Interesting now I am being told the Navien doesn't really modulate its flow rate to hold temp.
Does anyone know which is the right answer?
Meaning, let's say we open up 4 showers and are drawing 10 gpm of hot water, and because of the water inlet temp, the Navien can only heat 5gpm to 120 degrees. Does the Navien slow the flow to 5gpm itself so you will have 120 degree water but weak pressure at all 4 showers, or does the water just flow at the same rate thru the Navien so you have the same pressure but the water will be lukewarm and not 120 because it doesn't have enough time in the heat exchanger?
EDIT - I called Navien and confirmed that it is the first example (the Navien will slow the flow of water to whatever it needs to maintain the set temperature rise), so in my example above with 4 showers open at once, the water coming out would be fully hot, but the pressure would be lower than ideal if we were exceeding the GPM that the Navien was delivering at the time.