The principal is this from my understanding. Boiler is at steam temperature. A tube runs thought the boiler to the group head. When you turn on the brew switch water is pumped from your water reservoir through the tube, and it heats up and goes to the group whichis already hot from your preheating. You flush a HX because the water in the HX gets really hot sitting in that tube, it equalises to the steam temperature while it's sitting. To vary the temperature in the HX tube, you can lower the Pstat for the boiler, but this has the effect of reducing the steam pressure too, so you have a play a game, of getting the desired temperature at the group, and adequate steam pressure for steaming milk. Personally I think with all the faffing about, fitting a PID to a HX is a waste of time. Getting it calibrated would be a nightmare, and I doubt you will really see that much of a difference in the cup for your efforts/cost. Put the money towards a dual boiler if you want to faff around with temperatures so there are less variables. Personally I would just grind, lock n load, brew and drink and enjoy. The coffees I had at your place were better than any cafe around our area. Go visit Kelsey and try his L1 (but don't blame me if you get upgraditis). I don't drink espresso straight much, but everytime I go to his place its so nice, lots of flavour. I think he gets shocked that I say it's fine without milk when I visit! haha. If I had $4k lying about I would love one of these now that the niggles have been worked out of the machine.
