It means the furnace's fuel and air settings are adjusted for air that holds less oxygen. At Englewood's roughly 5,300 feet, a gas appliance set up with sea level assumptions burns rich: less complete combustion, more soot and carbon monoxide risk, and less heat from the same gas. Derating corrects the input to the elevation, and a proper install or major repair here verifies combustion with readings at this altitude. The question to ask any installer is short: show me the combustion numbers. In this market, that request is ordinary, and hesitation in answering it is information.
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