chappcc wrote:In the hot leg, I have installed a needle valve to control thermosyphon flow and thus cool or heat the group as needed. Without the needle valve, I was getting ~95C at the bottom of the group and the extractions were a bit acidic. Closing the valve, so that the bottom of the group was ~75C, I got slightly sour extractions.
Putting a restriction in the thermosyphon does two things. First, it restricts the flow of the thermosyphon when the machine sits idle. Second, it changes the balance of water from the top or bottom of the boiler that reaches the group. More flow from the hot leg gives water closer to the set point. Reduced flow from the hot leg gives water substantially lower than the set point.
As you noted, this gives you the ability to change the difference between boiler temperature set point and the temperature of the water that reaches the group. This difference will impact the boiler set point needed to get good-tasting shots. In addition, it will influence how long it takes for the group to reach the steady state temperature after pulling a shot.