Use a garden hose and a soft brush.

That’s all you need.

Turn off the gas and power, remove the access panels, and locate the heat exchanger. Flush it from the bottom up with a regular garden hose — no nozzle, just steady flow. That pushes debris out the top. If there’s scale or soot, gently brush it with a soft nylon brush. Hard bristles or a pressure washer will dent the fins or puncture the tubes.

Don’t use chemicals unless you know exactly what you’re doing. Muriatic acid solutions are common but easy to overdo. A simple water flush twice a year keeps most heaters running fine.

Future you will thank you for not using a pressure washer.