hot water in shower but not in tubs (tankless system)

Our house is about 14 years old, but we had a new tankless (Rinnai) installed 11 months ago. During the warm months, everything was fine, but the water pressure did seem low in the tubs. For about 3 months now (since the weather has been cool and then cold), we have been having trouble getting hot water in the tubs (both upstairs). The shower hot water seems to be fine (both upstairs also). The water in other faucets does get hot (over time) but lately not as hot as before. The plumber is here and he said there are no error codes on the hot water system. He checked the water temp, which is set to 120 at the system, but the tubs are only getting up to around 90 and then dropping. He checked the water regulator in the crawl space and turned that up. He said it was only coming out around 45PSI (I think he said). He has gone now to get a gas meter to check the gas amount (natural gas). We have a home warranty, but the water regulator is not covered (of course).

But my big question is, why would we still be getting the right amount of hot water in the showers, but not in the tubs if it is something like the water pressure or gas pressure? Thanks.

Shower heads are limited to a flow of 2.5 gallons per minute, which the heater can manage to heat successfully. If you had a way to reduce the flow from the tub spout, I am willing to bet that the water would be just as hot as the shower.

Update...seems that the flow in the tubs IS part of the problem. The plumber narrowed it down to something with our gas line. When the unit calls for gas to heat the water for a sink or shower it is fine. But when the unit calls for more heat for the amount for a tub, it can't handle it and shuts off. So if I run the tub at 1/2 pressure, we can get hot. Now I just need to figure out what is going on with the gas problem. (not sure if all that made sense, or even if what the plumber was telling me is true, but at least I can fill up the tub slowly now).

This could be one of many factors...you need to have this troubleshooted by someone on site. And if you don't trust their conclusions, then have another experienced installer come out and diagnose as the problem as well.

There are tools specifically designed to check the available gas pressure...if the pipe is too small or the line too long, you may not have enough supply. The tankless will also usually specify the required supply pipe size. Restricting it will cause problems.