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.

 

[Rossando]

Thank you, everyone, for your replies.
To GReynolds929... Water temp is set at 130 (we have not changed that from when we moved in). We did as you suggested - shut the inlet valve at the water heater. Opened a faucet to hot only. (Note that the water heater is in the garage on a wall that backs up to the master bath. We assume that the water flow is to the master bath first - tub is closest to the source - before flowing to other parts of the house.) The water slowed down to a very low flow in the master bath but never totally stopped. Just to be sure to completely empty the lines, we opened all hot water faucets; still had the very low flow occurring in the master bath. We opted to keep the faucets opened when we turned the inlet valve back on, to make sure any air got out of the lines. Interestingly, the master bath tub water got very hot while the sinks in the same room stayed ice cold. It wasn't until we turned off the master bath tub that the sink water turned hot.
We have searched EVERYWHERE for a crossover valve for using the cold water line to recirculate water back to the heater. We can't find one anywhere. We have been told that there is not a dedicated return line in the house, but I'm starting to wonder. All I know is that first thing in the morning, when no hot water has been in use, I can turn on the faucet in the bathroom furthest from the heater, and I have hot water in about 4 seconds (I actually timed it this morning).
Breplum, the dip switches set to external circulation is the way it was set when we moved into the house. And EVERYTHING WORKED FINE for the first 8 months that we lived in the house; so, something else has changed (or broke).
One last comment, on seeking professional help. We have been badly burned in the past with service technicians "taking" us - saying they did one thing and charging us an arm and a leg, just to find later that they didn't do what they said at all. We now try to research things on our own first. Believe me, we would never attempt anything of a very technical nature. But dip switches, parameter settings and changing a valve or two are pretty simple to tackle on our own before we call in the big guns. We know the "bad" technicians are the exception, not the rule, so we try not to lump everybody in the same basket.

You have a crossover at a fixture. I would suggest the shower cartridges are stuck open if calcium deposits get within them. It's simple but is a few step process. Flush out your tankless water heater and do this once a year or bi- annually. Second start pulling shower cartridges and replacing them with new ones one at a time. Or to find out which one has the crossover shut it down at the stops and see if you get hot water. Probably good to replace them all to be honest.