WHAT IS IT:
In any of our heat exchanger or dual boiler espresso machines that use a pressure stat for the steam boiler they can sometimes experience a big steam boiler pressure drop when the hot water tap or steam wand is opened for the first time in a day. This is a normal function caused by the machine's initial pressure build creating "false pressure" in the boiler when the boiler goes from not pressurized and open to pressurized and sealed.
WHAT DO I DO:
The solution is to simply open the steam valve once the machine is warmed up, as this will relieve the false pressure and allow the pressure stat to sense the true steam pressure of the boiler (you can also try doing this the night before when the machine is cooled off). The machine should heat back up to full pressure rapidly.
This can also be incorporated at the beginning of your drink making routine to avoid delays. When you approach your warmed up machine (tip: use a timer to have the machine warmed up when you want), open the steam valve first to "wake the boiler up", run 2oz of hot water through the group head and portafilter into your cup or shot glass (to ensure everything is toasty warm), and then prep your milk pitcher, grind your coffee and make your favorite drink.
WHY IS IT HAPPENING:
When a machine that uses a pressure stat builds pressure for the first time after turning on the machine on it is pressurizing both the water in the boiler and the air in the boiler. When the boiler is cold and off it has water and air residing in it. As soon as the the machine reaches a boil the vacuum breaker seals on the boiler, allowing the machine create a seal and build pressure above atmospheric pressure.
Because the boiler is heating both air and steam AND the machine starts and stops heating based on an internal pressure measurement, both air and steam pressurize inside the system creating a false (steam)pressure. Once the steam valve is opened you will usually see a 0.1 - 0.3 pressure drop at the steam gauge because a huge rush of pressurized air is escaping the boiler. At that point the machine has to reheat and create only steam pressure - not steam and air pressure. This is made worse with elevation. Because gases expand faster at higher elevations than liquids the preexisting air gases in the steam boiler create a larger false pressure.