Low volume on hot water

Hot water volume in one of my rental properties has dropped significantly. Has affected all units. However all valves are full open. Also, meter not moving, so no leak. Any thoughts?

As a workaround, I've cranked the temperature on the water heater so that tenants can create volume by mixing with cold water, but I don't like this for 2 reasons: 1) possibility of scalding and 2) it's kind of wasteful.

Thanks in advance for any input.

What year was the home built? Could it be the pipe nipples at the water heater? Sometimes those can either close off, or if you have a check valve, it can malfunction.
If the pipes are galvanized, the hot will close off sooner then the cold, but normally it's a little sporadic until it all closes down. On a repipe, I always recommend replacing the hot and cold together, or you wind up with too much hot and not enough cold to balance it down.

If it's on lav or kitchen faucets, and the pipe in the wall is galvanized, it can be the nipple that the shutoff is threaded onto. Where the brass meets the steel, it can close off. Sometimes replacing the shutoffs with new nipples is a temporary fix until the home is repiped.

If you've raised the WH temp above 120-degrees, you really should install a tempering valve. WHere I live, they are required, regardless of the temp you set the WH, and are a good idea, regardless.

There is no way for us to diagnose it based on your description. It could be a corroded pipe, a broken valve, or something else but we would have to be there and test it ourselves.

This is helpful info. House is 100+ yrs old but all visible piping in basement is copper. What has me is that all 4 units experience the problem. Even the laundry basin, where the supply lines are visible right from the meter all the way to the faucet, has low pressure.

I may have the water heater inspected.

Thanks.

Some water heaters have heat traps that can stick from sediment. The result is reduced flow.

Good to know. Sounds like my next step is to have the heater inspected.
Thanks again to all for the input.