the board is connected to relay module that control venta (in my bathroom)
and some lights.
my code operate the venta for 10 minutes when I turn off the light in my bathroom, if the light turn on in that time it will turn off the venta.
the problem is that sometimes the venta causes other relays to turn on \ off.
i think that the venta creates a spike on the power network that causes it, sometimes it affects on another arduino mega that is connected in another room.
This venta is a fan yes? If so its almost certainly a mains induction motor fan, which is the most inductive load you can have,
and will generate a massive spike if not correctly suppressed - normally such a motor would have suppression components
built in (a snubber circuit) to greatly reduce the severity of the spike, but it will still be present at switch-off and could easily be the culprit.
As to how other systems are picking up this spike, possibly an inadequately filtered or earthed powersupply, perhaps sensitive
sensor/low voltage cables are run near/parallel to mains wiring (never a good idea).
If the relay controlling the fan motor is inadequate for the task it may be sparking excessively and contributing more high voltage
noise - in general you have to derate relay contact ratings for an inductive load compared to resistive load - so is your
relay generously rated?