Use a template sensor. Make sure to take into account the availability of the sensors in your calculations, and in the availability attribute of your template sensor.
Use a sensor group, and set the options of the group to do average. Unsure how the availability of the underlying sensors affect the availability and calculations of the group.
Great thank you for replaying. How do I create tamplate sensor? once created how do you tight that in to thermostat to actually use that instead the one built in to the thermostat?