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