Maybe it was put-in for a future AC, a pre-duct per say. Easier to do it during construction than later,.
Combustion air for a gas appliance?
Apparently no system requiring it. Does baseboard heating require combustion? IDK because I rarely see them.
Could still have gas service to the home for water heater, dryer, etc.
Guess I should have asked if this was a bedroom
OP stated there were 2. Bedroom and adjacent bathroom. Could not verify ducting between the 2. Head scratcher.
Using a register or grill to cover an access hole is somewhat common.
Hey Steven -
When you say “a home that has always been baseboard heated” … is that confirmed, or is that “as far as anyone knows”. Usually when I see abandoned registers, it means that 1. An air-conditioning system was added, or 2. there’s been a change in heat source somewhere along the way. As in - somewhere in the past, the old oil-fired system expired and was replaced with baseboard heat. But anyway you look at it - IMHO if the registers are abandoned, they should probably be removed, and ceiling penetrations closed-off and finished. Cuz nothing good or useful is going in or out of those holes in the ceilings.
Depending on where it’s located, it could be a dummy vent to supply warm air from the living space to plumbing pipes so they don’t freeze in the winter.
The reason I asked that question is depending on the age of the house, a wind turbine with a bucket attacked to it in attic with usually 3 holes was used for ventilation. One of the holes was used to ventilate the attic and insulated ducts were attached to the others holes and extended to registers at ceiling and were used to ventilate the house. One register would be located on the upper floor at ceiling and the other would be located in basement.
Were you able to get into the attic? That should tell you all you need to know. Many times I see electric baseboard heat with a cooling only split system in the attic. The fact that you only see two supply vent grilles may be because the others have been removed. Also as was previously mentioned, sometimes we see grilles like this used as access covers for plumbing shutoffs or electrical junction boxes.