Older units work by heating up a large amount of water, then directing it to the shower, sink, or any other fixture. The water has already been heated and is ready to be used when you need it. The problem is that sometimes you need more hot water than your water heater is able to heat and store.
You may turn on your fixture expecting hot water, but all you get is cold.
This can be inconvenient and annoying, especially if you live with many people all waiting to use the shower, sink, and washing machine. Instead of water heater repair, we often recommend tankless water heaters. New units have been recently designed that avoid this tank system completely. Instead of heating water prematurely, tankless water heaters only heat water when needed - and do so quickly.