Refactor code - then do proper testing
The big lesson in 'refactoring code and relying on unit tests' is a bad idea.
A massive difference where the temperature of fermentation was swinging with 1-1.5degC differential. It turned out the fridge wasn't switching off when the temperature reach the target - instead it was relying on a fall-back protection which turns of the fridge after it has been running for 15 minutes - meaning excessive cooling.
Setting 1 FLAG gets the difference back to the +/- 0.3 deg intended.