{TAIL ~ Control features} prevent {HEAD ~ the burner} from firing unless there is liquid flow through the shell 
{ TAIL ~ Control features } prevent { HEAD ~ the burner } from firing unless there is liquid flow through the shell
TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH