Boiler failure notification

[iron crag]

I have an old gas boiler at home only controlled by a Zigbee thermostat and I have a Zigbee water temperature sensor attached to it.
The pilot light sometimes blows out, so when the thermostat starts the boiler it doesn't do anything. I want to detect that using the water temp sensor.

I thought I could do it by setting a helper variable to the current temperature when the thermostat starts heating, waiting a bit and comparing the new temperature to the one in the variable.

But in the UI I don't see any way to set the helper variable to the value of the thermometer. Is there a way to do that? Or at least to do it using YAML?

[dense glen]

You might want to look at a Derivative helper - if it's heating you'll get a positive output

[iron crag]

Ah, so I can just find out if it has been heating in the last 5 minutes or whatever I set.
Well if it does work like that then it is actually perfect!