Rinnai HDC1600i Heat Exchanger (P1600IKBX-KITB) The heat exchanger is effectively the core of a tankless water heater — its job is to transfer energy from the burner to the water passing through it, delivering hot water on demand without sacrificing efficiency or reliability. On the Rinnai HDC1600i, this is the primary heat exchanger, and its condition directly determines how well the whole system performs. This genuine Rinnai heat exchanger core is made to the original specification, built for reliable heat transfer, corrosion resistance, and long-term durability. Where the existing heat exchanger has worn or is starting to underperform, a genuine replacement restores the appliance's performance, reduces the likelihood of repeat service callouts, and helps the system keep running reliably for years to come. ⚠ Important: