Heat check completed on a horizontal gas system. Drain line was flushed and treated. Flame sensor was cleaned. All mechanical and electrical components were checked. Gas pressures were adjusted to spec. I did find buildup on the blower wheel. I also found significant rust in the external drain pan. I took the static pressure of the system and it is double what is recommended from the factory. I went over my findings with the homeowner. No other major issues were found. The system is running at completion of service.