Tankless Water Heater not delivering hot water to showers

[GReynolds929]

Take some pictures of the piping going into the unit. What is the two way valve set for? Should be set to external. Sounds like you probably have either a crossover return system or a dedicated system. If the Navien check valve is not the source of the crossover the next likely suspect is a single handle shower cartridge or an external check valve. I have seen a 2 handle utility sink faucet where the handles are left open and there's a shut off on the spout which lets water mix. You can probably rule the Navien out by shutting the outlet valve on the heater and see if you still have the same flow on the hot side as the previous test.

 

[Valveman]

Reading the problems from all three OP's in this thread, I cannot tell the water source. This is exactly the problem with instant water heaters when working with a well or booster pump. The pressure swing from 40 to 60 over and over causes the flow rate at the shower head to fluctuate. When the pressure is low, the flow is too low to keep the heater lit, and the water gets cold. After the pump starts at 40 and the pressure climbs to 60, the flow through the shower head increases and the heater lights back up and gives hot water. I have heard it called a "Cold Water Sandwich". Lol!

Many Cycle Stop Valves are added to well and booster pump systems to solve this problem. The CSV will hold a strong constant 50 PSI for as long as the shower is on instead of letting the pump cycle on/off from 40 to 60 over and over. Not only will this keep the instant heater lit (no cold water sandwich), but will make the pressure stronger as well as extend the life of the pump, tank, and everything in the pump system.