I have read over the years and on various sites, people talking about the pros and cons of stainless vs copper. Unfortunately the area is rife with misunderstanding and misinformation. So here is my take on it, having machines with both types of boilers and seen the problems/issues over a 20 year period.
The statements below are not quite accurate or truthful but are widespread on the internet. I will quote some of the things you will read on the internet….where there is overlap I have made it a single point
- Best thermal conductor, good conductors of heat.
- Copper is highly malleable, ductile, and most importantly has unmatched electrical and thermal conductivity (unlike stainless which performs very poorly in this regard)
- Most expensive (pricier than aluminum, brass, and steel)
- Antimicrobial properties Stainless is thought by some to be more hygenic than copper, but that is not the case either
- Less resistant to corrosion than stainless steel, Copper boilers do not degrade quickly. Critically copper will not rust as stainless will
- Flexible and can endure mechanical stress, does not suffer from metal fatigue from the repeated heating and cooling cycle that a boiler is subject to
- Copper boilers hold steady temperatures well.
- Copper boilers are cheaper to produce than other boiler types.
- Copper boilers can produce excellent espresso if they are made and treated well.
- copper does not have any lead in it, it is not an alloy, it is a pure metal
Stainless steel AISI 316L specifically:
Stainless steel boilers last for a very long time.
Stainless steel boilers do not negatively affect the taste of espresso (no copper taste)
Stainless steel boilers are not damaged by descaling chemicals
Stainless steel boilers produce very clear espresso.
cheaper than copper and brass boiler construction
Now sadly, other properties of stainless, that are not always mentioned.
Very good when you want to undo a boiler fitting (this can be problematic on stubborn fittings in copper)
Most fittings now, are confined to endplates, which if thick 8+mm can be directly threaded (no brass braised fitting, which is prone to leaks). Sometimes a reducer stainless fitting is used on larger holes, done correctly, still super resistant to leaking.
A correctly made stainless boiler should outlast a copper boiler by decades. I’m talking about the stainless boilers with a proper thickness, not the bean can thin variety you might find in a Chinese machine and a few others from yesteryear.
More manufacturers understanding galvanic corrosion of brass, when wetted in contact with steel. So it’s not usually a problem (e.g. a a brass thread in steel with water in the thread and thread to steel contact), because of sealants or PTFE tape which prevents corrosion by both occupying the space and preventing metal to metal contact. It does mean an external O ring or fibre washer should never be used for sealing a brass fitting to copper. Any coating of fittings is very helpful.
Clearly more and more AISI 316 boilers are appearing and companies of yesteryear who may have used different grades for certain steam boilers, are hopefully moving to AISI 316L. …for chinese machines..who knows?
So lets go back to copper and dispel (explain) why some some myths persist if we can
- Best thermal conductor, good conductors of heat. - This might be important in a steam boiler (Casey Jones), or other types of boiler that are externally heated. Get that heat to the water quick. An espresso machine boiler has the heating element on the inside, good heat conduction is the last thing you want and the reason we insulate boilers!
- Copper is highly malleable, ductile, and most importantly has unmatched electrical and thermal conductivity (unlike stainless which performs very poorly in this regard) - Copper is indeed highly malleable and ductile as people have found out when removing stubborn threadlocked or siezed fittings and damaged their boiler. It doesn’t really benefit anything for it to be soft! Electrical conductivity is a non issue and a non benefit and the terrific thermal conductivity has been covered above.
- Most expensive (pricier than aluminum, brass, and steel) - This really isn’t a benefit, being expensive and a lot of that expense is in fabrication by brazing and brazing brass endplates to copper (a source of week and leak points)
- Antimicrobial properties Stainless is thought by some to be more hygenic than copper, but that is not the case either - It’s a boiler, it’s really hot, bad stuff that will make you ill dies.
- Less resistant to corrosion than stainless steel, Copper boilers do not degrade quickly. Critically copper will not rust as stainless will - Partly correct, less corrosion than some stainless steel, but not less corrosion than all stainless steels especially e.g. AISI 316L. Many times I have seen copper boilers from machines less than 20 years old with pinholing around the brazed joints and in the body of the boiler itself (usually seen as green dots).
- Flexible and can endure mechanical stress, does not suffer from metal fatigue from the repeated heating and cooling cycle that a boiler is subject to - Stainless steel AISI316 boilers have shown no tendency to fail due to metal fatigue caused by heating. In the La Marzocco An FAQ where the answer is always “Yes.”, there is the question are your boilers stainless steel. Perhaps La Marzocco don’t know what they are doing, but in my experience they are not choosing stainless steel (AISI 316L) because it’s cheaper, if Copper was better, they would use it!
- Copper boilers hold steady temperatures well. - Complete rubbish, a correctly programmed PID system holds steady temperatures well
- Copper boilers are cheaper to produce than other boiler types. - It probably ends up cost neutral for the smaller boilers, but copper boilers are definitely more expensive for larger boilers.
- Copper boilers can produce excellent espresso if they are made and treated well. - It’s really hard to get away from that copper taste until they scale up (which you don’t really want). Try cooled water from a non scaled copper boiler…urgh.
- copper does not have any lead in it, it is not an alloy, it is a pure metal - Absolutely correct, but if the brass endplates are not Zero lead (not low lead), then it will. You can tell if an endplate is Zero lead, is it silvery, not yellow! The statement about lead was made in relation benefits vs steel boilers, but last time I looked steel has no lead in it either. Of course 316L steel doesn’t leach anything into the water, whereas copper boilers leach….well copper into the water.
Nickel plated copper and brass
Pinholes in boiler
I hope this has helped dispel some of the well meaning, but incorrect statements about copper boilers..clearly retailers and manufacturers who use copper boilers will sit one side of the argument and that’s probably not my side. Stainless steel boilers used to be very hard to make, especially for the small boutique espresso machine companies. The corporatisation of espresso machine companies and increased buying power led to the rise of the AISI 316L espresso machine boilers and associated manufacturers. As the quantities increased they became cheaper so all espresso machine manufacturers, small and large, could now use them. IMO they are a major advance in boiler construction, making in life maintenance of the machine much easier and reducing worries about descaling, or low mineral water.