Tankless water heater

[regularjoe]

I have a rheem natural gas tankless water heater. The house is ~3,000 Sq ft and the heater is located on opposite side from the master bathroom. Despite it being set on 120 degrees, the warmest it gets is ~95 degrees even after two showers. The bathtub also doesn't get warmer than this. It also takes several minutes to get up to that temperature. However, if we have two showers running, the temp gets up to ~105, which makes a big difference. Any advice on getting the water hotter when running just one shower/bath?

 

[GReynolds929]

How old is it? When was it last serviced? What size? If you shut the cold inlet to the heater off and open a hot faucet does the water stop flowing?