Industry-leading thermal efficiency up to 97.1%. Stainless steel, vertical firetube heat exchanger. Available in 750, 1000 & 1100 MBH sizes. 160 psi working pressure. Water | Natural Gas or Liquid Propane. Modulating burner with 5:1 turn down ratio (*Model dependent). PVC, CPVC, PP and SS (AL29-4C) materials approved for direct vent and direct exhaust. Full range of venting options.