Technical guide
How to set the pump pressure on an espresso machine (9 bar)
Pump pressure is read on the gauge during an extraction, and manufacturers of professional machines specify 8 to 9 bar. On a rotary pump it is set with the by-pass screw: tightening raises the pressure, loosening lowers it. Adjust only while a group is brewing coffee, in small steps, checking the gauge each time.
What is the correct pump pressure on an espresso machine?
Manuals for professional machines specify 8 to 9 bar, read on the gauge while a group is brewing coffee. The exact value and the installation limits depend on the model:
| Machine (manual) | Pump pressure | Mains water pressure | Expansion valve |
|---|---|---|---|
| La Marzocco Linea Classic | 9 bar (8 to 9 bar factory preset) | 2 to 6 bar | Must keep pressure from exceeding 12 bar |
| Astoria and Wega (C.M.A.) | 8 to 9 bar | 1 to 5 bar; pressure reducer above that | Limits the circuit to 12 bar |
| Nuova Simonelli Appia II | 9 bar | Up to 4 bar; pressure reducer above that | No value in the manual |
For any other machine, the manual for that model applies.
Where is pump pressure read?
It is read on the pump scale of the double gauge while a group is brewing coffee; the other scale shows boiler pressure. Some electronic machines show the value on the display, and some manufacturers offer a portafilter with a gauge for measuring directly at the group.
With the pump stopped, the needle does not show extraction pressure. Gauges and capillary tubes are listed under Sensors & Instruments.
How is the pressure adjusted on a rotary pump?
It is adjusted with the by-pass screw on the pump body: turning clockwise (tightening) raises the pressure; turning counter-clockwise lowers it. The La Marzocco, C.M.A. and Nuova Simonelli manuals agree on this, and the first two say to adjust while a group is brewing coffee.
- With the machine switched off, locate the screw; on the Linea Classic it sits under a plug, on the side where the pump's power supply is connected. If there is a plug or a locknut, remove or slacken it.
- Confirm the supply: tap open, filter clean and mains pressure within limits.
- Switch the machine on and start an extraction with coffee in the portafilter.
- Turn the screw in small steps, watching the gauge, until it shows the value in the manual.
- Stop the extraction, retighten the locknut or plug and confirm with a new extraction.
If reaching the screw means removing panels that leave live parts exposed, disconnect the machine from the mains before each adjustment and refit the panels before testing.
Does mains water pressure change the pump pressure?
Yes. The La Marzocco manual states explicitly that pump pressure, preset at the factory, may vary from place to place because it is affected by incoming mains pressure. So check the pressure after installing or moving the machine, and after fitting a pressure reducer, a filter or a water softener.
Also respect the inlet limit given by the manufacturer (see the table): above it, a pressure reducer is fitted before the pump.
Why does the gauge climb to 11 or 12 bar while the machine heats up?
Because water heated in a closed circuit expands; it is not a pump fault. La Marzocco explains that water enters the coffee boiler at 8 to 9 bar and that heating adds about 3 bar. The expansion valve should then release a few drops so that the pressure does not exceed 12 bar; the C.M.A. manuals give the same limit.
Do not lower the pump pressure to correct this reading. If the value goes above 12 bar, it is the expansion valve that needs adjusting or descaling; if it sticks open, the pressure no longer reaches about 8 bar. Over-pressure valves are listed under Valves & Pumps.
What if the pump does not reach 9 bar?
Before replacing the pump, check in this order the water supply, the expansion valve, the gauge and the by-pass.
- Supply: tap not fully open, blocked inlet filter or low mains pressure. A dirty filter causes cavitation and pump wear.
- Expansion valve stuck open and discharging continuously.
- Gauge faulty: confirm with a portafilter fitted with a gauge.
- By-pass dirty or worn: the pressure does not respond to the screw. On some pumps the by-pass piston can be replaced separately.
- Worn pump: if everything else is in order, the pump is replaced.
FAQ
- What if the machine has a vibratory pump?
- A vibratory pump has no adjusting screw. The pressure is limited by an expansion or over-pressure valve (OPV) fitted after the pump, and the adjustment is made on that valve, where it is adjustable, following the machine manual.
- Can I set the pressure with a blind basket?
- The manuals consulted say to adjust a rotary pump during a coffee extraction. With a blind basket there is no flow and, on a hot machine, water expansion pushes the reading up; use it only if the manual for your model says so.
- How often should pump pressure be checked?
- La Marzocco includes the 9 bar check in the maintenance carried out by a qualified technician every three to four months, and C.M.A. recommends checking the pressure periodically during extraction.
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