The ideal way to backup a process dependent device (eg- heating or cooling) is with 2 separate/independent controls. In series, so that if one fails in an ‘on’ state (closed relay) the 2nd will take over & turn it ‘off’ (open relay). It is also preferable to set them at least 2 deg F apart on their set points, to avoid having them interfering with one another’s operation (aka- ‘chatter’).
Using a two stage control that has separate heat and cooling control relays, and even separate temp. probes, is convenient but does not offer redundancy should say that control’s power supply fail. Which is also likely, in my experience. A second temp. controller would mitigate that scenario.
Fwiw,
I am also a fan or Ranco brand temperature controllers. They have never failed me.
