This week I had two 50‑gal atmospherics with high gas bill complaints that turned out to be a recirc timer stuck ON and a slab loop bleeding heat… My quick triage is a 1.5 gpm draw with a thermometer on inlet/outlet, confirm 120°F setpoint, check flue temp and manifold pressure, then feel the return line; nine times out of ten you can spot standby losses or scale without cracking the tank. What numbers are you logging on a clean unit to separate efficiency loss from combustion or venting issues?
On ‘1.5 gpm’ checks, clamp a probe on return; IR the flue confirms standby loss — wind skews temps.
Had one last week where everything ‘confirmed 120°F’ yet bills were nuts — turned out the heat‑trap nipples had failed and were letting thermosiphon on the return. Quick test: kill the recirc/timer and close the check, then feel the cold inlet; if it warms fast, you’ve got crossover or a bad trap, and I’ll shut the cold to the tank and open a hot to double‑check no flow. Small caveat: I also clock the meter for a 10–15 min no‑draw to put a number on standby before blaming scale.