How should I clean a heat exchanger?

Use a garden hose and a soft brush. Turn off gas and power, remove panels, and flush the heat exchanger from the bottom up with steady water flow to push debris out the top. Gently brush scale or soot with a soft nylon brush — hard bristles or a pressure washer can damage fins or tubes. Avoid chemicals unless you’re experienced, and a simple water flush twice a year is usually enough.