I've often read about how it's great to have a copper or brass boiler versus a stainless boiler because the copper and brass will heat up faster.

But isn't this backward--literally outside-in versus inside-out thinking?

Copper conducts heat much faster than any other metal, save silver. And so, for example, in cookware, copper takes the energy from a heat source and, far more quickly than stainless, transfers it to the food (or water) inside the pan. Notably, some copper cookware even has stainless steel handles--specifically because stainless conducts heat so much more slowly (and thus stays cooler for longer).

But, inside a boiler, we want the opposite. With a heating element inside the boiler, we want the water to heat, not the metal walls of the boiler, which will then conduct the heat of the water out into the air. Indeed, the better the boiler's metal resists transferring heat from the water to the outside air, the better. So, brass and especially copper, are the last things we want surrounding the hot water in our boilers.

This struck me while reading a Clive post about two machines, the Lucca M58 with brass boilers, and the ECM Synchronika with stainless. They state, in part, concerning the brass boiler, "This means it's a better conductor and warms up faster than stainless steel..."

I'm thinking that's exactly what I do not want. Instead, I want the water to heat and stay hot, helped by the walls of a boiler that are TERRIBLE at absorbing and then conducting heat to the outside.

After all, if we think it's wise to insulate the boiler, we think it's good to slowly conduct heat to the outside.

And for that, isn't stainless is king?

Of course I'm leaving out a discussion of the thickness of the metals involved. But thick copper isn't appreciably slower at conducting heat; it just conducts is more evenly. Hence, my highly coveted 2.5mm thick copper pans, lined with tin, that are worth a small fortune. Whereas thick stainless will indeed be a very effective insulator for the water in a boiler.

What you are missing is that in a service boiler or a steam boiler, we want the steam to be hot; the water is there to make steam. Thus we want the inside surface of the boiler that is above the water level to get hot as quickly as possible.

Of course, a boiler and a machine can be engineered to match the materials used, e.g., La Marzocco GS. However, some (E-61) machines just used the same designs that were used for copper/brass boilers and made them in (304) stainless steel, a.k.a., machines with wet steam .

Perhaps more than one would ever want to know about the subject: https://coffeetechniciansguild.org/blog ... omplicated

That also explains why stainless steel boilers were insulated long before manufacturers started worrying about energy conservation ... and also why stainless steel boilers only became popular when they became less expensive to make than copper or brass boilers, mostly due to increase in price of copper and the introduction of limitations on the amount of lead allowed in brass. (IIRC, the latest reduction in allowable lead in the EU came into effect January 2023 with mandatory compliance by Jan 2036.)

As an aside, I just picked up a small 1.5mm-thick copper saucepan lined with silver (Alex's) and was shocked at how heavy it was compared to my vintage Revere Ware (stainless steel with copper-coated bottom). I can only imagine how heavy 2.5mm would be.

The stainless boiler will heat up marginally faster, mainly because it doesn't conduct heat as well as copper. You may well want copper in the base of a pan, but it's not a superior material for conducting heat in a boiler. Copper is approximately 65% more expensive (counting cheaper brass end plates than the stainless in a boiler, for material costs, construction costs are probably similar).

Steel has some nice properties compared to copper and in general I have found stainless boiler to be lasting quite well since their introduction some 15 or so years ago...whereas copper boilers can show pin holing and leaks at seams before that. The history of stainless boilers in espresso machines is an interesting one and it wasn't just done to save money, even though it did and not for the reasons you might think. Many espresso machine manufacturers (assemblers if you like), don't really manufacture any more. The capacity for heavy engineering has gradually shifted to the metal foundries. Laser cutting, boiler manufacturer, polishing etching etc.. are have moved away from the more traditional factories. A few like ACS do still manufacture (Quickmill make their own cases), but most, including ACS, outsource their boiler manufacturer because it's more cost effective and quality can be better.

So a stainless boiler has some big advantages for the consumer over a bespoke copper one.

You can buy one and plug any extra holes
the boilers become more generic
All the fittings are on the end plates (as it's tricky to fix into the side of stainless)
corrosion isn't much of an issue with 316L stainless
It's a good boiler material.

Cheaper doesn't always mean worse, more expensive doesn't always mean good!

As for boiler insulation, that started originally with Olympia and some of the early manufacturers, mainly on some commercial machines and on the Creminas using asbestos (probably not the best choice). In the main though manufacturers wanted the excess heat to warm the cups above...this hangover dogged the prosumer market, and machines routinely had non insulated boilers...with the exception of Expobar machines, which had a token bit of rockwool wrapped around them. The main reason for this was the proxmity of the steam boiler to the brew boiler in machines like the Brewtus and an attempt to protect the fragile electronics and pressurestats of the Brewtus and the Expobar Pulser (which ran really hot).

I saw this insulation and figured it was a good thing, I included it in my top level specification to Izzo for the Duetto. The immediate response was negative, because of warming the cups. I said electronics would last longer and energy would be saved, plus the cups would still warm up enough I also wanted the steam boiler to be able to be turned off and got an argument about that as well!

The finally included it and for PID/temp control reasons back in the day, I purposely didn't specify it for the brew boiler...control systems weren't so good back then. In every machine I have been involved in over the years, I have insisted on insulated boilers, as more manufacturers did it, it became ubiquitous. Plus lots of manufacturers copied the Duetto design in terms of brew boiler sizing (yes, I gave them that as well 800ml), insulation, steam boiler off/on etc..I suspect now most manufacturers will routinely do it.....

So that's how it all came about on Prosumer machines.

P.S. A confession, I based my brew boiler sizing and ideal thermosyphon pipe positioning, based on what I found on the Isomac Zaffiro. Had the Zaffiro had a PID back in the day and some insulation, it would have been really good..

baldheadracing wrote:As an aside, I just picked up a small 1.5mm-thick copper saucepan lined with silver (Alex's) and was shocked at how heavy it was compared to my vintage Revere Ware (stainless steel with copper-coated bottom). I can only imagine how heavy 2.5mm would be.

omg lucky you, when I saw that it was already sold out..... Definitely interested to hear your opinion on that pan

Think about the refilling of the boiler as you use water and it will answer most of the heat transfer questions as it applies to a large capacity machine. You aren't as worried about speed as you are, temperature stability (thermal mass). A cafe wants to heat up the boiler and produce drinks without "running out of steam" . Smaller machines where quick heatup/low volume usage are concerned; different question and answer. Longevity is another question that can change decision on boiler material.