Both of my showers do not keep hot water very well. As I take showers in each one, I have to keep turning up the hot water. I will then get a burst of hot water and then it will slowly get colder and colder. The first time I took a shower in one of them, I ended up taking too long and ran out of hot water. The plumber who came and "fixed" it said that I don't have a big enough water heater (40 gallons), but that doesn't make sense. If I had run out of hot water, I wouldn't be able to get a burst of it when I turn up the dial. What else could cause this?