About 15 years ago, widow who lived across the street from me was having heater trouble; went over to took at it, dust on themostat, cleaned it for her; as I looked at heater, no CO detectors: got her 1, put it by flues for hot water heater & heater : 2 into 1 ducts: about 6 months later, she came over it wouldn't stop chirping, BIG OLE HOLE IN FLUE: opened window, fan in window; she had contract with SJ Gas for heater service, they fixed it for free. Poor lady had 3 gas line leaks at various times too, turned her manicured zoyza grass brown; 1961 gas main in street leaked each time.