Your automation triggers when the temperature changes from above 25 to below 25. If it is always below 20 it will never trigger.
Also if the temperature is always below 20 the switch should always be on. Why is there a need to automate it?
Anyway, here’s one way of overcoming this:
trigger:
- platform: state # this triggers whenever the temperature sensor changes to a valid number
entity_id: 3dd45aa1b936e8cdcfb15bab9d44edc6
not_to:
- unknown
- unavailable
condition:
- platform: numeric_state # this checks if it is below 25
entity_id: 3dd45aa1b936e8cdcfb15bab9d44edc6
below: 25
action:
- service: switch.turn_on
target:
entity_id: 3dd45aa1b936e8cdcfb15bab9d44edc6
The other way is to create a generic thermostat (nicer control on your dashboard too):