Guide to Coffee has an in depth look at dual boiler espresso coffee machines, the benefits and drawbacks and which machine will suit you best
Dual Boiler Espresso Machines
Firstly, what is a boiler? A boiler is a copper, brass, stanless steel or sometimes aluminium cylinder which stores water, it is fitted with a heating element inside which is used to heat and maintain a set temperature. Water from here will then be pumped out to your brew head on demand, alternatively if the boiler is used for steam it will then be heated to a hotter temperature where steam is provided when requested.
A dual boiler machine contains two such boilers, one is dedicated to brewing and one to steaming. This means that once your machine has pre-heated it is ready at any time to generate espresso, as well as steam simultaneously.
One of the biggest benefits that comes from this set up is that there is no delay between when you want to brew and steam, because you have two boilers with a dedicated function - they are specialists and that's all they do. For those of you familiar with more entry level machines, perhaps with a single boiler or Thermoblock you will be well aware of the delay you must endure when switching between brewing and steaming.
Further to this time saving there is the added benefit of knowing that you know the temperature within the brew boiler is consistent - one less variable you have to worry about when it comes to preparing espresso.
There's two main cons- neither of which affect the quality of the coffee which is to be produced but may be considerations when purchasing a new machine.
Firstly is the size, dual boiler machines will take up a greater amount of bench realestate due to the fact that the manufacturer needs more room for those two boilers inside. So if your kitchen is on the smaller side or you can't spare the space this may not be the machine for you, you may like to look at either heat exchangers or single boiler systems instead.
Second point is that of cost, these are not an entry level system and involve more parts when being manufactured. This inherently means that these will be a dearer machine - if you are a true coffee enthusiast and price is no object (or you've just saved really hard) then a dual boiler will be a much better option than a single boiler or Thermoblock.
Who's it For?
So in summary you may tend towards obtaining a dual boiler machine if you:
Experienced in or willing to learn how to prepare espresso
Require multiple milk & espresso drinks to be prepared quickly
Have adequate space for a slightly larger machine