Emil:

That said, you will notice that an insulated boiler will result in a higher boiler surface temperature for a given water temperature and therefor in a higher temperature win of the group, leading to overheating problems especially in times of frequent shots. Uninsulated boilers will be more forgiving in this aspect as that is the way they were planned and calculated to be used.

Interesting points, various here.

Emil,
Thanks for the graphic demonstration of the group design and that water also plays a role in limiting the heat that the boiler water exerts on the puck. I don't follow the point that insulation of the boiler would lead to a higher temperature of the group or greater heat transfer into the group. The central relation seems to be temperature and pressure governed by the setting of the pressure stat.

If you insulate your home hot water heater, the temperature at your spigot doesn't change. You have to adjust the temperature setting of the boiler to do that.

I'm not using the insulation on the group or it's boiler connection, just the boiler.

Great discussion. I would think that any of these Italian multi group lever machines were designed to be able to pull shots repeatedly under heavy use in a typical Italian espresso bar and have the group maintain a fairly constant temperature. Check out Rosario on that Bosco behemoth at Bar De Ga. Doesn't look like he's having any temperature issues and I can't imagine the number of shots he must pull in a day.

I am a beginner at the lever machine as I just got my izzo pompei one group machine not so long ago. The boiler is set at about 1.4 bar.

I am struggling to get consistency of quality with the shots. I can see an excessive blonding in many of the shots and I can taste too much bitterness in the cup.

Is the temperature of the water too hot so that it is burning the coffee? How does one cool it down then? It would seem to my beginner's understanding that if I flush the grouphead by lowering and raising the lever before I pack the portafilter and lock it into the grouphead for extraction, that I would be actually heating up the grouphead before extraction and thus reducing its abiltiy to cool the superheated water from the boiler when I do finally begin the extraction?

Thanks for any suggestions.

I have the new model Izzo Pompeii with the curvy sides and the PID for temp control. Mine is set on 122 deg which is giving me about 1.2 bar on the pressure gauge. Mine is pulling god shots every time so maybe yours is a bit hot. I agree that a flush with these designs will tend to heat rather than cool like an HX machine.

I assume yours is controlled by a pressurestat under the case somewhere, I would try reducing the boiler pressure back down slowly towards 1.2 bar.

emil wrote:Interesting discussion - here's my 2 cent:

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The superheated boiler water ist not immediately entering the group, as there is already some water present (see left image) - the dipper tube and the group channel are filled with water left over from the previous shot and therefor slightly cooler than the boiler water, because not actively heated. The steady temperature of this water is proportional to the group temperature, which itself has a given heat loss via its surface and the surrounding air temperature and a given heat win depending on shot frequency and temperature and, as it is directly bolted to the boiler, it depends higly on the surface temperature of the boiler. You can see the influence of this connection when comparing with other machines using the same group bolted to the structure instead of the boiler as Nuova Simonelli or Izzo, which tend to need much higher boiler settings (up to 1.5bar)

Thanks Galumay for your reply. I have the previous model Pompei which lacks the PID unfortunately. Setting the termperature for your machine must be comparatively easier as you do not need to reach into the machine to adjust the pressurestat but can just use the PID. I was also thinking that the 1.4 bar setting is somewhat high but I was told by the dealer that Izzo sets its boilers at 1.5 bar and prefers it at that pressure setting. Also I note Emil's post above. Will there be negative effects if I lower the pressure to 1.2 or 1.3?

galumay wrote:Will there be negative effects if I lower the pressure to 1.2 or 1.3?

Possible effects are - slightly lower steam pressure, and a slightly longer recovery time if you're pulling lots of shots or steaming alot of milk.Although Izzo states a preference for 1.5 bar, this is probably based on use in a commercial setting, or just to their taste preferences.
When using a commercial lever at home its quite possible to go as low as .7 - .8 bar, as usual, let taste be your guide.If you start to get sour shots, or very low steam pressure, start increasing the pstat setting.
A longer pre-infusion can also help lower the brew water temp

Remember that the Izzo recommendations will be for classic Southern Italian bar blends containing Robusta. If you are mainly using single origins and like fruit and floral notes 1.25 is likely to be more appropriate.

Many thanks to michaelbenis and to kitt for the advice - I will try it out and see how it works.

michaelbenis wrote:Remember that the Izzo recommendations will be for classic Southern Italian bar blends containing Robusta. If you are mainly using single origins and like fruit and floral notes 1.25 is likely to be more appropriate.

Yes, that was the basis for my setting. I know Izzo set them higher when adjusting them prior to shipping in Italy. Thats why i recommend working down gradually and checking for differences in taste.

Some further observations.

I tested the temperature of the group (outside the bell about 1 cm above the shower screen) with the cowling off and with the plastic cowling on and with the potmetal cowling on. I had wanted to know if the change in material or no cowling made any difference in brew temp. Result: apparently not. Measured after one hour, the temperature did not change when I removed the plastic cowling. Measured again after replacing with the metal cowling the temperature had declined 1 deg Celsius. I would not think that small difference would not affect the brew temperature to any significant degree.

Lately, I find the shots are a bit bitter if I brew at steady state with the temperature at 76 C (same probe site). The pressure of the boiler is 1.2 bar.

Now I am lowering the boiler pressure and making temperature measurements. I had noticed earlier that I like the flavors at about 65 C so I am trying to bring the boiler pressure down to reach that temperature at steady state. I will post those temps as I bring the boiler pressure down.