You need to have the tech clock the gas meter to determine the btu
input. Being a variable speed, it should also be two stage, so check it
in both stages. He also needs to check the dip switch settings for the
proper CFM. Also they need to check the return air temp right at the
furnace-- not just use the indoor temp. or the temp at the return
grille. You may be sucking in cold air on the return which is throwing
off your temp rise. Larry