Since I'm upgrading from a Gaggia Baby Twin to ??? the question of which is better HX or DB comes up. I got a crazy idea. But, isn't that the wrong question. Most of the descriptions of various machine show a difference in the size of the brew boiler rather than only the type of boiler.

The lower extreme of size is the Gaggia brew boiler at 3.5 oz. Some of the DB machines have a boiler of about 10 to 20oz. The other extreme is the manual/spring lever machines with the extreme size being about 7 liters IIRC in the PV Lusso. So, the method of controlling (?) the temp is probably less important than the size. The Gaggia boiler will lose up to 2/3 of the water in the boiler for every shot while in the Lusso the water loss is 2oz in the ~210 oz of the boiler or approx 1% of the volume which is not replaced by a reservoir. I don't have and never have had any machines other than the Gaggia which I equipped with a Auber PID. I've observed the BOILER temp varying approx 10 deg in every shot. I'm still contemplating whether I want a HX or DB for the upgrade. If my thought has any validity I should be choosing the largest boiler I can afford in the interest repeatability of the temp during the shot and between shots. I haven't had any experience with the purely commercial machines, i.e. LM, Slayer, synesso etc.so I don;t know what size the boilers in those machines are.

Another variable not accounted for is the variation in the refill water. A reservoir machine would have replacement water at room temp (~70 deg) having been in the tank for a long enough time for the water to warm up to room temp versus water from the plumbing system which in the winter will be real cold (40F to 50F) and in the summer could be >60 deg. IIRC the water temp in the boiler will change at a rate directly proportional to the difference in the the two bodies of water.

Lusso has a 3 liter boiler, still huge. The way it works, however, is by using mass in the group to cool water to brew temperatures. Given time ( a couple minutes) to reset, it will always cool to the appropriate range. Boiler temperature is adjusted by pressure, which is constant despite water levels.

Your logic is reasonable (especially for HX machines), but you are not the first person to think of this. Modern double boilers tend to refill the brew boiler with hot water (via HX through the steam boiler) to minimize the temperature impact. The brew boiler is often much smaller than the steam boiler (witness the new Linea Mini: 175ml), and recovers quickly from pulling a shot. Some machines (Breville DB) even have a grouphead heater, to further stabilize brew temperatures.

So... does boiler size matter? Yes, but it's not as simple as "bigger = better".

RapidCoffee wrote: Modern double boilers tend to refill the brew boiler with hot water (via HX through the steam boiler) to minimize the temperature impact.

This is common on the high end DBs but do any of the Prosumer DBs offer this feature?

Breville DB: http://www.brevilleusa.com/dual-boiler-espresso.html

Ok, there's one if you consider the BDB prosumer. What about all the others in the $1800-$2600 range?

The VBM DB uses had a pre-heating HX:

http://www.espressocare.com/equipment/i ... bed-in-tan

Thanks, so that's two but the VBM also allows you to turn off the steam boiler which would eliminate the preheat function.

...so don't turn the steam boiler off!

Besides, since the Vibiemme DB regulates power between both boilers as needed ("power-sharing" vs. one boiler on and the other off as other brands of DBs in this class of machine do) -- mainly so it can use 110v/15A as efficiently as possible (no need for a 20A circuit) -- it's best to leave both boilers ON anyway.

-- BR

FirstBetta wrote:I've observed the BOILER temp varying approx 10 deg in every shot.

The rather warm grouphead manages to take care of that quite easily.

On some of your other statements:

The temperature of the reservoir water in any "cold" machine starts off at room temperature but can easily approach 90-100 degrees F if the machine has been on for several hours. The temperature of the water feed in a plumbed in machine will be essentially slightly above room temperature as there is a whole lot of plumbing between source and machine and some capacity within the machine itself. It can get complicated but this should not be a determining factor in any choice one makes.

User serviceability, retailer responsiveness, parts availability are some factors I consider important.