Judith,
Just start taking things apart until you have the diaphragm valve in your hand. Then, and only then, remove the 5 screws on the cap. Be very careful when you take apart a connection made with bolts or screws because there is a fragile gasket in there that needs to be reused. I wet the gasket with plumbers grease or a silicon lubricant. Not too much, just wet it.
I have never had luck with vinegar to de scale the coils. Try a de scaler specially made for tankless water heaters. Home Depot and Lowes have them. also some tankless heaters must be plugged in and turned on for the modulating valves to open allowing the circulation of the descaler. Keep the gas off.
A phosphate feeder in the cold water supply may help keep the minerals in suspension as they pass through the coils. Some swear by the magnetic affect when two permanent magnets are bolted to the supply line. They are sold as a kit.
Another thing with tankless heaters is to set the temperature low and if possible shower with hot water only. We used to call them flash heaters and each person would set the temperature they liked when they showered.
