mysql - adding alias columns as a new column
Try this:
select x.timest,     
    max(case when x.devloc='outside' then x.value end) as outside,     
    max(case when x.devloc='hvac_main_return' then x.value end) as hvac_main_return,     
    max(case when x.devloc='hvac_main_supply' then x.value end) as hvac_main_supply,
    ((max(case when x.devloc='hvac_main_return' then x.value end)) -     
      (max(case when x.devloc='hvac_main_supply' then x.value end))) AS diff
from sample x where date(timest) = curdate() 
group by timest     
order by timest desc;

If you want a more detailled answer, please provide some sample data as in the fiddle I've done here. Also, please provide your desired result here - always provide everything you include in any fiddle here also (linkrot).