The short answer is no. New Jersey's residential code, built on the same model code most of the country uses, lists the rooms a return air grille is not allowed to draw from, and bathrooms and toilet rooms are on that list along with closets, garages and furnace rooms. A grille in the bathroom that pulls air toward it when the blower runs is not doing its job by accident, it is working against the room's own exhaust fan and pushing whatever that fan is supposed to remove into the return trunk and out through every supply register in the house.
Why bathrooms are on the excluded list
A bathroom holds warm, moist air after a shower, and often carries odor along with it. The rooms on that excluded list share a common thread: closets can trap stored chemicals or off-gassing from cedar and mothballs, garages hold vehicle exhaust and fuel vapors, furnace rooms can pull combustion byproducts into the airstream, and bathrooms add moisture and odor to the mix. A return system is built to circulate conditioned air through the house, not to collect what a room's own exhaust fan is meant to carry outside. Pulling supply air from a bathroom defeats the purpose of having a fan there at all.
A return fighting the exhaust fan
Picture both running at once: the bath fan pulling moist air up through a duct and out through a roof or wall cap, and a return grille a few feet away pulling that same moist air sideways into the trunk line instead. The fan loses part of the fight every time the blower cycles on, and the moisture that should have left the house gets carried to the coil, the plenum and every branch duct downstream. Over a heating season that adds a steady, low-level moisture load to parts of the system that were never sized to handle it.
How a return ends up in a bathroom in the first place
Nobody plans a bathroom return on purpose. It usually happens because a room's use changed after the ductwork was already in place. A bedroom or hallway with an existing return grille gets remodeled into a bathroom, and the grille stays where it was because moving it means opening a wall or a floor. In Bound Brook, where Census estimates count a large share of homes as two-family or small multi-unit buildings, a single house divided into separate units often keeps the original duct layout while the room uses shift around it, and a return that made sense in a 1950s floor plan ends up behind a shower curtain. North Plainfield has a similar story from a different direction, since more than one home in four there predates 1940, and decades of small renovations can leave a return grille stranded in a room that was never a bathroom when the ductwork went in.
How to tell if a grille is pulling return duty
Look at the grille itself. A supply register usually sits high on a wall or in the ceiling and has adjustable louvers to aim airflow. A return grille tends to sit lower, often in the floor or near the baseboard, and is a plain, flat grid without louvers. The test that settles it takes a tissue or a strip of toilet paper: hold it near the grille while the blower is running. If it gets pulled toward the opening, that grille is a return, and if it is in a bathroom, it is one that should not be there under the state's residential code.
Closing it off and giving the system a real return
Once a bathroom return is confirmed, the fix is Air Duct Repair on the branch that feeds it, sealing the opening at the register and capping the duct back at the trunk so no air is drawn from that room. That alone solves the code problem, but a house still needs enough return path to move air properly, so the next step is usually Air Duct Installation of a new return grille and duct run from a hallway, a central room, or another space that is not on the excluded list. Matching the new return's size to what the old bathroom grille was carrying keeps the system balanced instead of just closing one opening and calling it done.
What a visit looks like
We start by walking the house room by room, noting every grille, checking louvers, and running the tissue test where a grille's job is not obvious. Any return found in a bathroom, closet, garage or furnace room gets flagged, along with a plan for where a replacement return can go and how the duct run gets there from the trunk or plenum. That plan, along with the split between Air Duct Repair and Air Duct Installation, goes on the written estimate before any work starts, so a homeowner in Bridgewater or Bound Brook knows exactly what is being closed off and what is being added before a single duct gets opened.