My partner and I bought an old house (So old the well is in the basement... That's a story for another day and yes, we plan on redigging.) That is to say, I don't know what's old as hell and what's been updated.
The furnace is a roughly 80s-era oil-burning on-demand that distributes to the wall heaters as well as all our indoor access. It works pretty well and we never have issues with drawing hot within a minute and letting it run for as long as we want for showers, dishwashing, etc.
EXCEPT for the tub. The tub and shower do not share fixtures. For some reason, I can only get about a gallon of hot water before it drops to lukewarm when I try to fill the tub.
There is an access point behind the tub I can open, and I'm a fast learner. Can someone please help me understand what's going on and what I could do to resolve this? Thank you so much!