How long does it take the shower to run out of hot water? What is the "bath faucet"? Tub? Vanity?
How long can you run them without running out of water? Back to you, Tom

Sorry, my original post didn't go out with the full question -- here it is: I have a shower/bathtub setup where you pull a lever to divert the water from the bathtub faucet to the showerhead. Lately I have been having a problem maintaining hot water from the showerhead, but not from the bathtub faucet. Hot water comes out of the showerhead for anywhere from 3 to 5 minutes and then turns ice cold. When I divert the water back to the bathtub faucet it takes just a few seconds to get hot again, but if I leave it on the shower setting it remains cold. Once it's hot through the bathtub faucet I can again divert the water to the shower and it's hot again, although it always comes out in spurts for a few seconds from the showerhead after I do the diversion. I have already replaced the faucet valve and this does not happen in my other shower.I have a tankless water heater, if this matters at all. Any thoughts?