Cleaning of your model Rinnai Vented Gas Heater is not difficult. But will require the removal the front of the heater's cabinet.
What you'll need: A can of compressed air with a straw nozzle (like you clean your computer with). A vacuum with a hose and brush attachment (a Shop Vac is perfect for this, if you have one), and soft, clean rags.
To walk you thru all the proper steps of cleaning the interior of the unit, I have provided a link to the Owner's Manual for your model, in case you have misplaced yours. You can download and print the manual for future reference. What you're looking for starts on page 35.http://www.rinnai.us/documentation/downloads/431F-1700_%28RHFE-431FAIII%29.pdf
I hope this helps you and thanks for choosing FixYa.