A flow switch 70 detects {HEAD ~ the movement} of {TAIL ~ liquid} through 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 
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A flow switch 70 detects { HEAD ~ the movement } of { TAIL ~ liquid } through 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 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 OTH OTH OTH OTH OTH OTH OTH OTH