Context-aware temperature sensor for sauna (glitchy inside sensor when off)

Configuration
1

Hi all,

I’m trying to build a context-aware temperature sensor for an electric sauna, and I’d like some feedback on the right approach in Home Assistant.

Background / problem

I have two temperature sensors:

  • sensor.temperatuur_sauna
    → inside the sauna
    → reliable when the sauna is ON
    → unreliable when the sauna is OFF (it keeps reporting very low values like ~2 °C instead of becoming unavailable)
  • sensor.temperatuur_buiten_sauna_auro
    → outside, near the sauna (still inside a building)
    → always reliable
    → good proxy once the sauna has fully cooled down

I also have a binary sensor:

  • binary_sensor.sauna_operating_message
    → on when the sauna is actively operating
    → off when it is not

This combined temperature is used for ETA calculations and scheduling, so it must be predictable and not jump to nonsense values.

Functional goal

What I want conceptually:

  1. Sauna ON
  • Always use the inside temperature
  1. Sauna OFF, short time ago (grace period, e.g. 15 min)
  • Still use the inside temperature - use the last meaningful value (any value above 20 degrees is valid);
  • Do not immediately switch to outside
  • The sauna does not cool down instantly
  1. Sauna OFF, longer than grace period
  • Use the outside temperature
  • At that point it’s a better approximation than the stale inside sensor

Important nuance:
The inside sensor does not become unavailable when wrong — it keeps reporting plausible but incorrect values (like 2 degrees Celcius). So this cannot be solved with simple availability checks.

What is the clean, maintainable HA way to model this kind of contextual sensor?

If anyone has solved a similar problem (e.g. ovens, saunas, hot tubs, boilers with bad sensors when idle), I’d love to hear how you approached it.

Thanks!

2

So add an availability key to the sensor that if it reads below X, make it unavailable?

3

Here’s one way:

template:
  - sensor: 
      - name: Combined Sauna Temperature
        unique_id: fe60a754-5039-4e59-9031-456f137a099e
        unit_of_measurement: "°C"
        device_class: temperature
        state_class: measurement
        state: >
          {% if is_state('binary_sensor.sauna_operating_message','off') and now() - states.binary_sensor.sauna_operating_message.last_changed > timedelta(minutes = 15) %}
            {{ states('sensor.temperatuur_buiten_sauna_auro' }}
          {% else %}
            {{ states('sensor.temperatuur_sauna' }}
          {% endif %}

If the sauna operating binary sensor is a template sensor then the template above can be simplified by including a delay_off option of 15 minutes in the template binary sensor. That way there would be no need to check for last_changed and the template if condition test becomes:

          {% if is_state('binary_sensor.sauna_operating_message','off')  %}
4

That was indeed the solution. I was trying to solve everything by building all logic in one template sensor.

But now I created one template sensor that is only available when the sauna is actually on (availability: {{ is_state(‘binary_sensor.sauna_operating_message’, ‘on’) }}).

And then I could create a second, contextual sensor that can work with the last valid state of the previously created sensor.

Thanks!

Thanks for trying to help. However, the logic I needed is more complex than that. In that 15 minute window, it should not just take the current temperature from the inside-sensor, but it should take the temperature from (up to) 15 minutes ago.