Raytherm® Low NOx Hydronic Boiler, 181-401

Consult local emission requirements prior to ordering.
Model Number Full Input in
 Flue Dia.
H-181 181,000
H-331 334,000

Available Firing Modes
H3 2-Stage
H4 On-off
  *H3-401 (2-Stage) has input 375,000 BTUH


  • Cast-Iron Headers with Copper Finned Tube Heat Exchanger
    Single-bank, straight-through design delivers lowest pressure drop. Integral fins have ten times the heat transfer surface of plain tubes.
  • 80% efficiency, CAT I venting.
  • Corrosion Resistant Steel
    Fittings are brush galvanized against rust. Heat shields are corrosion-resistant steel. Jacket has Polytuf powder coat finish.
  • 24-Volt Controls
    Are totally enclosed for protection. Door is easily removed for inspection and service.
  • Floating Return Header
    Immunizes heat exchanger from thermal shock.
  • Tube Sheet and “O” Ring Construction
    Eliminates the repair and maintenance problems associated with rolling tubes directly into a casting.
  • Interlocking Refractory Panels
    Sealed corners reduce radiation loss.
  • NOx emissions under 55 PPM (40ng/j). Consult local emissions requirements prior to ordering.
  • Stainless Steel Burners
    Maintain precise combustion at all firing rates, burns quietly and won’t clog or corrode.
  • Slide-Out Burner Tray
    Allows easy inspection and service.
  • Spark-to-Pilot Ignition
    The most dependable ignition method available.
  • Available for Natural Gas Only.
  • Built-in Controller With Outdoor Reset, STD On Sizes 181-261