Well, your results are what I get ...:
SELECT offmin=m.tmpmin,
offmax=m.tmpmax
FROM(Select t.year_ID,t.mnth_ID,t.day_ID,tmpmin=min(t.l),
tmpmax=max(t.l)
FROM( select n.year_ID,n.mnth_ID,n.day_ID,l=n.load_1
FROM temp n
UNION ALL
SELECT n.year_ID,n.mnth_ID,n.day_ID,l=n.load_2
FROM temp n
UNION...