I have an Eve Weather outside to record our temperature. This is the Eve device made for placing outdoors. I don't recognise the name Eve Temperature.
As you can see from my screenshot I can use the Home app on my iPhone to create a negative temperature trigger to start another device.
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I achieved this in the Home app by adding another automation, touching on the button for my Eve Weather (Patio Temperature as in the screen shot) and then follow the prompts to set a temperature to trigger the actions which follow.
However, would it not make more sense to have an automation to turn on a heater when that room falls below a set temperature? It is the comfort of the room you are regulating, not the outside temperature.
You can set the HomePod Mini to monitor the temperature (it has a thermometer and humidity sensor too), and it does not need to be in the room with the heater either. Of course, this would not work if you have the HomePod mini in a room which is already heated and not falling in temperature. Sounds like an opportunity to get some more smart devices?