Breville Bambino Temperature Stability (Breville Buyers Be Aware)

There are lots of conflicting reports on the internet about the Breville Bambino. Some say it runs too cold, many reference a Lance Hedrick video and say it easily gets too hot, and everyone else says it is the obvious choice for any beginner setup. Who is right? After lots of testing and considerations for the engineering choices of the machine, I believe everyone is correct and I will explain why. I invite this to be a discussion more than a declaration of fact. This is simply my opinion as someone with a mechanical engineering degree and about a year of home espresso experience on the Breville Bambino.

First is understanding how the Breville Bambino works. I will be referring to Tech Dregs’ fantastic teardown that I have linked below and the Breville owner’s manual. The bambino and many other Breville machines use a “ThermoJet” heating system to heat a small volume of water between two metal plates “in 3 seconds”. On the outlet of this flat, spiral shaped, continuous heater is a thermocouple/thermistor (I believe it is likely a thermocouple, but the teardown calls it a thermistor. It's not really relevant to this discussion). The important thing to note here is that this sensor measures the temperature of the water leaving the HEATING ELEMENT.  Once it goes past the temperature sensor it runs through plastic tubing to a solenoid-controlled valve (more plastic) that directs the water to either the steam wand or the group head. Not once does the machine measure the temperature at the group head. This temperature sensor then informs a PID controller which controls the heat released by the heater. The PID is NOT attached to a boiler. This allows for near instant ready time but means there is not a large thermal mass of water or metal keeping the heating element and water within it at a constant temperature. The only thing providing any kind of temperature stability is the PID controller. This is the PIDs only job: temperature stability in the heating element. As I have mentioned, it is only concerned with the current temperature at the heater outlet, which is before even the solenoid. This allows the heater to provide a constant stream of water at a constant temperature.

Test #1) does the PID work and will the Breville Bambino provide water at a constant temperature?

Yes, quite well actually. This can be tested by using the steam wand to produce boiling water. After about 10 seconds the water made its way out of the steam wand and the temperature stabilized at about 95C +/- 0.2 C and continued at that temperature for the remainder of the minute before the machine filled my mug and timed-out. This means that the water took a few seconds to make it through all of the internal hoses and to heat all of its pathway to 95C. Once at steady-state, the machine was able to maintain its planned temperature. 95C is higher than the advertised 93C, but I am not a Breville engineer and maybe they put in a slightly higher set point for the steam wand. Regardless, the PID works, and the Bambino is able to produce constant temperature water.

Test #2) Is my puck getting the promised 93C consistently? A much more complicated question. In short, not really, but to fully answer this we need to understand a few things.

First, Breville (and everyone else) recommends running an empty shot first. The results of the last test demonstrate this is a REQUIREMENT. Ignoring the fact that the tubes and group head are cold, you need to get warm water to shower screen of the group head, or the first few seconds of your shot will be just like the first few seconds of the previous test (no water or cold leftover water). This also means anyone here on reddit suggesting that you should run the steam wand first to get the heater “warmed up” then pull your shot is ignoring this priming effect and how the heater works. The heater does not need to “warm up”, the group head and portafilter do. By design the heater gets to temperature near instantly.

Second, most if not all of the prosumer machines have a way to pre-heat the group head and keep it hot. Whether that’s with a thermosiphon in the e61 group head or a PID in something like the Bezzera BZ10. These machines are able to do this because they have large amounts of metal in the group head that can act like a giant thermal battery and buffer. That thermal battery takes time to charge. So, if your expensive machine uses a boiler, you are waiting the 20ish minutes for the water to come to temp, but you’re also waiting for the group head. This thermal mass at the group head allows for the temperature stability of these machines. Breville designed the Bambino and Bambino plus to be affordable, so they did not include all of the expensive metal or the heating elements at the group head. The bambino depends on the heat of the water destined for your coffee to warm the group head. Now take a look at the teardown video. You’ll notice that the bambino is basically all plastic after the heater until the group head. Tech Dregs discusses this at the end of his video too, but this all-plastic design is a feature. It was done on purpose.  Not only to make the machine more affordable, but it allows the Bambino to achieve the advertised 3 second warm up time. You cannot have 3 second ready time and the thermal stability at the group head of a prosumer machine. Adding all of that metal would mean a 10-20 minute warm up period (hence why a $4000 machine cannot get around the warming period and give you 3 second ready time). A machine can have temperature stability but temp stability at the heater and at the group head are NOT the same thing. Many people are spending thousands to get around this.

So, with the Breville, what can we do to get temp stability, not just at the heater, but at the group head? Pull an empty shot to heat it up like Breville says. They designed the machine to work this way, that’s why they suggest it. If you want to get that tiny piece of metal in the group head as hot as possible follow the advice of u/rmanalan (their post linked below). They recommend using the double walled pressurized basket for the empty shot. Run the empty shot using manual mode and do not cancel the shot. This will allow the machine to run for 1 minute. Repeating their tests on my own I can confirm their results, after a minute it gets very hot. (Note when they test with the steam wand that is for heating water in a mug from cold and not measuring the output of the steam wand like I did in Test #1).

