How to convert strings extracted from JSON to a numeric type for aggregates?
You need to extract them as text using jsonb_array_elements_text(), then you can cast it to an integer:
select min(x.val::int), max(x.val::int), avg(x.val::int)
from the_table
  cross join jsonb_array_elements_text(data #&gt; '{temperature,boiler_temp,values}') as x(val)