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 {TAIL ~ the furnace} and the exhaust line outlet 
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 { TAIL ~ the furnace } and 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 TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH