Balancing Hot and Cold Rooms

Q: When the bedroom doors are shut at night and the heat is running, my master bedroom gets unbearably hot. Other than keeping the doors open, what can be done to remedy this problem?

A: Keeping interior doors open may be the cheapest solution, but it is not always feasible. Between family members going to bed early, late-night television and pesky pets, we feel your pain.

The Problem

When interior doors are closed, the heated air cannot circulate enough to maintain a consistent temperature to match the thermostat setting.

Most homes with forced air systems have one air return. If 10 supply registers blow 100 cubic feet per minute (CFM) each, the air return will pull in about 1,000 CFM. One cubic foot is the volume of five Crispix cereal boxes.

When interior doors are shut, the air return still tries to pull the air. Door undercuts are often too small because they are typically 1/4 inch by 30 inches (7.5 square inches), and most supply vents are 4 inches by 10 inches (40 square inches). Small rooms such as bathrooms are the only spaces for which door undercuts work fine.

When the air supplied to a bedroom cannot circulate back to the air return, the HVAC unit sucks air from somewhere else, which seeks the path of least resistance.

When visiting family over the holidays, I felt a cold breeze coming from an unlatched window and attic access panel in my bedroom. The bedroom door was shut and the air return that serves three bedrooms was in my room. All the air supplied to the adjoining rooms couldn’t make it back to the air return, so the air was getting sucked from the easy sources, the window and attic. Easy sources of air are also the most expensive because it takes more energy to heat a house that is sucking in vast quantities of 40 degree air.

The Solution

Have you ever noticed operable transom windows over interior doors in historic homes or schools? Transoms enable indoor air to circulate and help equalize room temperatures throughout a building, which is called pressure balancing. A transom would allow you to keep the bedroom door shut in the winter and prevent the room from getting blistering hot.

Modern solutions to pressure balancing include transfer grilles, jump ducts and multiple air returns.

Transfer grilles are a rectangular hole through an interior wall that is covered with a grille on each side. This “hole in the wall” allows bedroom air to cycle back to the air return. There are a few products made specifically for this purpose that also limit the amount of light and sound transmitted between rooms. This is an inexpensive solution and a good weekend project.

Jump ducts are grilles installed in the ceiling that connect a room without an air return to a room with an air return. This method is a little more time consuming in an existing home, but jump ducts don’t transmit light or sound like transfer grilles.

Air returns located in bedrooms and the main body of the home are also a terrific way to create circulation.

In any of these scenarios, it is best to consult with an HVAC contractor to make sure that the air supply is correct and that the pressure balancing technique is sized to match. It is possible to live in a house where the bedroom doors can be closed at night and everyone is comfortable. Sleep tight!