Some home pump espresso machines use a single chamber both to heat water to brewing temperature and to boil water for steaming milk. However, they can perform only one operation at a time, requiring a warm up period between the execution of espresso pull and the milk frothing process. Since the temperature for brewing is less than the temperature for creating steam the machine requires time to make the transition from one mode to the other. Moreover, after the brewing process, a single boiler will expel (usually minor) quantities of water through the steam wand that were left over from brewing, which can cause the steam heated milk to then have a slightly watered down taste. To avoid this, the leftover water needs to be collected from the steam wand before steaming of the milk should begin. SB/DUs are generally found within the lower tiers of enthusiast home models, with steam wands being a simple and valued addition.
You might have heard that purchasing a double-boiler espresso machine or a proportional-integral-derivative (PID) controller will give you more accuracy and control over temperature. They’re definitely features to dig into for advanced espresso crafters, but expect to pay an additional $400 for a PID and $800+ for a second boiler. Since we wanted to focus on beginner machines, we stuck to single-boiler models without PID controllers for this review.
All the machines we tested came with either insulated carafes or glass pots with built-in warmers. Both have pros and cons. Glass pots are typically easier to clean because they tend to have wider mouths, and the lack of internal insulation means that glass pots will have a greater interior volume relative to its exterior volume — basically, it’s easier to get your hand or dish sponge into a glass pot. On the other hand, glass pots are more fragile and have to be heated from a base plate. In our tests, those base plates could even raise the temperature of the coffee, like with the CuisinArt, which can make coffee taste burnt.
It can take a little while to get to know the ins and outs of a new coffee maker. In CR’s labs, each drip coffee maker we test brews roughly 65 cups by the time our engineers are through with it. And brewing is just one of many aspects we look at. We test for handling and convenience, too, so you can choose a model that helps you sail through morning madness. 
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