Thus, {HEAD ~ the check valve} is between {TAIL ~ the furnace} and components 4 8 in the feed line 
is between
Thus , { HEAD ~ the check valve } is between { TAIL ~ the furnace } and components 4 8 in the feed line
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH