On Wed, 4 Sep 2019 08:59:27 -0700 (PDT), wrote:

It is a Valley single handle shower valve. A similar Valley model (with possibly the same internals) is shown at https://www.youtube.com/watch?v=caZnXs42Zo4 .

How likely is it that replacing only the cartridge will fix the low hot water flow problem?

How likely is it that the entire assembly needs replacement (requiring soldering et cetera)?

Hot water flow in the two other faucets in the house (kitchen and bathroom basin) seems fine.

I once replaced a 20-year-old shower valve (Harcroft) for that same reason. Once the new valve
was in use and working well, I got the old one on the bench and found a rubber washer jammed in
the hot water circuit. If I could have seen that before ripping it out, I could have saved a
lot of $ and sweat!

--
croy