Brief temperature swings during a low-flow draw

Does the temperature swing stop when another hot-water fixture is opened? If it does, that’d be a useful clue that the heater behaves differently at a higher draw; if the swing stays limited to this fixture, I’d look more closely at the fixture side.

Opening another hot-water fixture is a useful clue, but it won’t isolate the cause by itself. It raises the total flow through the heater and can also change the pressure balance at the affected faucet or shower, which may shift its hot/cold mix. So a change in the swings could fit either low-flow operation at the heater or a fixture-side mixing issue.

That pressure-balance point matters: opening another fixture changes more than the heater’s operating conditions. If the heater displays flow and burner status, comparing them during a swing with the model’s stated minimum activation flow could help show whether it’s cycling near that threshold. It wouldn’t rule out a fixture-side mixing issue, but it’d make the heater-side explanation more concrete.

One distinction with the display comparison: the minimum flow that activates the burner and the minimum firing rate are separate limits. A displayed flow above the activation minimum wouldn’t, by itself, rule out the heater cycling because demand is too low for it to modulate steadily. The model’s specs may give both thresholds.

The one-fixture-versus-several pattern is a useful clue, though it won’t settle the cause. If the swings recur only at one outlet while other outlets stay steady at similar low flow, I’d look at that fixture’s mixing or flow control first. If several outlets swing under low demand, the heater becomes a more plausible place to investigate.