I have a Navien NPE-240A tankless hot water heater delivering hot water throughout the house. All of a sudden, I am only getting luke warm water at each of the showers (which have a single mixing valve for hot and cold). However, I get piping hot water at sinks and the master bath (which all have individual hot and cold faucets). My first thought was the cartridge in the shower valve was restricting the flow of hot water. I was going to replace the cartridge, but this is happening at all three showers simultaneously. I had the heater set to 125 deg F outlet, and increased it to 130 deg F with no change in symptoms. The Navien has a flow sensor. When I run a shower at full hot, it's showing a flow around 1.2 gpm of hot water. The hot water outlet is reading around 105 deg F. When I run a single sink faucet at full open, I get around 4 gpm hot water flow, and the outlet temperature is at the set temperature of 125 or 130 deg F. I cleaned all the water filters in the hot water heater. There was no dirt in any of them. This seems like an issue with the showers, but why would all three cartridges fail at once? I pulled one cartridge out and it looked clean as a whistle.