Whether you call it DD, double boiler, dual boiler, or the best machine you ever owned, a double boiler machine is a fairly sophisticated piece of machinery, but the basics of how it works are fairly straight forward.

As you can see, a double boiler has, as the name suggests, two boilers. The light blue arrows indicate the direction of flow.

1) The steam boiler contains steam and hot water

2) The Brew Boiler holds nothing but hot water for the brewing of espresso. All water that comes through the brewing group comes from this boiler, and in normal operation that is the only exit path for water in this boiler.

3) Water enters the brew boiler from the pump. In some cases the designers have run the inlet tubing through the steam boiler. The supply line, being immersed in the hot water of the steam boiler, is preheated to lessen any thermal changes as the water enters the brew boiler. This design feature goes a long way towards assuring that the brew temperature of the water is as controlled as possible.

4) Water entering the brew boiler displaces the heated water which is forced out of the boiler and through the brew group. Of course, these paths are sealed until the brew function is selected. There is no need for a water level sensor in the brew boiler since it is always completely filled. An external pressure relief valve is located somewhere along the inlet line to relieve excessive pressure from the heating of the water in this sealed chamber.

5) A supply line for the steam boiler allows the pump to refill the steam boiler. The level of water here is sensed by a probe which signals an electronic control box when the water level droops below its tip. Water is then sent to the boiler until the water once again touches the water level sensor. This maintains a water level that leaves enough room to hold a steam reserve yet still covers the heating elements with water to prevent them from being damaged.

6) Steam is drawn off the top of the boiler through a copper pipe which leads to the steam valve.

7) Hot water is dispensed from the bottom of the boiler through a copper pipe which leads to the hot water valve. Hot water is pushed through the pipe by the pressure of the steam in the steam boiler. Hot water is not pumped out. If the pump runs when dispensing hot water, it is only to replenish the steam boiler's water level.

      There are numerous parts which are not shown here. Some of which are:

The Benefits
      Since the steam boiler has to be kept at a high temperature to maintain a reserve of steam, the water contained therein is too hot to be used for making espresso. The brew boiler is filled with water only, and is only going to be used for brewing espresso so it can be easily regulated to supply water at the perfect brewing temperature, or very nearly so. Two boilers, each designed for specific functions.