Do most single boiler machines have such small boilers when compared to HX machines so they can easily transition from brew temperature to steam temperature? If that is the case then why does the Silvano by Quick Mill only have a .7L boiler?

I would say, yes to be able to "quickly" switch between steam and brew temperatures, and probably to keep the cost down since most of those machines are geared towards lower budgets.

fredm wrote:Do most SBDU machines have such small boilers when compared to HX machines so they can easily transition from brew temperature to steam temperature?

A larger boiler may mean more stability, but as you noted, it also demands a higher-wattage heating element to reach the setpoint in a reasonable amount of time. Higher wattage relative to volume increases the likelihood of overshooting the setpoint, especially if the heating element's control is via a slower-acting thermostat versus a thermocouple + PID combination.

fredm wrote:If that is the case then why does the Silvano by Quick Mill only have a .7L boiler?

As a point of reference, the Quickmill Alexia brew boiler is only 0.8 liters and the La Marzocco Strada brew boilers are a mere 1.3 liters in volume. In terms of brew temperature, they're both super stable, demonstrating that bigger isn't always better.

I can tell you from just going through the exercise that it mostly come down to cost and space. The Silvano has a heat block for steam so it does not use the brew boiler for steam.

The Strada boiler is small because it is fed with preheated water from a pre-heater in the steam boiler. This way they can get away with a smaller boiler and less power (because the water is already heated to about 200 degrees F) .

BillC

HB wrote:As a point of reference, the Quickmill Alexia brew boiler is only 0.8 liters and the La Marzocco Strada brew boilers are a mere 1.3 liters in volume. In terms of brew temperature, they're both super stable, demonstrating that bigger isn't always better.

I am a newb and not familiar with those models but the .75L on the Alexia seems to be the largest size for the single boiler machines which fits its e61 group and $1200 price tag.

Billc wrote:The Silvano has a heat block for steam so it does not use the brew boiler for steam.

Which blows my theory about 'change of state' being a factor as it only has the job of brewing. I was just curious as it seems like the 500-1000 dollar machines could do well to increase the boiler size from their average .3L sizes.

You simply do not need a big boiler. Most single boiler machines are espresso only. You have for example a 0.7 liter boiler. If you pull a double shot you are running 0.06 liters of water through it. You simply don't need a big boiler.

If this is indicative of anything, Vibiemme has just reduced its Domobar Super DB brew boiler from 1.4L to 0.5L. They probably also address the overshooting issue between shots (associated with big element size compared to boiler size) by using a double heating elements setup.

La Spaziale also uses a small 0.8L brew boiler in its double boiler machine. The DallaCorte Mini (DB) even use a really small 0.45L brew boiler. As for the La Marzocco GS3, they use a quite large brew boiler but this may be required to keep the massive saturated group at a stable temperature.

Hence, it rather seems like a small brew boiler is a good/better solution.

In the case of SBDU, having a small boiler also provide a faster rise to steam temperature and faster temperature drop to get back to brew temperature.

The boiler only need to be big enough to provide adequate steaming power.

Steam generators for ironing like Rowenta DG 8760 has a large, beautiful (0,8 liters and 1350 watt ) stainless steel boiler with solenoid release valve and CEME 15 bars pump inside .
It heat steam after one minute - is faster than many espresso boilers .
I found mine refurbished at 50 euros . Can be very useful for upgrading your espresso machine .

That boiler is made from thin steel and would probably explode if pumped up to 9 bar.

No thin steel at all - is heavy - default pressure is 5-6 bar measured with gauge . Is more thicker and

resistant that stainless steel boiler used in Tschibo Cafissimo .

The steam exit tube cord is more fragile and it has no problem , or the ptfe tube between pump and boiler in a regular espresso machine is way more fragile than steel .

For increasing the resistance of ecuatorial weld this boler can be compresed between 2 circular metal plates fixed with radial screws .

If you are afraid , it can be used as steam boiler as is or for use with a steam heat exchanger .