I've noticed that once the machine is cooled at room temperature (approx. 30C) & upon power on, the PID will displayed a low temperature reading of 12°C. Strange, since my home water supply temperature will never be this value. Any idea why? Thks.

I assume it's the measured temperature minus an offset of 18°C so the displayed temperature is more "friendly".

For example, if the desired temperature at the grouphead for brewing is 90°C, the actual temperature required in the boiler as measured by the probe is 108°C (to compensate for heat loss between boiler and grouphead). Rather than confuse the user by displaying 108°C, they program in an offset (-18°C). So, when the boiler cools to room temperature, the probe in the boiler reports 30°C, but the displayed temperature is 12°C.

Thanks Dan for the explanation . Finally, well understood the offset thingy. Does anyone here knows how to goes into the PID & check on the pre-set parameters? Thks.

See Vibiemme PID adjustment and Stefano's Vibiemme Double Boiler Supplemental Instructions (PDF). FYI, according to Stefano's Vibiemme page, the offset is 19°F.

I've just checked the PID parameters according to Stefano's supplementary instructions & it's of version 1.01. The strange thing is that the values are different from the user's guide . The displayed values are P:1.8, I:0, D:1.7, F04/Offset:20C whereas stated on the user guide are P:0.5, I:1.6, D:2.1, Offset:20C. Should I changed back according to the guide or? End users pls. share your input. Thks.

There's a great primer on setting your PID by Greg Scace. It takes at least an afternoon and a Scace device/Fluke setup (or something similar). The unique way we use PID's require unique settings and normally letting a PID set itself will rarely work.