But what does my testing say? First the parameters, I am placing the thermocouple inside of the stock portafilter but outside of the basket. Without a pressurized basket, my machine produces water from the group head at about 65-70C very consistently (both with the portafilter and without it just holding the thermocouple to the water and sacrificing my hands). I believe Breville knows the machine is doing a casual warm and rinse. It does not need to waste energy on going to 93C because there’s no coffee to brew. With a pressurized basket, it is a very different story. Running u/rmanalan’ s minute test I get a range of temperatures. At the beginning of the shot I am still on the cold side starting at 70C (running the test starting with a cold machine), but by the end of the minute I agree with Lance Hedrick, I am over 93C in the 95C+ area. This means everyone on reddit feeling like the bambino is cold and under extracting their shots is right, but so is everyone that says the bambino gets too hot. The water in the portafilter starts out too cold and gets too hot. If you are pulling very long shots, then you will start to over-extract at the end. This means if you pull a shot without priming the system at all then you are in trouble. At the beginning of the shot when you’re expecting to get the sour part of the espresso, the temperature will be extra low, extra under-extracted, and extra sour. By the late end (if you are pulling a 50 second shot) then you are too hot and over extracting the part of the shot that is already prone to be bitter. This is the worst of both worlds. But what happens in the middle? In the middle the bambino is able to hold a temperature in the mid to low 80C range at the outside of the basket. Pre-heating the group head and then immediately pulling your shot allows this 80s range from the beginning and to be held throughout the ~30 second range where most wisdom says to pull your shot. So yes, Lance is technically right, but functionally you are running colder than expected and the bambino is not a machine that runs hot (particularly for the 1-2 shots the machine is designed to pull before it is turned off). Before you run to the comments about my testing method, I will concede that I did not test the temperature inside the basket or with a puck in. This is going to affect the results, so do not take my results to mean that you are brewing in the low 80s. I am here to understand how the machine works and analyze the trends so we can better understand how to use it, not measure the exact brew temp. I’ll leave that to someone else. I expect the machine, in actual use while I am not measuring, is operating in the high 80s. An area that has the potential to make acceptable dark roasts.

My theory for why this is all happening: I’ve alluded to it, but I believe the answer is thermal mass. There is a reason that Breville uses nearly all plastic internals, provides a lightweight aluminum portafilter, and recommends an empty shot. It all has low thermal mass, and the empty shot will quickly ~warm~ everything. The plastic tubing and half plastic portafilter will no longer be cold and a beginner can quickly pull a half decent shot through it. Breville is banking on the fact that nothing will be in contact with the water for long, so if they are fast enough then the constant temp water leaving the heating element will result in an extraction where the temperature is stable throughout. (in the high 80s/low 90s). This is PERFECT for the average joe who just wants to stop drinking Keurig coffee. He’s already drowning out any minor variance in taste with milk and/or sugar, only pulling one or two shots, not buying any extra equipment, and doesn’t know what r/espresso is. He doesn’t need anything more than this $300 machine can do and it does exactly what he wants for a great price that doesn’t gatekeep espresso. That is who Breville engineered this machine for. Unfortunately for me, I have the taste and perfectionist behavior that craves a $4000 machine, but my wallet disagrees, so as a beginner I bought one too. So, what is happening to me and everyone else upset with the Bambino online? I bought a stainless steel bottomless portafilter (larger thermal mass), a WDT tool, an IMS basket, and started drinking my espresso without any milk. Now I am in a tough spot where I feel unhappy with the machine because I am trying to act like buying fancy accessories changes the way my machine is designed to brew. It is not a high-end machine. So, I will be brewing with dark roasts and using the machine for what it was designed for until I have the savings to invest more.

