Now I agree with you that the overall thermal situation of a group is somewhat complex and hence not easy to calculate or even estimate. But what I was objecting to was your remark that the heat loss through the showerscreen would be about the same regardless whether or not the portafilter is removed. Which simply is not the case...

Walter wrote:But what I was objecting to was your remark that the heat loss through the showerscreen would be about the same regardless whether or not the portafilter is removed. Which simply is not the case...

if we figure that the vast majority of the heat going into the portafilter while it's locked in the group is radiant heat from the showerscreen, and the portafilter is radiating that heat to the room, then wouldn't the overall delta-T be the same whether the portafilter is locked in or not?

barry wrote:if we figure that the vast majority of the heat going into the portafilter while it's locked in the group is radiant heat from the showerscreen

I'm afraid you have lost me here. Are you saying that the locked portafilter is only heated by radiation from the showerscreen? Conduction through the metal contact and convection can be omitted? Frankly, I am not sure about that...

barry wrote:..., and the portafilter is radiating that heat to the room, then wouldn't the overall delta-T be the same whether the portafilter is locked in or not?

I am not sure about that either (though I never bothered even trying to calculate or measure)...

But by removing the portafilter the showerscreen becomes an "outer surface" and will transfer heat to the surrounding air faster than with the portafilter inserted (proportional to ΔT) and thus its surface will cool. The heat loss will be compensated (entirely or in part) by heat transfer from the hotter parts (depending whether we have a saturated or thermosyphoned group or barely conduction through the metal surfaces).

The remote portafilter will also cool (without any compensation). Overall we have - by removing the portafilter - significantly increased the surface area of our thermodynamical system which is exposed to room temperature.

Thus, we have an overall cooler system when we reinsert the portafilter. On the one hand the surface parts of showerscreen will be cooler and on the other hand the portafilter certainly is cooler. Additional heat transfer is required to bring the system into equilibrium again and to warm all surfaces that will come into contact with coffee...