A couple of weeks ago, a tenant complained that she wasn’t getting enough hot water out of her three handled shower.
Meaning, when you first turned it on, there was plenty of hot water. But after a minute or two, it slowed to a trickle. This didn’t happen with the cold water in the tub. And it didn’t happen with the sink next to the shower.
So we turned off the water, removed the hot water stem, and attached* a hose to the shower valve, and ran the hot water for a few minutes. We blew the debris out that was clogging her hot water, and all was well.
This is the second option that Larry mentioned.
* This attachment wasn’t strictly necessary. We could have done this without the hose, but it made the job a lot less messier. She had a tub with just a shower curtain, so this kept water from spraying all over and getting on the floor and wall. Attaching said hose to the valve involved a certain amount of jury rigging with nipples and adapters and what not.