It's not digital, but according to Honeywell, the T87K
is not a simple pair of contacts on a bimetallic spring, like mine:
Power Method - Uses stored power from heating controls.
Sensor Element - Thermistor
and most importantly:
Also, there are at least two versions of the T87, but
The T87K heat only works with 2 or 3 wire heat only systems
Unfortunately, it expects one of those wires to be from a 24 VAC transformer.
This is a vastly more complex thing than my simple thermostatic switch, which might be fine if it were powered by its own battery
... but apparently it's not.