After digging a Dirt dobber nest out of the fan on my RV hot water heater I decided to make a plug to keep the little boogers out. They make a screen for most things that need plugged but I could not find one to fit this that would not have to be removed to open the access door. You do have to remember to remove this plug before using your hot water heater. This is for a Suburban tank-less hot water heater.