Usually the quick answer is "The OPV is doing what it's designed to do" - but the reason why this can be an issue needs some explanation:

The Classic is a "single boiler, dual use" (SBDU) machine - so the single boiler is used for both brew and steam.

When brewing an espresso shot, the boiler has a pump pushing water in at pressure, and an output that is opened and closed by a "solenoid valve".  So when brewing, the pump is on and the solenoid valve is open - allowing water to exit the boiler.  At this time the entire system (from pump output to coffee puck) is at pressure - including the boiler itself - of 9+ bar (depending on your OPV setting).

When you finish brewing a shot, the pump stops and the solenoid valve instantly closes the output from the boiler - and this can result in the boiler being left pressurised (often at anything between 1-6 bar).  Sometimes the way the pump stops, or the way the solenoid valve closes, or the pressure exerted by the puck during that shot can mean that the boiler is left at 1 bar and at other times it may be 6 bar - or anything in between.

This is usually fine, and with a 9 bar or lower OPV setting then the OPV will bleed off any excess brew pressure water via the OPV outlet and everything will be fine.

HOWEVER, if the boiler is left with residual brew pressure of 5-6 bar and THEN you press the 'steam' switch - as pressure is directly proportional to temperature, and you are now increasing the temperature, the pressure within the boiler MUST increase. Usually this will result in the boiler pressure exceeding the 5 bar pressure spring, and often also the 6.5bar pressure spring - though as pressure typically peaks at around 8 bar, using the 9 bar spring is normally issue free.

When the pressure in the boiler (and OPV) exceeds the rated spring pressure (5 or 6.5 bar) then the OPV will open - releasing the contents of the boiler (steam and water) via the OPV outlet pipe - back into the tank.  This usually makes a frighteningly loud noise - as it's high pressure steam and water - and the process usually leaves very little (if any) water in the boiler.

So - if this happens (and you get the loud noise as the boiler pressure exceeds the OPV pressure) you then find that you have zero power (or actually zero water/steam) for steaming - as you have no water.

This ISN'T a problem of the OPV - as stated above - it's doing its job correctly.   It's a side effect of the residual pressure from the previous brew session.

What can you do about it?  Well, a number of things:

1. If this is happening with the 9 bar spring:
   
   
   1. firstly make sure you are using the correct spring (it's the firmest of the two short ones - NOT the longest one, as many people who don't read the instructions think!).
      
      
   2. Then make sure that the valve cap is properly located at the end of the spring (see the image below).  Some people change the springs and don't notice this small free-floating piece of metal fall out - and frequently find it in the bottom of the machine.
      
      
   3. It's also possible for the valve cap to rotate sideways or completely, and therefore to not be properly seating in the OPV. So make sure that it's positioned on the end of the spring as shown in the image below.
      
      
      
2. With the 5 bar and 6.5 bar spring, the various things that you can do are as follows.  One of these may work better for your workflow than others, or may be more effective for you than others.  The essential action we're looking for is to stop the pressure exceeding the OPV limit and causing the frightening noise as your water and steam are dumped. So..
   
   
   1. The easiest thing to do, is to wait around 10-15s after pressing the steam button - and start steaming your milk.  Purge the wand initially, and then steam as normal - but don't wait too long after pressing the button (this is good practice anyway, as you can ensure that the elements are being powered for the longest period of time).  If the noise still happens, reduce the waiting time before you start.
      
      
   2. Another thing to try is to remove the excess pressure by pressing the steam switch and the briefly purging the steam valve - opening it, purging some water (and excess pressure) from it, and then closing it again.
      
      
   3. A final thing that works for others is to remove the excess pressure by pressing the brew switch on and then off about 0.5s later - this starts the pump, but more importantly it opens and closes the solenoid valve - again releasing excess pressure.

Hopefully one of the above tips will ensure that you have the correct spring fitted, the valve cap is properly installed, and you are able to avoid the excess residual pressure.

If you're still struggling after all of the above, then please contact us and we'll do our best to assist further.

This is how the OPV valve cap should be located - on the top right of the image: