HVAC systems and methods are shown which use volumetric air flow and static pressure calculations that correct for different air densities encountered at different altitudes. With respect to the volumetric airflow rate, the systems and methods disclosed herein adjust the calculation of airflow rate to better match the BTU content of de-rated natural gas used at higher elevations, and therefore they are usable in HVAC systems that combust natural gas. The adjusted static pressure calculations may be used in any type of HVAC system.