In my house, 2 rooms lights are on the same circuit, I noticed that for some reason whenever I install a preheat fluorescent in one of the rooms it sometimes causes the light in the other room to flash for split second when turning it on and off, this can be annoying for various reasons, the room with the preheat light has a standard on-off light switch, the other room has a leviton wifi smart switch and boob light with 3 phillips 100w equivalent LED bulbs. Does anyone know what is causing this or how to fix it while still being able to install preheat fluorescent lights? Thanks in advance.


NOTE: I cannot modify anything in the other room for personal reasons.

It would be helpful to pinpoint whether the "smart switch" is toggling the lamps or the lamps are doing that by themself.

Otherwise I would also suspect something fishy in the installation: The flashing may be a result not of the preheats being turned ON/OFF, but by the vibration in the wall when operating the switch because somewhere in some junction box is a loose connection, or something similar. Review where the wiring is routed, whether there isn't a common junction ox on the wall with the switch (so operating the switch shakes it), or above the preheat fixture (so the magnetic field from the ballast during preheat is shaking the wires with current in them,...). Or a loose contact, already nearly sparking, changes its characteristics when the passing current through it changes.

For how little I do the electrical work (not my main job) I've seen this problem way too many times. Even when I was initially refusing to accept a bad contact (because of "other symptoms"), I was proven wrong (again), it was exactly another case of a connection being loose.

Screw terminals are frequent offender, by far not exclusive in old aluminium wire installation
Bad assembly (someone tried to fit two wires underneath one screw,...)
Riveted joints in sockets/switches/... (when lying on the desk, the internal working springs tension the joints so the rivets appear tight, but when a bulb is screwed in, it becomes lose)
Soldered joints in electronics (improper mechanical fixing of wires, abuse when assembling, abuse when handling the part before installation leading to fatigue cracks)
Push-in terminals with fatigued springs (mainly when someone "fixes" a wiring mistake by attempting to remove the wire and then pushing in another one)
Wires being cut by those springs because exposed to movement (mainly in drywalls beside door where slamming the door shakes everything there)
Damaged "Wago" style terminals (because someone did not assemble the connection carefully)
And many more...

Interesting, thanks, the smart switch controls the LED bulbs, the bulbs are "dumb" ones. I'll try to check for loose connections, but I can't really check where wire is routed because that would require ripping up drywall and or flooring (other floor above). I don't think vibration from the switch itself is causing it since when using non-preheat lights, the other rooms light doesn't flash.

also for simplicity and convenience the room with the preheat fluorescent light is (A) and the room with the smart switch and LED bulbs is (B).

But did I understood well, both are on the same breaker, right?

To me this sounds like the other room's lights are switched with single pole switches, or that the relays or triacs in the smart stuff is single pole. The burst of RF high voltage that a glowbottle starter and inductive ballast creates, can capacitively couple through 'floating' wiring.
Because LEDs are so incredibly sensitive, they'll flicker on the burst of RF. This is especially noticeable if they're capacitive dropper powered, because the capacitor will let through RF much easier than the mains frequency. Capacitive dropper circuits will also make the LEDs fail quickly if you have a 'dirty' mains.

The proper solution to this problem is mains filtering at the fluorescent light.

You can test all this by putting an incandescent (ideally halogen) lamp in parallel to the LED lamp. The low cold resistance of a halogen bulb will dampen the voltage spikes that may be there.
If the halogen *also* flickers, then there's another problem, more likely to be the smart stuff suffering from the RF interference.

If everything is exactly in reverse (the lights flicker OFF instead of flash ON) then a fault in the installation is quite possible, but i'd first remove the smart thingies from the equation.