Water Heaters - Tankless Either Too Hot or Cold

I recently bought a house with a gas-fired, tankless water heater. Discovered at the first shower that it was too hot for comfort, but when I turned the mix to add some cold after a few minutes it went all cold. From what I read in the install manual it requires a minimum flow to keep the heat on, so I have turned the temp setting to its lowest hoping to need less cold in the mix and keep the heat on. Still a bit too warm.

I will say this, the shower is only about 12 feet away. There is another at the other end of the house that I have not tested yet.

Is this just a "feature" of the tankless technology or is there something not set quite right?

I had the same problem with my Bosch Aquastar after I installed it about 4 years ago and I chalked it up to the shower valve as I have never had any problems with any other hot water spigots in the house and eventually it went away 95% of the time in the shower. Because it almost never happens anymore I never got around to changing the shower control to see if that is the problem.

I talked to a guy who also installed the Bosch Aquastar series and he complained about the same problem, spent weeks on the phone with Bosch, replaced a few things that Bosch said could be causing the problem and he said it still never solved the problem. He said eventually he gave up and about a month or so later it quit acting up on him and had been fine for months since his last problem.

I don't know what caused his problem or mine but they were very similar and eventually went away. I do know the troubleshooting says low gas pressure and improper gas supply line can cause it to go cold but I don't know if that is an issue for your unit.