So I have called my plumber but his turn around time is slow.

I have a new house (1 year old) with a recirc loop for hot water and 80 gal tank with solar and electric back up.

The tank is hot (the solar portion has a display of tank temp). Hot water works downstairs. Upstairs in one room in particular, the water starts hot and then cools down. I tried it this am and I could take a shower for about 60 sec and then it was too cold to continue. I moved to the master bedroom and the shower was hot but colder than usual. The sink in guest room with single handle was cool - just like the shower.

There are antiscald - pressure balanced in upstairs rooms except the master which has thermostatic valve. When this happened, a direct hot water valve (sink) was hot but would get cold for a few secs. The pressure is normal in the sink and shower and tub.

It is like a valve is allowing cold water to get into the loop. Is that possible? Could a single valve cartridge causes multiple points to not have hot water? The loop coudl be blocked by debris but then the whole house would be affected.

Any guesses?