What is this "on" cap that's adjacent to my tankless water heater's service valve?
i need a side pic but as of right now i think that valve is closed. so if this is the case it is a valve to open or close the T pipe. im guessing the black screw cap is there just to cover another connection. so the valve is closed. you could take that black cover off and add another pipe there and open valve. from your other question your trying to clean it. thats the hot water side of the water heater. so this is im guessing you would clean it from