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SW input as a sensor?


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I have a use case for using the mini purely as a sensor, perhaps this is not possible.

My thermostat wiring has 4 wires.

L, N, a switch output to turn the heater/cooler on and an fourth input of 230v which feeds the thermostat. If this fourth wire is ON the system should be in cooling mode, if it is off the system should be in heating mode.

I want to change the existing thermostat with a smart thermostat, but the smart one does not have this fourth input.

My idea is that I can split the L and N lines and feed both the thermostat and the mini, obviously the control wire for the thermostat will go to the thermostat, but the fourth wire which is either ON or OFF could perhaps go to the SW input of the mini? My assumption, is that if this wire becomes "live" the SW input would register as being "live".

Is this a viable possibility or am I going about this the wrong way?

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Thanks for the response, I thought that I had listed the thermostat but I guess I was wrong.

It's a Bosch room thermostat 2 230v edition.

The heating/cooling signal comes from the main contr room of the apartment block. Around 100 apartments.

I know that it is purely an on/off signal as I have tested with a multimeter and in cooling mode it is a live 230v and heating mode it is 0v between this wire and neutral

Edited by GeneralPILK
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well .. consider that switching between heating and cooling happens twice a year. so you can do manually in your smart thermostat. The additional device would be useful ONLY if you can send an http command to your thermostat telling "hey, it is cooling time". 
Do you have coil fans ?

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The problem is that it doesn't switch as you would expect. The switching is based on some sensor reading and randomly will change between heating and cooling based on the average temperature over 24 hours.

This has caused issues like coming home to a sauna after a couple of days away.

Of course was all related to the old thermostat that had no way of being switched off, you could only set it to one extreme it the other based on if it "should" be in coming or heating mode, as it was a simple dial with 6 positions related to temps between 20-30 degrees.

Oh and more info I left out, everything is set up in home assistant, so this would be the missing integration that would inform the thermostats of which mode they should be in.

I have no fans in the apartment, the heating and cooling is a water loop that again is centrally connected. The thermostat controls the valves that allow the water to flow or not. 

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If it is based on the temperature in teh last 24 hours it is not "randomly changing" .
So i assume it is made for seasons as late spring or early autumn when it is possible that on some days is necessary heating, in other cooling.
It may work but the question is: the shelly can send a messge when there is a switching between heating and cooling, but is the other device ready to get and process this information ? if the second answer is yes, no problem to use it (the relay is till usable for other purposes ...)

 

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Randomly is a bad word, however, the system does not behave as one would expect given past experience along with actual outside temperature.

It also cannot actually switch because the same loop carries both the hot and the cold water and yet another system decides when the heated water even enters the whole complex and when the cooling system is actually cooling the water versus cold mains water coming in.

And the second answer is yes.

So would I do what I had planned and add L and N to the L and N poles and the heat/cool signal wire to the SW pole?

Edited by GeneralPILK
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It is your place, you may do what you want, but if the other system is able to understand that signal, commuting from heating to cooling you can do.
This system has a flaw:
You can set your thermostat to "confort zone" so for example set minumum as 18 and maximum as 27. so if is in heating mode would open the valve if the in temperature is below 18, and if cooling if is over 27. of course the thermostat must know if water is cold or hot.
I would suggest a different approach: Have an additional temperature sensor on the arrival of water:
IF Troom<Comfort temperature and Twater>Troom THEN open valve;
IF Troom>Comfort temperature and Twater<Troom THEN open valve;
ELSE keep valve closed
(this algoritm works even if you put absurd limits

Edited by wooly
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I had at some point considered adding a temp sensor on the intakes as a backup, but you're absolutely right that it makes more sense to use the water temperature as the main contributor to the heating/cooling info.

Food for thought overall.

Thanks very much.

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