Here's a thought that many of you will already know, but some may not, so bear with me.
The most common design of domestic espresso machine uses an electric pump to shift cold water from the reservoir through a pipe that runs through the inside of the boiler to heat the water to the correct temperature for espresso. Unsurprisingly this design is known as a heat exchanger, often expressed as simply 'HX'. When a machine of this design is on but not being used the water in the HX quickly rises to the temperature of the water in the surrounding boiler that the HX pipe is running through, which is too hot for espresso. For this reason a cooling flush of 5 or 6 seconds is required if the machine has been standing idle for more than a minute or so. If the pump on these machines is run for too long the HX will struggle to heat the water fast enough and at some point it will become too cold for correct espresso preparation.
The simple lever espresso machines that Londinium Espresso offer differ not just in the absence of an electric pump to drive the water through the coffee, but also in the way in which they regulate the temperature of the water prior to it making contact with the coffee.
Both the Bosco machines and the Olympia Cremina use what is known as a dipper design. That simply means that when you move the lever, raising the piston in the group, which allows water to exit the boiler as a result of the steam pressure in the head space at the top of the boiler forcing down on the water in the boiler and pushing it out through the open port. Here lies the fundamental difference between a HX design and a dipper.
As outlined above, HX machines sit too hot at idle, yet run cooler and cooler if they are used continuously. In contrast, dipper designs sit too cool at idle yet run hotter and hotter if you draw enough water through the group to exhaust the thermal capacity of the group. So it can be seen that both designs have their limitations.
The extent to which the simple dipper design is successful in delivering water at the correct temperature to the ground coffee depends on the ability of the group to bring the water as it exits the boiler at well over 100C down to say 92-94C by the time it makes contact with the coffee. To achieve this you want the group made from a material with high thermal conductivity (chrome plated brass) and lots of it (to create a heat sink). It also means the first shot (on a small group like the cremina) or the first two shots (on a monster full commercial group like the Bosco) will be too cold to produce optimal espresso if the group has not been used for a while.
Testament to the quality of the design of the small group on the Olympia Cremina is that you can pull six double shots before the group's thermal capacity is reached and the water becomes too hot to make optimal espresso. On the Bosco the significantly greater mass of the group means it is difficult to saturate the thermal capacity of the group, assuming you have the boiler pressure set correctly (1.2 bar). The length of time it takes for the shot to be extracted, to move the cups away, remove the portafilter, knock out, wash, wipe, reload, tamp, and reattach to the group is sufficient for the group to recover its thermal capacity, ensuring the group does not deliver water that is too hot to make optimal espresso.
The important point is that under no circumstances should you try and bring the group of the dipper design up to temperature after it has been sitting idle by embarking on what I call 'open port flushing'. That is to say, by simply moving the lever on the group and allowing water to gush from the boiler unrestricted. This will almost instantly exhaust the thermal capacity of the group as a result of the high flow rate that occurs through the group when no coffee is in the portafilter to regulate the flow. Rapid flushing of water at boiler temperature through the group not only over heats the metal immediately surrounding the bore in the group but also leaves cold spots in the group.
This is how bad experiences with dipper design machines start; open port flushing rituals that are appropriate for HX designs but ruinous on dippers. Please desist if you have a dipper machine and astound yourself with the improvement that arises in your espresso.The correct way to bring the group in a dipper design is to use the stale coffee that is sitting in the internals of your grinder (a double shot dose). It will have dried out a bit and the water exiting the group will be cooler than it needs to be so expect it to gush through a bit faster than it should.
On domestic grinders and groups the next dose of coffee out of your grinder will be fresh and the water will be hitting your coffee at the correct temperature so the second one should be good to drink. This approach not only ensures the bore is at the correct operating temperature but that the entire group has had a chance to slowly heat through evenly, eliminating any cold spots in the group.If you are running a commercial setup with say a mazzer robur and Bosco then the robur will need to serve two doubles before it clears the stale grinds and the big Bosco groups will need two double shots to get up to temperature, so they pair up rather well.
So if you are fortunate enough to be enjoying the design purity of a dipper espresso machine (which includes for example the ubiquitous La Pavoni lever machines) be sure to quit open port flushing. Try it, and taste the difference.
If you over heat the group on a dipper machine just leave it to rest for 15 minutes and come back to it, but if you follow the procedure outlined above it should never occur at startup.
It is my firm view that open port flushing on dipper designs is more to blame for the horror stories you hear of people struggling to get a good shot from them than effects of all other poor techniques combined.