A flow switch 70 detects {HEAD ~ the movement} of liquid through the shell 12 by testing for {TAIL ~ flow} in the shell liquid outlet tube 56 and prevents the burner 16 from firing when there is no flow 
by testing for
A flow switch 70 detects { HEAD ~ the movement } of liquid through the shell 12 by testing for { TAIL ~ flow } in the shell liquid outlet tube 56 and prevents the burner 16 from firing when there is no flow
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH