Performed 2 gas boiler pms Left back door Brushed and vacuumed burner tubes Inspected and vacuumed burner compartment and surrounding area Checked electrical connections and safeties Checked gas pressure Performed combustion test Inlet pressure- 8.725 in.w.c. Manifold pressure-3.4 in.w.c. 7.4%o2 38 ppm co Right back door Brushed and vacuumed burner tubes Inspected and vacuumed burner, blower compartments and surrounding area Checked electrical connections and safeties Checked gas pressure Performed combustion test Inlet pressure-8.84 in.w.c. Manifold pressure- 3.591 in.w.c. 5.3%o2 30 ppm co While performing the maintenance for the left back unit we found the cast iron heat exchanger is starting to develop cracks in between the sections of the boiler letting extra oxygen into where the flames are. Filled the cracks with furnace cement and took pictures and measurements for a replacement quote. Recommend replacing the unit in the near future before the cracks become worse.