Where are you located?
-6°C and -9°C are quite cold, not to cold maybe, but it could lead to condensing water on the logic board which in itself could harm the computers.
Minus Celsius temperatures are below zero, the point where water solidifies into ice, so I think maybe the sensors are a little bit off.
What are using to monitor temperatures?
My iMac once or twice had temperatures of around 30°C, it was -12°C outside, the window was open and the heater was off. So for your Macs to be that cold it must be really cold inside your flat/house.