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Heat Exchanger vs. Double Boiler

A common myth for newcomers to the espresso machine world is that one needs a double boiler machine to extract espresso and steam/froth milk simultaneously (at the same time). This myth is false. The following machines can extract espresso and steam milk at the same time:
  • Espresso machines with a heat exchanger.
  • Espresso machines with a double boiler (independent coffee boiler and independent steam boiler).
  • Espresso machines with a independent coffee boiler and independent steam thermoblock.
We have worked on two generic visual representations - one for a heat exchanger model and one for a double boiler model.

Immediately below is a visual representation of the heat exchanger design with a thermosyphon grouphead. It is called heat exchanger because there is a transfer of heat between the steam/how water in the larger boiler vessel and the closed tube that passes through the boiler.

In the first diagram, the heat exchanger tube is the larger tube inside the single boiler with upside down 'V's. This is the water that is flash heated to make espresso - incoming cold water is mixed in with this hot water when the extraction starts. When not extracting espresso, this same water inside the closed loop system is heated in the heat exchanger and is pushed through to the grouphead which forces the slightly cooler water back to the boiler for re-heating. If the machine has a brew pressure gauge, this is one reason why the gauge will fluctuate and change while the machine is idle. This is why the brew pressure gauge should not be read while the machine is idling and is nothing to worry about.

In some cases, the larger boiler is laid down horizontal, and the heat exchange tube remains vertical. In some cases, the horizontal boiler incorporates a horizontal tube from inside to another. In most vertical boilers, the heating element is at the bottom of the boiler. There are a few cases where the heating element is on top of the vertical boiler with extensions internally to keep it under water. We have never seen a vertical boiler with a heating element on the side. On horizontal boilers, the heating element is usually on the side.

In some cases where there is no thermosyphon tubes into the grouphead, there is on one tube running to the grouphead. This grouphead is usually attached directly/welded to the boiler for passive heating by way of the metal.

Vibiemme Hx Diagram

Immediately, below is a visual representation of the double boiler design with a thermosyphon grouphead.

Espresso quality in the cup - The double boiler version has superior control over its heat exchange counterpart in the temperature control of the water in the boiler. This better temperature control allows the user to change the boiler water temperature to match better with the coffee blend being used. The superior temperature control offers a better extraction in the quality found in the cup. Therefore, 1st-line 'considers' the PID double boiler machine is a better choice for espresso-only consumption.

There have been some arguments made by some that the size of the coffee boiler on a double boiler espresso machine can determine the quality of the espresso in the cup. For example, there are some who argue that a smaller boiler will have faster turnover of the water, and this fresh water makes superior espresso. Although we were a believer in this camp before we carried double boilers, we no longer believe this theoretical model holds true. Our reasoning is that if the smaller coffee boiler theory held true, then a heat exchanger machine with a much smaller capacity would make extract superior espresso over a double boiler machine.

Vibiemme DB Diagram

Steaming capacity - From our observations, the heat exchanger model is a more powerful steamer than its double boiler counterpart, If frothing or steaming milk for milk-based espresso beverages, the heat exchanger model is superior as the heat exchanger model seems to have more steaming power and more steaming capacity than its double boiler counterpart. The reasons are,
  1. that the higher steaming performance of the heat exchanger system design will allow quicker turnover in producing cappuccino and latte drinks, and
  2. that the espresso quality is not 'that' important as the milk in the drink dilutes the espresso and the difference in the quality of the cup (milk and espresso) is very insignificant.
Warm up time - The Vibiemme heat exchanger model takes about 20-30 minutes to warm up, and the double boiler version takes about 30-45 minutes to warm up both boilers. Our Vibiemme espresso machines can be placed on a timer. However, the timer MUST be plugged in the wall outlet first, and then a surge suppressor (1080 joules or higher), and then either model can be plugged into the surge suppressor. On the La Spaziale double boiler machines, only the Version 2 can accept a specialized Spaziale timer. The Version 1 units can not be on a timer at all.

Energy Savings - The double boiler version allows the user to independently turn on/off the coffee boiler or steam boiler. If the user only operates the coffee boiler, electrical usage is only 600 watts for the coffee boiler heating element. The heat exchanger has an one on/off switch for the entire machine.

Delta (difference or offset) of Temperature between water temp in Boiler and Water temp at Grouphead

On average, the difference of the PID programmed temperature and the temperature of the water at the grouphead is about 20 degrees F. However, we have done a lot of testing with PID double boiler machines in several different environments, and we can say in our opinion with good certainty the following:
  1. if an offset is programmed in one environment, the offset is most likely to be different in a different environment
  2. the offset can change even with the machine in the same environment, but one of the variables change. Environments include variables such as ambient temperature, drafts, actual voltage to machine, etc. For example, we tested a PID machine in a basement - one area where there was no draft, and another area where there was a draft. The offset was different by as mush as 5 degrees. We tested a PID machine with a room temperature of 76 d F and then again with a room temp of 66 d F. Again, a different difference in the offset.
  3. the offset can change if the PID sensor has limescale build up - if the PID sensor in the boiler is caked with limescale (and they have a higher probability to do so on PID double boiler machines), and we have seen up to a 40 d F differential between the grouphead water and the programmed temperature. Although some manufacturers use the offset as a selling point, it can literally drive one nuts as this becomes a false pretense. In our opinion, it is best to use the average of 20 when initially programming the machine, and the best way to determine your optimum taste is to change the programmed temp based ont he taste in the cup leaving all other parameters is the espresso extraction process being equal - just like one would only change grind settings with all other things being equal. This is really the best way to get the machine to the right temp for the coffee bean/blend that is being used. This is why we do not believe that offset programmability is an important factor for PID machines at this time.