Condensing Gas Heating Boiler is available in four sizes: NHB-55, NHB-80, NHB 110 and NHB-150 with turn-down-ratios respectively of 7:1, 10:1, 11:1, and 15:1. The 15:1 TDR in the NHB-150 is achieved with an advanced burner system. Efficiency is in the newly developed dual Venturi gas delivery system. With a 95 percent AFUE rating, the NHBs carry Energy Star’s Most Efficient designation. Ideal for residential and light commercial use, features include: large turndown ratios; timed hydronic supply water boost; adjustable heat capacity; adjustable anti-cycle timer; freeze protection; adjustable Delta T ranges; pressure LWCO with manual reset; adjustable minimum burner time setting; and adjustable turndown ratio timing. Navien.