Last reviewed: September 2026 · By the HVAC Brief Editorial Team · Our sourcing and editorial standards
A ducted system has two kinds of vent: supply registers, which blow conditioned air into rooms, and return grilles, which pull air back to the equipment. Supply registers usually have adjustable louvres and are smaller; returns are larger, have fixed grilles, and often have the filter behind them.
The most common mistake is closing supply vents in unused rooms to save energy. In a typical system it does not reduce what the equipment does; it raises pressure in the ducts, can increase leakage, and can freeze a coil or overheat a furnace.
Supply versus return
| Feature | Supply register | Return grille |
|---|---|---|
| Direction of air | Out of the vent, into the room | Into the vent, back to the equipment |
| Typical size | Smaller | Larger |
| Louvres or damper | Usually adjustable | Usually fixed |
| Filter | No | Sometimes, in a filter grille |
| Test | Air blows out when the system runs | A tissue held near it is pulled in |
Why closing vents backfires
A single-speed blower moves roughly the same air whether a vent is open or closed. Closing supply registers does not make the system produce less heating or cooling; it forces that air through fewer openings.
- Static pressure rises in the duct system, and the blower works harder or moves less air overall.
- Duct leakage increases, because higher pressure pushes more air out of every gap, often into an attic or crawl space.
- Low airflow over the coil in cooling can freeze it; see why an AC freezes up.
- Low airflow over a furnace heat exchanger trips the high-limit switch and shortens equipment life.
Variable-speed and ECM blowers respond by ramping up to hold airflow, which uses more electricity rather than less; see blower motors. If uneven rooms are the real problem, the answer is balancing or a zoning system designed for it, not closed registers.
Many houses have plenty of supply registers and too little return. Closed interior doors make it worse: a bedroom with a supply register and no return pressurises when the door is shut, so air stops flowing into it. Transfer grilles, jumper ducts or door undercuts restore the path. Blocked returns, behind furniture or under rugs, starve the whole system.
Placement, and why vents are where they are
- Supply registers under or near windows are traditional in heating climates, to counter cold surfaces and draughts.
- High supply registers are common in cooling-dominated homes, because cool air falls.
- Central returns are cheaper to build; returns in each room, or transfer paths, work better with doors closed.
- Kitchens and bathrooms generally do not get returns, to avoid pulling odours and moisture into the system.
Registers that whistle, or rooms that never reach temperature, often point to duct sizing rather than to the vent itself; see ductwork and airflow.
Simple things that help
- Keep every supply and return clear of furniture, curtains and rugs.
- Leave supply dampers open unless a technician has balanced the system deliberately.
- Vacuum grilles; replace filter grilles on schedule.
- Avoid magnetic vent covers as an energy measure.
- If a room is always too hot or too cold, ask for airflow to be measured at the registers.
Frequently asked questions
Should I close vents in unused rooms?
Generally no. In a typical ducted system the blower moves the same air regardless, so closing supply vents raises duct static pressure, increases leakage and can cause a frozen coil or furnace overheating. A system designed for zoning is the proper way to condition rooms differently.
What is the difference between a supply vent and a return vent?
Supply registers deliver conditioned air into rooms and usually have adjustable louvres. Return grilles pull air back to the furnace or air handler, are usually larger with fixed grilles, and sometimes hold the filter. A tissue held near a return will be drawn toward it.
Why is one room always hotter or colder?
Common causes are too little supply airflow to that room, no return path when the door is closed, long or leaky duct runs, or large windows and poor insulation. Measuring airflow at the register is the starting point; closing vents elsewhere rarely fixes it.
Can blocking a return vent damage my system?
It can. A blocked return restricts airflow for the whole system, which can freeze the evaporator coil in cooling, trip a furnace’s high-limit switch in heating, and make blower motors work harder and run hotter.
Methodology and limitations
This page describes airflow behaviour in residential ducted forced-air systems in general terms. Design practice for register and return placement follows industry procedures such as ACCA Manual D, which we name but do not reproduce.
- No federal source measures the effect of closing registers on energy use in existing homes.
- Systems designed for zoning behave differently from single-zone systems.
- Nothing here replaces airflow measurement on a specific system.
Sources
- Air Conditioning Contractors of America, Manual D residential duct design, named as the industry procedure.
The HVAC Brief is an independent trade publication. We do not sell equipment, take manufacturer advertising, or accept payment for placement in our research. Corrections welcome.
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