We live in a six year old apartment complex, where the maintenance guy should fix things like this, but he has exactly one year of experience in maintenance in general, which plumbing is only a small part of. If I can tell him what to do, he'll probably do it, though.

When we first moved in in November, the hot water would work everywhere, except in the bath/shower. It would be hot for a while (30 seconds to 5 minutes, depending on how cold it was outside), then it would go cold (this temperature varied). If we turned on any faucet while the water in the shower was cold, they would still be hot.

We noticed that it worked when the dishwasher, washing machine, or heat was on (the heat works from the water heater, somehow). He came and drained it a few times, until I called the corporate office and complained. They responded by replacing the water heater with a new one.

With a new water heater, it seemed to work better for a while (the rest of the winter, basically), but it stopped working as well when the heat wasn't on all the time. Now, the water in the shower gets cold pretty fast again (about the same rate), but running the dishwasher or the washing machine won't help anymore, and neither will turning the heat on (even though it's warm).

Now, when I run the water in the shower/bath, it will work for about 30 seconds, to 5 or so minutes, (it's as hot as it should be, depending on how we adjust the heat) then it will start cooling down. If I shut it off and wait for a bit before turning it back on, it will be hot again for a shorter period of time (10-15 seconds), then get cold again. When it's cold, every other faucet in the house is also cold.