Addendum:  The 4000' ASL O2 shutoff I quoted is for the Mr. Heater

30,000 BTU NG Ventless Heater (which I use).

The Mr. Heater "Big Buddy" 18K BTU propane heater manual states

that the O2 Depletion shut-off equivalent is ~ 7000' ASL.
--=20
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