When I run the hot water in the bath tub, it starts hot but then becomes cold when the tub is about 1/4 full. The shower water is always hot, and we always have hot water from the other faucets. We have this problem with both tubs. We have a 50 gal. water heater.

If the water heater's electric I would shut the power off, open up the bottom inspection plate and check the bottom element with a ohm meter for resistance. If you have none the element's faulty and must be replaced. Let me know what you find. Good luck, Tom