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Flexible Watertube Boiler

  • Steam or hot water
  • 1.5 to 25.0 MMBTU
  • Natural gas, #2 Light Oil, propane, digester
  • 15 to 250 psig on steam, 160 psig on hot water
  • Available to < 9 ppm NOx

The model FLX Flexible Watertube Boiler is designed to minimize thermal stress and provide quick response in a compact unit. There is also a field-erectable option.

View Presentation on Flextube 12.5 - 25.0 MMBTU

The FLX has distinct design features that include non-welded tube attachment for ease of tube replacement. The package includes a ProFire® burner for a completely integrated and engineered solution from a single source. Removable sectioned side panel design permits full access to all tubes.

The compact footprint design offers 160 psig hot water applications or 15 psig and 150 psig steam applications with optional design to 250 psig.


  • High turndown up to 10:1 on natural gas for maximum efficiency
  • Hinge-mounted burner and removable panels allow easy access for service and maintenance
  • Thermal shock resistance
  • The integral package for 30 ppm NOx is UL-listed
  • 20-year limited pressure vessel warranty


  • Lower Fuel Cost
  • Lower Emissions


  • Process Steam
  • Industrial Process
  • Building Heat
  • Sterilization
  • Hospital and Healthcare

Case Study

Cleaver-Brooks Large Flextube Boiler is Engineered for High Efficiency and Low Emissions

When Cleaver-Brooks engineers commenced to design a large flexible watertube boiler, capable of 12.5 to 25 MMBTU, they wanted to engineer a product that would raise the bar for both efficiency and emissions in the category.  Their goals were for the Large FLX boiler to obtain 81% efficiency on 125 psig steam and 85% efficiency on hydronic heating applications. 


FLX 1250-2500 PPT

MODEL FLX 1250 -2500 Flextube Watertube Boiler Steam or Hot Water ApplicationsFLX1250 -2500FLX1250 -2500 Model Designation: FLX or FLE Gas [Heat Transfer] Flow: 5 Pass Factory assembled package or Field Erectable Package. SIZES 1250 = 300 HP 1450 = 350 HP 1650 = 400 HP 1850 = 450 HP 2100 = 500 HP 2500 = 600 HP Standard