Rinnai HDC1600i Latent Heat Exchanger (P1600IKBX-KITB1) The latent heat exchanger — also known as the secondary or recuperator heat exchanger — on a Rinnai HDC1600i captures heat from the combustion gases that would otherwise be lost through the flue and puts it back into the water heating process. It's a core part of what makes a condensing appliance like the HDC1600i efficient, recovering energy that a non-condensing design would simply waste. This genuine Rinnai component is made to the original specification, restoring reliable secondary heat recovery and helping the system deliver both efficient operation and consistent hot water output. Where the latent heat exchanger is ageing or has started to underperform, fitting a genuine replacement restores the system to its intended efficiency and helps prevent further issues developing.