The water in my shower goes from lukewarm to very hot. It does this consistently during a shower. I live in a high rise so I don't have a hot water heater. Help

It sounds like the balancing spool (or diaphragm) has gone bad in the shower valve.

If you replace the cartridge assembly/balancing spool it should solve the issue!

If it fails to solve your issue then there could be a problem with a one of the single handle faucets in your unit (or even in another unit... makes it harder to solve issue).

My bet is on the shower valve cartridge... easily replaced if you are handy AND you can isolate the water to the shower valve (or your unit).

If you go to YouTube and enter replacing balancing spool in the search bar you'll get some good videos showing you how to do this job!

Back to you...

Mark