The Espresso Machine Restoration site
A non-commercial site for those interested in espresso equipment repair and restoration.
From the workbench 2
Solenoid valves
What are they?

Solenoid valves control the flow of water throughout the espresso machine hydraulic system. The valves are
electronic. With regards the water hydraulics they have two main functions.

1. Controlled by the electronic level control they open and close to fill the boiler on a HX and a steam boiler on a

2. Under manual control by the user or by automatic control from the volumetric dosing system the valve opens to
allow water from the HX to the coffee grounds (in a HX machine) and allow water to pass from brew boiler to coffee
grounds in a Marzocco.
What are they comprised of?

3 parts.  Coil, Valve body and the plunger.

The valve body

The valve body refers to the main brass component of the valve and the stainless steel component.  The two screw
together and should form a watertight seal.  Some valves have an o-ring between brass and steel parts to get a
good seal whilst others use a compression type arrangement.   The body of the valve can be a 2-way or a 3 way.  2
way valves are generally used to control water in one direction, for example the filling of a boiler.  When the valve is
opened water may flow through the valve.  Water flows through the brass part of the valve, the steel part is blocked

3 way valves as the name suggests have 3 holes for the water to travel through but only 2 are open at any given
time.  The 3 way valve bodies have the hole in the stainless steel part open.  3 way valves are generally used at the
groups of coffee machines.  The third hole is used as a way to release pressure from the group when the brewing
process has been stopped.  

The brass body of the valve may vary in shape between machines.  The 2 most common are pictured above and
they are the "T" shape and the square block shape.    
The coil

Coils can be 110V or 220V and when energised these create a magnetic field which moves the plunger downwards.  
This opens the valve.

The coil is held onto the stainless steel part of the valve body.  It is held in place by a 14mm nut.  The nut should not
be too tight and when tightening it only needs to hold the coil in place to stop it moving.  It is not necessary to
over-tighten this as you may crack the plastic case.  There is a washer between the nut and the coil.

The coils have 3 poles.  The two poles that face each other are the AC input.  The middle one, which should be
marked, is the earth. So the wire with the green/yellow stripe goes on that.

The small screw hole inbetween the poles is to be used for securing a valve connector/filter to the unit.  This is a
watertight connector/filter.  It may be seen in the picture of the square valve body above.

A good solenoid coil should have a reading of 0.7-0.8kOhms.
The plunger

The plunger may also be referred to as the bobbin or piston.  The plunger sits inside the steel part of the valve and
it is this that the coil attracts with the magnetic field.  

The plunger has a gasket that runs through it, the green part in the pictures.  It is this that seals with the valve body
to divert and block water flow.  In the power off position (coil not energised) the plunger makes a seal with the top
brass part of the valve.  In the power on position (coil energised) the plunger moves downwards (acting against the
spring force) and the gasket at the bottom of the plunger seals the hole in the steel part of the valve.  
Problems and overhauling

Solenoid valves are very reliable.  There are a few common problems that occur.

Dead coil - Coils do die and symptoms include no movement of the plunger even though the correct voltage is
going to it, and on a few rare occasions valve chattering.  A new coil is required.

Water leaking from a normally closed water route - e.g. water leaking from grouphead, or a contant drip from
the relief port on the valve.
 - This would indicate that the seals of the plunger are worn. It may also mean that the
spring no longer returns the plunger to the desired position.  Replace the plunger.

Lack of flow from the valve - The holes inside the valves are small.  Group valves require regular cleaning by
backflushing with detergent to keep them clear otherwise the coffee oils and crud will block it up.  2 way valves that
are used for controlling the filling of the boilers will have limescale build-up inside.  The only way to remedy this is to
dismantle and descale.  
This website is created by Paul Pratt, Hong Kong 2004. If you would like to use any of the images or text I am
sure I will say yes, but please ask first!  

Email me here.

3 way Solenoid valve body.
Another 3 way valve body.
Solenoid coil.
Solenoid poles.
The middle contact is the earth.
A new plunger.
The plunger has a gasket at the top.
The spring returns the plunger after the coil has been turned off.