Single boiler PID espresso machine (ECM Classika) vs HX

#1: Post by **Mdl55** » October 28th, 2021, 4:36 am

I've been looking for an upgrade to an old Espresso Gaggia that I picked up on the cheap a few years ago and threw a bit of money at to get up and running. I've done a fair amount of research (including on this site) and had landed on the ECM Classika PID, as I almost exclusively drink espresso and long blacks and rarely need to steam milk. I was attracted to the supposed temperature stability of a single boiler machine with a PID. (BTW I have a Mazzer Mini grinder.)

So I got in touch with a reputable local dealer and asked if they could import an ECM Classika as nobody seems to stock them here in NZ. The dealer made a number of comments about single boiler PID machines that seemed inconsistent with the research I had done, and now I'm starting to question whether a single boiler PID machine is in fact the right machine for me. I'm interested in the community's views on the following points made by the dealer:

1. "if it is a single boiler machine, the general opinion is that there is not necessarily better temperature control (the main problem with single boiler is the immediate introduction of cold water into the mix) with PID, but more a monitor of what is happening with temperature... The PID is basically a feedback of the poor temperature stability of single boiler machines."

Is this right? My understanding was that a PID provided better temperature control and was not merely an indicator of temperature.

2. "This is why single boiler machines are not suitable for multiple coffees (temperature plummets once you start to run a shot, regardless of control system)."

I understand single boiler machines are not quite as good as HX machines at producing multiple espressos, but is it true that they are "unsuitable" for multiple coffees?

3. "If you want hot water added to your espresso this is also a problem with single boiler machines."

It seems that a single boiler machine will be less convenient for adding hot water if you've just raised the boiler temp to steam milk, but shouldn't be too much of a problem if you've just pulled a shot? The Classika does not have a dedicated hot water outlet but I've read that the steam wand can be used with the pump at brew temperature to produce hot water.

#2: Post by **DeGaulle** » October 28th, 2021, 6:20 am

The benefit of a PID on a single boiler such as a Classika is that it responds by modulating the heating element duty as soon as there is a measurable drop in temperature, as opposed to a thermostat that responds full-force only when the temperature has dropped by say 10 degrees C or more. That doesn't mean that the PID will keep the boiler at a flat temperature profile throughout the extraction, but a sizeable boiler volume and the thermal mass of the E61 group will certainly help.

As for making multiple coffees in succession:
I think a HX will hold more thermal buffer because of the latent heat content of the steam volume, so the potential for a faster recovery is there. Also the temperature gradient from boiler to group-head is higher, so there is more driving force for thermosyphon water circulation. That said, E61-based HXs are also known for the need to recover after a number of shots in succession.

A HX machine with dedicated group heating, such as the Bezzera BZs or the Xenia are not depending on aTS water circulation and recover very quickly; in fact, the BZ group tends to heat up from mutiple shots rather than decline thermally.

Edit on the hot water tap:
If you pull hot water from the steam valve on a single boiler machine, it has to be topped up with fresh water as with a regular extraction, so the hot water will see the same potential temperature decline that the PID is trying to counter. For an americano this is just additional brew water being added after the fact, I don't see the problem with that.
When you draw hot water from the boiler of a HX machine, it is flash boiling vigorously, so it always ends up in your cup at 100 deg C with the steam pressure in the boiler as a margin. To make tea that would be my preference, so I am sure that the water has boiled.

#3: Post by **Jeff** » October 28th, 2021, 11:51 am

At least as I read that dealer's responses, they seem to be more focused on selling up-market machines than providing accurate information.

On (1), the introduction of cold water into the boiler happens on every machine. The amount of temperature drop is related to the volume of the boiler, volume of a "shot", and incoming temperature. For example, take a tiny boiler, 250 mL, and say 50 mL for a shot, you've got roughly 200 mL at 95°C (brew temp) and 50 mL at 30°C (reservoir temp), so the mix is somewhere around 82°C. The greater drop with a small boiler is offset by the ability to heat the smaller volume of water quickly with the heating element. Well designed, either "micro boilers" or large boilers can work well. Remember, a dual boiler is a single-boiler machine with a second boiler for steam.

(2) Flat out untrue. The heat stored in a well-designed group mitigate the drop in temperature before it gets to the coffee. Not to mention, there's some thought that a declining temperature profile may be a good thing. (See also the notes on recovery time above.)

