S4 Conditions:

25.2 C, 35.3% RH, Td = 8.8 C

Water temperature: 20 C

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Rotating the dewpoint sensor on the pipe seems to have made it switch over to solid red = humidity OK.
Waiting confirmation to see if we should check a FEE (as per PJCS suggestion) before water is turned back on.
Will check the outside copper after lunch.

13:04 : Copper seems warm and dry, humidity sensor still solid red.