Temperature Profile Potential (HX vs. Double Boiler)

[TimEggers]

A common theme that comes up when users are seeking advice on a new machine purchase are the merits of temperature management and the brewing practices of HX machines versus those machines utilizing a dual boiler arrangement.

Commonly HX machines are praised for steaming on demand at less cost then dual boiler machines but then almost in the same breath cursed for the need (and debatable difficulty) of the flushing required before making espresso.

Dual boiler machines are heralded as the simple easy way to make espresso as needed and steam whenever needed without the need for flushing or manual brew temperature management. Yet they do represent the upper end of most users budgets.

Now the questions I'm curious about are in regards to brewing temperature profiles:

1.) HX and Dual Boiler machines are designed quite differently and I am wondering if the same temperature profiles are possible on both machine setups. Humped, flat, curved you name it can they all be executed on both designs?

2.) Does one or the other lend itself better to certain profiles?

3.) Does one design lend itself to better switching between profiles (useful when trying out a new coffee)?

4.) Where would a single boiler like the Alexia or Zaffiro fall in all this?

[RapidCoffee]

Theoretically, both double and single boiler machines should be able to achieve reproducible flat brew temp profiles. HX machines, at least when used as designed... prolly not. A humped profile is more likely. OTOH you can manipulate brew temps on the fly by varying the HX flush.

Which is better? Who knows? Amazingly, no good studies are available on this subject*, although there are certainly plenty of strong opinions.

* Please correct me if I'm wrong about this.

[cafeIKE]

Given enough valves and sensors, one could design a machine with configurable profiles. If a definitive correlation between coffee, profile and a pleasing cup exists, surely by now the crema would have risen to the top. A great steak has difference nuances when grilled over gas, oak or charcoal, but none will turn shoe-leather into charolais.

Single and double boiler machine profile flatness will depend on the size of the boiler relative to the injected volume of cold water. To some extent, the heat input capability will have an affect.

Quickmill Alexia, for example, injects the full volume of cold water and passes the excess hot water back to the reservoir. This will likely give a slight droop to the tail of the shot.

Vibiemme Domobar Double Boiler, OTOH, only injects replacement water and since the water from the thermosyphon exit is cooler than the boiler water, a slight rise at the tail of the shot might be expected.

In any case, the delta will be a function of how long the shot is pulled, the shot volume and the shot interval.