Burnham SteamMax STMX399

The SteamMax STMX399 is one of the twelve sizes in U.S. Boiler Company's current SteamMax gas-fired steam line. Its PDS ratings row prints 399 MBH input, 323 MBH output and a 242 MBH net AHRI steam rating covering 1013 sq ft (1.333 piping and pickup allowance), at 81% AFUE with 82.5% combustion efficiency.

The boiler uses a cast-iron sectional design with metal push nipples, wide internal waterways, heat transfer pins below the water line, no heat transfer pins above the water line and a large steam chest, with optimized stainless steel burners, a step-opening gas valve, a probe-type low-water cut-off, an industrial-grade pressuretrol, a rear low-profile draft diverter, a vent damper (models 075–299) and 2 in. risers. Supply and return are 2 in. NPT with a 2 in. skimming port tapping, the gas connection is 3/4" NPT, and the unit ships with a 15 PSI relief valve and gauge glass; near-boiler piping kits are documented for the residential sizes.

The 350–550 MBH sizes are documented as commercial builds with dual combustion controls, manual-reset ignition and alarm (UTEC 1003/1145-2) plus an additional auxiliary LWCO and manual-reset pressuretrol. U.S. Boiler prints a 10-year limited heat exchanger warranty and a 10-year limited waterside corrosion warranty. No official Canadian-sales statement was found, so canadianStatus stays 'unknown', and gas, combustion and steam work belongs with a licensed contractor.