This kit contains the appropriate burner orifices and pressure switch for the application of 90% furnaces in installations above their maximum (as shipped) rated altitudes. The burner orifices in the high altitude kits have been selected as a result of agency certified testing at high altitude and will provide the appropriate derate. If the BTU content of your gas supply has been artificially changed to account for altitude, contact your gas supplier for orifice sizing (orifice may not require change).