· by Michael Blair, Nashville's Air Duct Cleaning
The logic seems sound: close the registers in rooms nobody uses and the conditioned air goes to the rooms people are in, with less to heat or cool overall. A forced-air system does not work that way. The blower moves a fixed volume of air through a duct system that was sized for every register open, and closing some of them changes the pressure in the ducts rather than the destination of the air.
Where the air actually goes
When a register closes, the blower keeps pushing the same amount of air into a system with fewer exits. Static pressure in the supply ducts rises. Some of the extra air does reach the open registers, but a large share goes out through the duct leaks that every system has, at joints, at boots and at the plenum. In most Nashville houses those ducts run through an attic or a vented crawl space, so the air you paid to condition is delivered there.
Duct leakage rises with pressure, so closing vents does not just fail to save the air from the closed room; it increases the loss from the whole system. A house with typical duct leakage can lose more to the attic from closed registers than it would have delivered to the room.
What it does to the equipment
Higher static pressure means less airflow across the coil and the heat exchanger. In cooling, reduced airflow lets the evaporator coil get too cold, and it ices over; the system then blows warm air until it thaws. In heating, a gas furnace with restricted airflow overheats and trips its limit switch, cycling on and off, and repeated overheating shortens the life of the heat exchanger. Heat pumps with resistance backup run the strips more.
The blower itself works harder against the restriction. Fixed-speed blowers move less air and run hotter; variable-speed blowers ramp up to hold their airflow target and draw more power doing it, which erases the saving the closed vents were supposed to produce.
- Frozen evaporator coil in summer
- Furnace short-cycling on the high limit in winter
- More duct leakage into the attic or crawl space
- Higher blower energy on variable-speed systems
The closed room itself
A room with its supply closed and its door shut still has a return path, or it does not, and either way it causes problems. If the door is closed and there is no return, the room depressurizes slightly or, with the supply closed, sits at whatever the outdoors is doing. In summer that means humidity migrating in, and a closed-off room in Middle Tennessee in July grows mildew in the closet and on the north wall.
In winter an unheated room on an outside wall can get cold enough for condensation on the windows and, in an extreme cold snap, for a supply pipe in that wall to freeze. Keeping a modest flow of conditioned air to every room is how the house as a whole stays dry and even.
What works instead
If the goal is comfort, the fix for a room that is too warm or too cold is balancing, not closing: adjusting dampers in the ducts, not at the registers, so each room gets the share it needs, with the total airflow unchanged. If the goal is saving on a rarely used part of the house, the answer is zoning, which uses motorized dampers and a control that reduces blower output to match, or a ductless unit for the space that is used differently from the rest.
Short of that, the most effective savings are the ordinary ones: sealed ducts, a clean filter, a thermostat schedule, and attic insulation that stops the house losing what the system delivers. Closing a few registers is not on the list, and on most systems it is on the list of things not to do.