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The water pressure is unstable after flushing, and the impact of positive and negative pressure waves in the water flow in the pipeline.
This can only determine if there is a leak between your inlet and return, and at what stage the leak needs to be further checked.
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The small yellow valve you said, is it on the cold water inlet of the water heater, if so, it should be a pressure relief valve, if your cold water valve is open and it is dripping, it is recommended that you let the after-sales service overhaul it.
It may be that the starting circuit of the water heater is falsely connected, and secondly, it is best to separate the water pipe of the flush valve from the pipeline of the electric water heater.
The circuit is connected incorrectly, it is recommended to find your decorator to reconnect it.