Per the installation manual for your fireplace:  This fireplace has been tested and approved to ANSI/IAS/AGA Z21.11.2 for use with an unvented gas appliance having a maximum rating of 26,000 BTU and complies with the Standard for Factory-Built Fireplaces, U.L. 127 when installed with unvented gas log sets. The limit may be increased to 40,000 BTU providing that minimum combustible mantle heights were increased to at least 18 inches when the fireplace was installed.