# Temperature control



## salty dog (Jun 6, 2015)

Hello folks a question which probably shows my naivety as much as anything,

Apart from a good grinder, consistent application of method, quality ingredients, so far one of the other things that I have learned is that the temperature of the brew water is very important.

I understand that in something say like a Gaggia Classic and entry level machines this is a little problematic as the thermostat that is fitted has a wider value range and it can be something of a lottery as to what temperature you actually get - various methods such as 'surfing' can be adopted to try to get as close to optimum as possible.

Other machines such as the more upmarket single boiler machines can have a PID (either as standard or as retrofit) as can some of the twin boiler machines and this enables a much more precise control of temperature. I also understand that boiler temperature is often slightly higher than optimum to ensure that when the water reaches the brew head having passed and being cooled along various waterways it is at an optimum. However, it doesn't always seem the case that these latter machines have PIDs and I can only conclude that the thermostats that are fitted have a much closer range.

The matter then (for me at least!) becomes more complicated with the HX type of machine. Here the brew water passes from the water tank through a heat exchanger in the boiler and out often through an E61-type brew head which is pre-warmed/heated with a thermo-syphonic flow. I know that like a pressure cooker the temperature of the water in the boiler rises above the nominal boiling point of water and the temperature in the main boiler is therefore regulated by the pressure regulator in the machine. This is fine up to a point, but somehow (again as I see it) seems to ignore the fact that the temperature of the feed water from the tank/mains varies and that the water in the boiler may also be at a lower temperature having had water drawn off for say steaming. OK this is the point of having good heaters well placed and designing well balanced water volume to watts ratios.

I have looked at various schematics in say the Bella Barista reviews, but maybe, because I am a 'green bean', I am missing something or some things have been left out for the sake of simplicity.

I will also offer an apology as this very topic may well be covered somewhere else on this site - its just that I haven't found it!

Is there a simpler explanation to something I am making too complicated?

Many thanks:confused:


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## risky (May 11, 2015)

I'm not able to offer any expert advice on this, but as I understand, the main reason 'beginner' machines like the Classic suffer with temperature stability is because the boiler is so small. HX machines will have larger boilers and therefore once the machine is at temperature there is far greater temperature stability. Steaming etc. doesn't cause a noticeable temperature change like it does on a small single boiler machine.

No doubt an expert will be along shortly to correct me.


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## 4085 (Nov 23, 2012)

The lack of an 7lb group head might make a difference!


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## hotmetal (Oct 31, 2013)

Risky is pretty much bang on there. I'm no expert either but the general gist is that the small single boiler machines suffer from temperature fluctuations due to their small size and low boiler capacity. So a Classic has a 100ml boiler, but when you pull a shot (say 36ml) you're pulling 36ml of cold water in. The group isn't a huge piece of brass like the E61 so the temp of the brew water will decline through the shot. Temperature surfing is a kind of 'finger in the air' way of trying to establish a routine of switching on the heater manually to try to mitigate the effect of the cold water draw. Classics are often modified with a PID which tries to be a bit more scientific. I had one on mine and it made it easier but beyond that it's the laws of physics (which 'ye cannae change, cap'n!')

HX machines often have larger boilers and heavier groups with more thermal stability. The thing with HX is that even drawing a normal amount of steam won't dramatically affect the brew temp (I think) because the steam boiler is so far above what's required for the brew that it's not an issue. However HX machines can overheat the brew water after a while so it is often necessary to flush a bit of water through the group to get any overheated water out of the hx pipe before you pull a shot. The E61 group is several kilos of brass etc and therefore has good thermal equilibrium to even out fluctuations. Dual boilers avoid the necessity of flushing by fairly obvious means of having a PID to control the brew boiler directly.

Did it really take me 14 mins to type that on my phone lol! DFK's post wasn't there when I started!


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## salty dog (Jun 6, 2015)

Aaaah! The Grasshopper (aka greenbean) is much more wise in the ways of the coffee machine. Many thanks to all those who have made life clearer for me!


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