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@bombcup
Yes, the temperature of the water idling in the heat exchanger pipe will equalise with the boiler water and be too hot for brewing. You need to flush this out before inserting the portafilter and pulling the shot. This means fresh cold water will be drawn through the hx and heated to the correct temperature.
HX machines are designed to be busy. There is no need to flush if you are pulling shot after shot as the water in the hx tube has no chance to reach parity with the boiler temp.