Filter,heat ducts,and best heat we can get from...

Question

PLEASE HELP WE HAVE A RUUD MODEL UGTA-09EZAJS ACHIEVER 90PLUS.WE CAN NOT FIND A FILTER IT WAS INSTALLED IN OUR ATTIC IT IS NOT UPRIGHT.WE HAVE HAD MANY MANY PROBLEMS WITH OUR INSTALLER.WE HATE TO CALL FOR INFO.BUT FEEL ALSO THAT WE AT NOT GETTING THE BEST HEAT FROM OUR UNIT.ANY HELP WE WOULD LOVE TO HAVE.ITS ALL VENTED IN ATTIC WITH BLACK PLASTIC HOSEING.THAT WE AT TRYING TO KEEP ATTCHED TO VENTSTHANKS :)