(3) I've got to chalk this one up to, "Let me give them all the reasons not to buy." Water from an HX tap is usually somewhere between nasty and disgusting, due to mineral concentration from boiling the steam. There are other reasons to prefer options other than an HX, but this certainly isn't one that is in an HX's favor. A kettle, electric or otherwise, works just fine, and is a lot less expensive than the price difference between a good SB and a comparably good DB.

I think a good, PID-controlled SB unit is a good choice for those on a budget, or those that want a pump machine and don't need steam.

I think a new HX, with or without PID-controlled steam-boiler temp, is more challenging to learn on (you have to learn to manage the brew temperature, even with a PID), and that flush routine becomes part of every shot. I prefer something with direct control of the brew/group temperature, rather than the PID controlling the steam-boiler temperature*. Buying new, I think that there are better choices than HX machines for most users.

* Standard notable exception disclaimer for the MaraX, an HX that controls the brew-head temperature.

#4: Post by **cafeIKE** » October 28th, 2021, 12:24 pm

Gee, I guess I've been dreaming these past 13 years with my Vibiemme DB PID.

The secrets to continuous back to back shots:

heating element style and location: The element must be of sufficient water and design to enable rapid equilibrium restoration
cold water injection location: The cold water must mix well with the boiler so as to mix thoroughly and not striate the boiler
PID tuning: Always a compromise and frequently flat out wrong as delivered. With the correct tuning, on the VBM DB it's possible to pull shots at 40s intervals with about 0.5°C temperature droop over 5 shots.
Hot water: I only engage the steam boiler for milk drinks. For Americano, the microwave suffices nicely.

ECM Classika II PID - In Depth Review goes over the hardware, but nothing on the shot stability or repeatability.

#5: Post by K7 » October 28th, 2021, 2:31 pm

Temp drop has nothing to do with single vs dual boilers. What matters is the size of the brew boiler. Many dual boilers being higher priced machines do tend to come with large brew boiler but not always, e.g. Profitec Pro 300 with 330ml. ECM Classika has a 750ml brew boiler which is quite large for a home machine -->should be plenty good. Even Lelit Anna 2 PID (PL41TEM) with 250ml boiler has no trouble . Important: don't confuse the PID temp reading drop vs actual brew temp as seen in the video. IIRC its rebound time is also pretty quick so it's ready to go by the time you grind and prep the next puck. But I suppose the ECM would rebound even faster because its temp wouldn't drop as much.

If I rarely make milk drinks, dual boiler would actually be a huge con--more things to go wrong/maintain, worse form factor, wasted energy/money etc.

#6: Post by **Jeff** » October 28th, 2021, 3:27 pm

Hmmm, just how fast can a 1000 W heater heat water?

Let's see if I can keep my units straight.

29°C water in the reservoir, 99°C water in the brew boiler, 70°C temperature rise

50 g of water, 70°C temperature rise in liquid form, 3500 cal * 4.19 Joules/cal = 14,665 J

1 Joule is 1 Watt-second for about 15,000 W-sec

So a 1,000 W element, with no other losses, can heat the water for a shot in about 15 seconds, faster than it takes the shot to flow.

I suspect this temperature-drop thing is a "non-issue" with either a conventional thermostat or a properly tuned PID, no matter the boiler size.

#7: Post by **cafeIKE** » October 28th, 2021, 4:42 pm

Ya gots ta figur da losses...

I did that about 15 years ago and IIRC figured 600w for a 2 minute brew cycle with time to spare.

#8: Post by **jgood** » October 28th, 2021, 5:10 pm

The disadvantage of a single boiler is that it's a bit of a production to pull a shot and steam milk for a cap. IMHO if you are just pulling shots for espresso there is no downside -- if you had a double boiler you wouldn't be using the steam boiler anyway. In terms of hot water for Americanos or tea, I think it's simpler to just use an electric kettle. As mentioned a HX puts you in the temp management business.

#9: Post by **CamBam_HB** » October 28th, 2021, 9:35 pm

I'll post an update tomorrow on recovery time from my Classika PID. Currently its set to 200 degrees on the PID and I usually pull 3 shots fairly quickly, so recovery time must be relatively fast.

As a sidenote, i'm surprised that no one has thought of either prewarming the reservoir tank or water line to reduce the gap. Perhaps it isn't as big of a deal in modern machines.

#10: Post by **cafeIKE** » October 29th, 2021, 10:00 am

The reservoir gets plenty warm in a machine that is on all day.

The higher the water temperature, the higher the evaporation rate and the higher the mineralization increase.