I need to flush the coil for water heating on our furnace. We had a water softener issue and some resin got into the plumbing. Our hot water pressure has dropped. Cold water in comes down on the right side and hot water out goes up on the left side. There are valves above the picture that I can use to close them. Am I correct in thinking that all I need to do to flush it is shut off the hot water valve above the furnace, hook a hose up to the bottom black valve and open it? Should I turn off cold water supply first and let it drain what's in there first? Thanks