I have been looking for the technical specs of such thermistor but I didn’t find anything.

Any idea about the tech specs for the thermistor (TH1)? NTC or PTC? Resistence?

I tried a NTC 10k Ohm but I got a measurement of 5C instead 21C.


Replying to myself…I found the below info in a manual:

Thermistor R0 = 15kΩ ± 3%
B constant = 3480 ± 2%

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Can you share a link of such generic cheap thermistor?

Sorry for the delay, just saw the message today.

In case you are still interested, I found a customized thermistor in Aliexpress but I´m still waiting for it.

On the other hand, I ended up making a “fake thermistor” with a digital potentiometer (DS3502).

My Home Assistant server is monitoring several thermometers I have distributed around the house. I created a select input that allows me to select one of those thermometer and an automation sends via MQTT the temperature of such thermometer.

A ESP8266 board gets that temperature and configure the DS3502’s wiper to adjust the variable resistor to a value that is equivalent to a thermistor 15K and B=3480.

That allows me to fool the Ecodan with the temperature it’s seeing via CN20 connector.


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@Eduardo_G.D this last piece of info is awesome. I will try to fool Ecodan between my boiler and hot tub. Can you share the code for the ESP of how exactly it gets the temperature and sets the DS3502’s

Also can you please share the link to the DS3502. Thanks

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Please check the following link for DS3502

ESP gets the temperature by subscribing to a MQTT topic, how are you getting the temperature?


This looks interesting, I may experiment with such setup. @Eduardo_G.D do you mind sharing some details?

  1. what is the CN20 connector part number you used?
  2. what is the wiring setup to use with the digital pot?

Thanks :slight_smile:

I think the connector might be JST XA, the JST XH that I found in my parts bin worked for me by cutting of the 2 locking “tracks” on the outside.

If you havent ordered a digital pot yet you could try a DIY vactrol. I 3D printed a case and used black tec-7 as filler, works pretty good. If you try this use a resistor to limit max GPIO current + a potentiometer in series to fine tune current for the led you choose. I sanded a led flat then glued it to a LDR.
Got very good resolution by adjusting the current and PWM frequency, from what I remember I ended up with a pretty high value resistor for the LED.
If the code I have on my laptop is the latest version I settled at 10bit PWM @ 15.6KHz.

This vactrol has the benefit of being electrically isolated, I did the same with the ESP on the CN105. Isolated power converter and optocouplers on the serial lines.

Here is the information I found about thermistors on indoor and outdoor unit
eco_t.pdf (255.0 KB)

That’s interesting. I already ordered the DS3502 and will go that way, but an isolated approach like yours is probably better… Regarding the connector - I was told the attached connector should work; I have 4 pcs on order and will report back.
product-2338183-2.datasheet.pdf (241.7 KB)