HAVE THE TECHNICAN CHECK THE GAS PRESSURE. THE GAS PRESSURE IS VERY IMPORTANT ON THAT UNIT, IT SHOULD BE SET AT 3.5WC. THE FLAPPER MAY NEED REPLACE AS WELL. PLEASE PLEASE HAVE THEM CHECK FOR CARBON MONOXIDE. LENNOX HAVE HAD PROBLEMS WITH THE HEAT EXCHANGER CRACKING ON THE PLUS FURANCE. P.S LENNOX DO NOT MAKE THAT HEAT EXCHANGER ANYMORE. GOOD LUCK.