Your bedroom really doesn’t aspire to be a balloon. Yet, because of the way your heating and air conditioning system was installed, it may be acting like one. At least to an extent. It doesn’t expand the way a balloon does, but it does get blown up.
Think about it. If your bedroom has a supply register from your HVAC system but no return grille or other pathway for the air to make its way back to the unit, what happens to that air blowing into the room when you close the door?
The problem with bedrooms
Forced-air heating and cooling systems use ducts to move conditioned air from the furnace, heat pump, or air conditioner back into the living space inside the house. The air that it’s conditioning, though, isn’t just whatever it can get. Ideally, it’s a closed system with as little duct leakage as possible, and the air that it’s conditioning comes from inside the house. The furnace or air handling unit (AHU in the drawing reproduced as Figure 2, below) pulls air through the return ducts, conditions that air, and then sends it through the supply ducts back into the house.
The problem with bedrooms occurs in houses where the HVAC design did not include return grilles in each bedroom. Well, I should say it’s only a potential problem because a good installer will be working from an actual HVAC design, which should specify exactly how air gets back to the air handler. If a home has a central return vent for all the air going to the air handler, a good HVAC design will include return air pathways from the bedrooms. That could be jumper ducts (shown in Figure 3, below) or transfer grilles. Door undercuts typically won’t allow enough…