Losing hot water temporarily before it returns on a tankless system is frequently caused by the cold water sandwich effect, fluctuating water pressure, or temporary flow sensor dropouts. When you turn off a hot water tap briefly and turn it back on, residual cold water trapped in the piping passes through the unit before the newly heated water arrives. Additionally, low flow rates can cause burners to cycle off and on. Verification: Run a steady stream of water to evaluate whether temperature stabilization returns after a few seconds of flow.
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