Closing the register in a guest room that never gets used sounds like it should save fuel, the same way turning off a light in an empty room does. Ductwork does not work like a light switch. A forced air system is built to push a set volume of air out through a set number of openings, and closing some of those openings does not stop the blower from pushing that air, it just forces the system to find another way out. In a lot of farmhouses in Delaware Township and Tewksbury, where the ductwork was added decades after the house itself was built and often runs through an uninsulated cellar or a crawl space, that other way out is a seam or a joint in the duct that was never airtight to begin with.
What happens behind a closed register
Shutting a damper or a register grille raises the static pressure inside that branch of the duct. The blower keeps moving the same amount of air it was built to move, and air under higher pressure finds the path of least resistance, which in an older duct run is usually a seam in the sheet metal, a gap where a duct meets a boot, or a joint that was taped once and has since dried out and cracked. Air pushed out through one of those gaps, into a cellar or a crawl space instead of into the room it was meant to heat, is air the furnace or boiler already paid to heat and never gets credit for. Closing one register in a tight, well-sealed duct system barely matters. Closing several in a duct system with loose joints sends more air out through the leaks instead of the registers, and the house as a whole ends up calling for more fuel to make up for what left the system early.
What the California duct study found
A study from the Lawrence Berkeley National Laboratory looked at houses in California with ducts running outside the living space, mostly through attics and crawl spaces, closer to what a lot of older Hunterdon County farmhouses have in a cellar or under an addition than to a house with ducts built entirely inside conditioned walls and floors. The study found that closing registers in unused rooms raised energy use rather than lowering it, because the closed registers pushed more air out through existing leaks in the duct system instead of cutting the total air the blower moved. Closing the registers farthest from the air handler cut the penalty somewhat, since air traveling the longest distance through the duct has more chances to leak out along the way regardless of whether the register at the end is open. The study also found that closing off more than about half the registers in a house risked pushing the furnace onto its high-limit switch, a safety cutoff that shuts the burner down when the air moving across the heat exchanger drops too low to carry the heat away fast enough, which can shorten a furnace's running cycles and leave rooms colder rather than warmer.
Why a leaky cellar duct system makes this worse
The farmhouses common in Delaware Township and Tewksbury typically had the furnace and ductwork added well after the house itself went up, run through a cellar, a crawl space, or along the underside of a first floor that was never built with duct chases in mind. Joints in that kind of system were often fitted around joists, posts and old stone foundation walls rather than laid out in straight, sealed runs, and connections made decades ago have had plenty of time to loosen, dry out, or pull apart at a seam. Closing a register in a bedroom over that kind of basement run sends the air the furnace already heated straight into an unconditioned cellar instead of the room, and the house as a whole works harder to make up the difference. A duct system with tight, sealed joints does not have nearly as much to lose that way, which is part of why the same closed register behaves differently from one house to the next.
A different case: ducts built for cooling only
Some houses in this area heat with a boiler and baseboard or radiators and only have ductwork at all because central air conditioning was added later, with ducts and registers built just for cooling rather than tied into the heating system. Closing a register in a room that only gets cooled through that ductwork, during a stretch when the air conditioning is not running anyway, does not carry the same penalty as closing a heating register, since there is no burner cycling and no blower moving warm air through a leaky duct run at the time. The duct study's findings apply to a system moving heated or cooled air on a regular cycle, not to a cooling-only branch sitting idle in the off season, which is worth keeping separate when deciding what to close and when.
What Air Duct Repair does instead of shutting vents
Sealing and balancing the duct system does more for a house than closing registers ever will. Air Duct Repair covers resealing the seams and joints that let air escape into a cellar or crawl space in the first place, so the air the furnace heats reaches the rooms it was meant for instead of leaking out along the way. Balancing the system by adjusting dampers at the branch takeoffs, rather than shutting registers fully at the room, lets a technician send less air to a guest room or a rarely used parlor without creating the high static pressure that pushes air out through a leak somewhere else in the run. Air Duct Cleaning is often done alongside a repair, since a duct run that is being opened up to find and seal leaks is also the easiest time to clear out any dust or debris built up inside it. Together, the two address a duct system losing conditioned air before it reaches the room, instead of treating the register as the only thing standing between the furnace and the fuel bill.
Older farmhouses and fuel oil in Delaware Township and Tewksbury
Census estimates for Delaware Township and Tewksbury show fuel oil heating more than half of homes in one and about a third in the other, a reminder that a lot of houses here are running older systems added to an existing farmhouse rather than ductwork and equipment installed together as one project. A homeowner trying to cut an oil bill by closing off rooms that rarely get used has a reasonable instinct, but the duct system underneath the house is usually the better place to look first. A technician checking the branch runs in a cellar or crawl space for loose joints, open seams and dampers that were never set right in the first place finds the leaks a closed register cannot fix, and seals them before deciding which rooms, if any, are worth running at a lower airflow.