An exhaust line 11 is connected to the feed line between check valve 10 and the furnace and includes two power controlled shut off {HEAD ~ valves} 20 and 21 between the furnace and {TAIL ~ the exhaust line outlet} 
between
An exhaust line 11 is connected to the feed line between check valve 10 and the furnace and includes two power controlled shut off { HEAD ~ valves } 20 and 21 between the furnace and { TAIL ~ the exhaust line outlet }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL