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.
In recent years air-pump-driven espresso machines have emerged. These machines use compressed air to force the hot water through the coffee grounds. The hot water is typically added from a kettle or a thermo flask. The compressed air comes from either a hand-pump, N2 or CO2 cartridges or an electric compressor. One of the advantages of the air-pump-driven machines is that they are much smaller and lighter than electric machines. They are often handheld and portable. The first air-pump-driven machine was the AeroPress, which was invented by Alan Adler, an American inventor, and introduced in 2005. Handpresso Wild, invented by Nielsen Innovation SARL, a French innovation house, was introduced in 2007.
Why it's great: As much as we appreciate freshly ground coffee beans for our espresso, let's be honest: when you *accidentally* snooze your alarm (five times) in the morning and don't even have time to brush your hair, chances are the last think you're gonna think about doing is grinding some freakin' beans. This could be a problem if you're someone (like me) who can't function without a strong dose of caffeine in the morning. However, not a problem with Nespresso. This life-saver heats up in less than 15 seconds and gets you tasty espresso at the push of a button. You get your quick fix without sacrificing quality. 
All of the espresso machines we tested only included a single boiler for heating. This means that there's only one mechanism for heating both the water for brewing and the steamer wand. Because of this, it'll take the steamer wand a while to heat up after pulling the espresso shot, since the steamer wand operates at a much higher temperature than the brewing temperature of around 195°F. It is possible to buy a home machine with a double boiler (Breville makes one) but this raises the price significantly (we didn't test the Breville because it's over $1,000), and the minor wait time wasn't a major concern for us. We figured it wouldn't be for home brewers either.
For many of us, the daily grind can't begin until we've had that first cup of piping hot coffee—so the right brewer is at the top of the list. The most basic coffee makers make at least a decent cup, but a little more money buys conveniences such as programmability, a thermal carafe to keep coffee hot longer, settings that let you adjust brew strength, and more.