Looking for the ability to add a slab sensor

General
1

In floor hydronic heating systems use a slab sensor. This is typically a wired sensor to the thermostat.

What would it take to add another pair of wires to the thermostat. Also is there an available A/D input? The slab sensor is resistive.

2

Thanks from continuing here so that others will see and maybe respond to.
I see two issues that need addressing here. 1. is hardware and 2. is software.
Let’s look into those one by one…

  1. Hardware
    Pi does not natively support analog signals like Arduino does. This means that an additional little board/chip is needed that can fit inside HestiaPi’s case. Here is a little article about the options you have:
    https://learn.adafruit.com/raspberry-pi-analog-to-digital-converters/overview
    It is a relatively easy hack but not sure if it is your cup of tea. What I am sure is that it will definitely work as it is a tried and tested area by many Pi users across the globe.
    Our open design enables the users to add missing functionality. Buying a commercial thermostat that does not support this would end the conversation there.
  2. If you want the slab reading to replace the current temperature reading then you only need to replace the reading of the internal sensor (BME280) with the slab’s. This will need one extra file as described in the above link that will be called instead of the file for returning BME280 values.
    But if you want to keep the internal sensor AND the slab’s value it is doable but we need to know what logic will be based on these 2 values. Feel free to explain.
3
  1. hardware. I’m comfortable with hardware and software.
  2. Logic, sorry I have left some details out. The hydronic system has a forced air component and a heating coil in the furnace. So primary control is with the slab sensor which has 2 functions. 1) Max slab temp as some floors can be damaged if too hot. 2) keeping the slab at a specific temperature ( normal stuff although the feedback math is a little different as it takes a while to heat up ) Secondary control is if for some reason someone opens a door or window and the air temperature drops I’d like the heating coil to be activated and the furnace fan turned on (this is only one switch)
4
  1. If you can physically place your own sensor appropriately a cheap DS18B20 would be enough to avoid the extra board as it provides digital readings. Alternatively you need to add one of the boards and feed that to an unused GPIO.
  2. openHAB allows complex scenarios, called rules, that accommodate what you describe (hysteresis etc). Right now the existing rules are simpler as there is only one (air) sensor.
5

interesting. I’ve never actually tried to take the sensor out of the floor. I’ve played with the DS18B20 before though. Thanks

6

Think about it and come back here to talk about what steps you are stuck.
Interesting addition though for a future version…