I own an old REPCO TOS 100 oil fired tankless water heater w/ low hot water pressure at the taps. I've had the coils flushed 2 years ago with little or no results. The condition is getting worse..... Is there a simple fix to this i.e. valve replacement or coils swap? Or am I looking at a complete system replacement?
Follow the complete water circuit. Pay special attention to where copper meets steel or galvanized pipe. If you have these type of junctions you may want to take them apart to check for electrolysis or plugging. Dissimilar metals when joined together with water flowing between them can plug up or become restricted. What you are looking for is any place in the water system that has a restriction. When you find it you will always know where to look the next time and yes it can be a time consuming problem.