I bought a 1 group commercial HX machine. The machine fills the boiler slowly with the main water supply pressure.
In general it works as expected. When the water level is too low, the pump engages and fills untill the level probe is in contact with the water. But the main water supply pressure keeps on filling the boiler, but how I don't understand.
I've done some trial and error setups to understand the problem.
A. When the main power switch is off and main water supply is on -> no filling of boiler
B. When the main power switch is on and main water supply is on -> yes filling of the boiler

A. checked boiler fill solenoid. Took it apart (very clean) nothing abnormal. Disconnected the piping between boiler and solenoid and put main water pressure to it. No leakage.

B. Replaced the two way solenoid (not the brass part) put the power switch on and it still slowly fills the boiler.

So how to proceed further.

Thanks, Willem

ok so I'm understanding

when the water supply is on and the machine MAIN SWITCH is off
the boiler IS NOT slowly filling/overfilling the boiler

when the water supply is on and the machine MAIN SWITCH is ON
then the boiler is filling/overfilling the unit

IF SO check the voltage on the coil of the solenoid valve is it getting energized?
furthermore or simplifying
remove the connector of the SOLE and let the machine idle (taking all the good precautions)
and see if water is still sipping through the valve or not

Yes, correct on both.

The boiler fill solenoid opens up when energized, it does that when the water level is too low.
First I thought the HX is cracked and fills the boiler, but I checked that already.

ok check that is not getting current when is not suppose to
we are assuming that is doing a slow fill with the pump/motor NOT engaged

If it were a manual fill valve leaking it would fill the boiler even when the power was off. Since it only happens when the power is on, something is getting energized. Is the boiler filling to the point of over flowing or just filling slightly when hot.

Yes,
It only happens when energized. Eventually the boiler completely overflows.

Water pressure from the mains is stable. (I have another HX 24/7 running with no issues)
Pressurized the HX with water to check for leaks into the boiler, no issues

So it all comes back to the fill solenoid.
I'll do the following setup to check if the solenoid works accordingly and report results.

A cold machine (no pressure in boiler)
Disconnect the pipe between fill solenoid and boiler. (cap it if necessary)
Ground the wire from the level probe to the machine (to prevent pump engagement)
open main water supply
power switch on (fill solenoid is energized)

still confusing here:
obviously you already figured out that has water when get hotter it will expand so it will raise a little the volume
so

the motor and pump are running WHEN THE FILL CYCLE SHOULD,
THEN STOPS SUPPOSEDLY when the level probe is reached.
at this point it stops motor/pump and supposedly voltage to the solenoid valve coil in a normal scenario

if you problem happens ONLY when the power is turn ON
and there is no pump/motor RUNNING

yet if the main switch if OFF then is NOT happening?

IF SO try
disconnect power to the solenoid coil
and test the connector to see if is getting energized when it should NOT
in other words is your solenoid valve getting current when should not be any???????

anyway what machine and what control board you have on it?