As most lever machine users will already know, the ambient temperature affects the brew temperature significantly
For example, at home we sat the Cremina on the window sill as it is one of those old window sills that is deep enough to double as a seat
However we quickly learnt, this is not the place for an espresso machine – the single glazing coupled with the breezy old sash window frame meant that in winter you really needed to turn the wick up on the pressurestat to get the brew temperature up to the correct level
So, ideally you want to place your espresso machine away from windows, particularly if they are windows that are draughty or often open
At the factory the ambient temperature is around 50F for at least 4 months of the year; again this means the groups on the machines have been too cold on the first shot, and even second shots on the large lever groups
That might sound like a small price to pay for the excellence of lever drawn espresso, but after a while it really irritated us
This is what sparked our search for a better design. Enter the thermosiphon
What is really clever about the thermosiphon, despite its simplicity, is the rate of flow adjusts in response to fluctuations in the group temperature
So, if it is winter and the ambient temperature is 50F/10C, when the water in the thermosiphon returns to the boiler after it has travelled around the group it will be cooler than it would if the ambient temperature was higher. This means the pressure difference between the hot side of the thermosiphon and the cool side is higher, which causes the water in the thermosiphon to circulate faster. As a result the group temperature is raised up to where it should be, ensuring the correct brew temperature
Conversely if the group is a bit too hot, either as the result of a higher ambient temperature or from shots having been drawn, the temperature of the water in the thermosiphon when it exits the group and returns to the boiler will be higher. This means the pressure difference between the hot side and the cool side of the thermosiphon is reduced, or even eliminated, and the rate of flow in the thermosiphon will reduce, or even cease, allowing the group to cool. As the differential between the temperature in the hot and cold sides of the thermosiphon grows again, the thermosiphon starts to circulate again
So despite its simplicity, a lever espresso group equipped with a thermosiphon is self-regulating in terms of temperature control
Hopefully we haven’t bored you to tears, but I thought it was worth emphasising why we are so enthusiastic about thermosiphons, not forgetting their simplicity and the long term reliability that comes from that simplicity
Thermosiphons are a very elegant engineering solution to the problem of maintaining the correct group temperature in a lever espresso machine