No hot water / mixing valve replaced

hot_rod

We have seen tankless coils cause thermostatic mixers to be inaccurate. The pressure on the hot side can sometimes be less due to a scaled coil. Whenever there is a big pressure imbalance mix valves tend to misbehave. How is the pressure on a cold faucet compared to flow with just cold, on the same faucet?

A noticable pressure drop is another indication of a partially plugged coil. Has it ever been flushed?