Thanks very much for any help with this.

I have a tankless HWH that has basically been working as advertised.

Took a hot shower this morning but came home to discover that I can't get the water to warm beyond luke (that's a term, right?) at the shower although it's piping hot at the lav. Went upstairs to the attic and the HW feed to the shower is hot to the touch.

The only thing that's happened recently is that I turned off the whole-house valve to replace a hose bib outside yesterday. And FWIW, it's all copper pipe soldered everywhere except for two sharkbite fittings. And the tub hardware is a single handle sweeping from 7 to 12.

Again, many thanks in advance.