For a more technical theory about the temperature curve please indulge me a little longer. The thermal mass of the group head is nearly all concentrated in the portafilter. There is very little metal in the actual group head of the machine. This means that if you want the best temperature stability in the group head while using a Breville “ThermoJet” machine, you need to warm both the shower screen and the portafilter. If you want to do a complicated maneuver with a minute, a pressurized basket, and 10-12oz of expended water every time then be my guest. It will get the group head as hot as possible. The issue is that while you are changing the basket and preparing your puck, the tiny piece of metal (shower screen) in the machine is cooling down FAST. Also, all of that heat energy that you just put into the pressurized basket is now sitting on your counter. All of the heat energy you put into the portafilter also just flooded into the new basket (through thermal conduction. It’s metal on metal) and now both are a little warm but mostly cold. With the Breville machines, your only friend is speed. Speed getting to temp, speed warming the portafilter, speed in your puck prep, and speed in pulling the shot. That said, and as I’ve mentioned before, an empty shot is STILL needed. Don’t let the system hold stale water from yesterday and don’t needlessly let anything be room temperature. Any warming that can be done before you pull your shot is heat that is not coming out of your brew water during the shot and is letting your extraction happen closer to the planned 93C that is coming from the heating element. Now that you’ve warmed everything to the best this machine can, let’s follow what happens. The water gets to 93C in the heating element, gets pumped out, and hits your group head, coffee, and portafilter. All 3 act like a heat sponge, and they pull heat from the water lowering its temperature (but hopefully not too much because you pulled the empty shot). As the shot goes on, this continues and the heating element and PID do not care. They are sending constant temperature (93C) water out (constant heat). Think of the water as a constant allocation of energy per second from the heater to the group head. Once the water (and energy) gets to the group head, the water gives energy away into the colder elements (group head, coffee, portafilter), until they are at the same temperature. This means the water has been cooling down on its way from the heater to your cup and is no longer 93C. While that change has been happening over a distance, there has been another change happening over time in a single location. The water in the portafilter has been getting warmer. As the group head gets warmer the amount of energy that leaves the water every second to go to warming the group head decreases. This means more heat energy the water gets to keep and the higher its temperature. This is seen in the steady but slow climb in water temperature in everyone’s experiments (the climb is slowed by the thermal mass of the everything the water is warming). At approximately one minute though, everything is the same temperature. I confirmed this by using electrical tape to connect the wall of the basket to my thermocouple. This is where you wanted to start but unfortunately your espresso finished pulling 30 seconds ago. But once this happens the water in the group head is finally hot and 93C and there is thermal feedback to the PID. The water is no longer cooling down as it goes to your cup, it is staying constant until it leaves the portafilter. This means that the heater is producing too much heat, so the PID has to respond and stop heating the water so much or the temp will climb. Unfortunately, the PID cannot cool anything, just stop adding heat, so everything starts getting very warm at the end of the minute test. This is what Lance notices.  Now the thermal mass of all of the water in the tubes and the group head are working against the machine to keep it overly hot. You have run the machine for too long. Breville did not design their machines for this!

At the end of the day, everyone is right. Breville designed this machine for beginners. Beginner habits will play well with it. If you try and use it like an enthusiast machine you will notice it runs a little cold and struggles with medium roasts and I can only imagine also light roasts. But if you are a coffee youtuber or someone like me then you will notice it gets overly hot when you run it continuously for longer than it was designed (no coincidence it auto-shuts off at 1 min!). Please enjoy this machine for what it is!

As a side note, I don’t own a Gaggia Classic Pro, but I imagine that it suffers from similar thermal mass issues regardless of if you PID mod it or Gagguino it. Compared to a stock Gaggia I think the Breville bambino is better especially at $300. With the PID, I think the GCP is VERY similar to the bambino because the PID is still only on the heating element. The thing to note with a PID GCP is that it only has a 100ml boiler so if you run 70ml preheating things then you only have 30ml to make your drink which is not enough. At least the bambino is “bottomless” when it comes to producing hot water from the heater.

Teardown video (Tech Dregs): https://youtu.be/etxYC9AlBXM

u/rmanalan’s experiments: https://www.reddit.com/r/espresso/comments/ivx60s/more_temperature_experiments_with_the_bambino_if/

Lance Hedrick’s Video: https://www.youtube.com/watch?v=U2TNEhrBU5Q&t=655s

If you’re not tired of reading what I have to say, here’s my story of how I got here. I am new to making espresso and like many people on this subreddit I bought a Breville bambino as my first machine. For a long time, I was blaming my puck prep, my grinder (a fellow opus), and my portafilter. I could make shots that tasted okay, but I never made something that I was confident wasn’t sour. Pouring 8-10oz of steamed milk on top made a latte that was better than any drip coffee I’d ever made so I was happy. Unfortunately, I am on r/espresso and I have a girlfriend who lived in Italy for a year so sour espresso wasn’t acceptable for long. After a couple of weeks drinking sour shots that I had dialed in to the best of my ability, I decided to go to a local roaster and order a real double shot espresso. I noticed two things: first is the shot was delicious and I am not crazy for doubting my espresso and the second was that it was HOT. Suddenly I realized my shots were all sour because I was brewing too cold. I went down a reddit rabbit hole, ordered a set of thermocouples, and began testing.

TLDR: My opinion is that the bambino is a great beginner machine, especially for its price. Just use dark roasts, follow the instructions and run an empty shot immediately into your portafilter and basket before pulling your real shot, and don’t expect it to perform like a $2000+ PID machine with an e61 group head just because the letters PID are in the marketing material. If you do something different and love your bambino, keep doing it!