I am in the process of refurbing and installing the used EFM 520 I bought. Inspecting the DWH coil and I see signs of calcium/hard water build up inside the coil pipe. I haven't hooked it up to a water source to see what the flow looks like yet. Is there something I can use to cleam the build up from the coil? Also, does it matter which connection is the cold water inlet and which is the hot water outlet on the coil?