I'm no professional but I've always done my own plumbing repairs.
I bought this house earlier this spring. I wasn't getting any hot water to the one shower, which is a good ways from the water heater, but I was getting hot water to the sink.
Leaving the shower on full hot getting zero water we were looking for a valve to get this thing going(house was forclosed and not occupied for months).
We tried turning what we thought were valves under the sink(shown in attached picture, with a flat blade screwdriver). But then something let loose in the shower valve and I've had hot water ever since.
Sadly though, those parts in the pictures have slowly leaked ever since(June)
Now lately the sink aerator and shower head have been cloggin up with fine black material. I'm guessing these are rubber seals deteriorating inside. The black material isn't coming from any other water fixture in the house except in the house except this bathroom. Must be these components, right?
But what are they? Pressure balance? Check valve? I couldn't tell you which pipes are hot or cold at the moment though until later today when I get home.