After about 10 minutes with both wings running, the hot starts sputtering and temps swing; well pump holds steady at 55 psi. I flushed the 80-gal heater last month and swapped one showerhead today — thinking the mixing valve is gummed up with sediment; any quick fix I can try before the 60-person group rolls in Friday at 4 pm?
Had this in our lodge showers — after a few minutes the hot would spit until we pulled the hot-side “heat-trap” nipple (the little check-ball insert) and swapped to a straight nipple; it stopped instantly. If your tank has those, pop the ball out or replace that nipple and retest before the group arrives; do you have unions on top to make it a 10‑minute swap?
If you’ve got a recirc loop, “burp” it: open the return purge at the heater and run the recirc pump on high for a minute until the stream is bubble‑free, then crack the return balance a touch more — like burping a radiator… If that calms the sputter, the mixing valve may be fine; also peek at the hot‑side vacuum relief for chatter. @chen56’s bypass idea still helps as a test — do you have a bypass you can open while purging?