Some machines use a single boiler kept at steaming temperature, but water for brewing is passed through a heat exchanger, taking some heat from the steam without rising to the same temperature. Although the water for brewing remains at a lower range than that required for steaming milk, it is still too hot for proper coffee extraction without first cooling; thus this type of machine requires a cooling flush of 4–6 seconds prior to the first espresso pull. Once the machine is dialed into the proper temperature, as many shots can be pulled as required without refreshing. However, if the user leaves the machine idle again for some period, the flushing process will need to be repeated. The HX variety is found in many mid-range machines and many users install thermometers to assist them is dialing in correct temperatures. There is some controversy as to the temperature stability of the brewing water, since it is indirectly converted from steaming temperature to brewing temperature, rather than kept at a brewing temperature.
The term dual boiler is used narrowly for machines with two separate boilers, and more broadly for what are more properly called dual heater (DH) machines, featuring a boiler for brewing and a separate thermoblock (TB) for heating brew water to steaming temperature – opposite to HX machines, where the boiler is at steaming temperature and is cooled to brewing temperature.
The very first espresso machines worked on a steam-pressure basis, and they’re still in use today. With this type of machine, steam or steam pressure is used to force water through the coffee grounds and produce espresso. Some steam-driven machines can produce a measure of foam “crema.” But they can’t generate enough pressure or provide the precise temperature control necessary to produce true espresso: They simply make a very strong cup of coffee. However, they cost considerably less than pump-driven machines. Our verdict is that if you’re a true espresso lover and seeking to make a good shot at home, we recommend you steer clear of steam-driven machines. They’ll likely disappoint you.
Try a new way to brew coffee if you’re tired of the same-old, same-old. We offer a variety of ways to brew up tasty hot beverages to fill your favorite coffee cup. Choose from standard drip machines with high volume carafes or durable thermal carafes. Pour-over brewers come in manual and automatic versions to create fast, hot cups of java goodness. Automatic versions heat water to industry standards in mere seconds and allow plenty of contact between hot water and beans to develop the coffee essences you love. Manual pour-over brewers have everything you need but the hot water to make your own coffee wherever you are. Order a teapot or automatic kettle to make quick cups of coffee without waiting for a machine to brew it for you. Go retro with a siphon-style coffee maker. You never have to use the stove for the electric versions of siphon brewers. A flip of a switch starts the process. Hot water vapor pushes through the top of the appliance to soak the ground coffee beans, and then the brew slowly drips back down into the carafe for a perfect cup of coffee.