Closing air vents in unused rooms costs you money

Your air conditioner is not a faucet. Closing a vent doesn’t tell it to move less air, it tells it to move the same air through a smaller opening, and everything downstream of that gets worse.

The instinct behind it is completely reasonable. You’re not heating and cooling a room nobody uses, so why pay for it. But the machine doesn’t work the way the faucet in your kitchen works, and the research says closing air vents raises your total energy use instead of lowering it.

Here’s what actually happens, and what to do instead if you’re trying to fix a room that never gets comfortable.

The thermostat said 66. The house was 77.

We went out to a Valley home with three separate systems. Her husband had booked the appointment. She didn’t know anything was wrong with any of them.

Two of the three weren’t working correctly.

The first thing we saw was the thermostat calling for 66 degrees in a house sitting at 77. That system had been running and running and losing. Most of the supply vents on it were closed.

The outdoor coils on that system and on one of the others were completely caked over. Two of the three had clogged condensate drain lines. All of it on systems somebody had serviced earlier that summer.

We opened the vents, cleaned the coils, cleared the drains, and left with all three running.

Why closing air vents doesn’t reduce the work

Allison Bailes, who holds a PhD in physics and trains energy raters through RESNET, put the mechanism plainly: “When you start closing registers in unused rooms, you make the duct system more restrictive. The pressure increases.”

That pressure has to go somewhere, and where it goes depends on what kind of blower you have. An older single-speed blower simply moves less air than it’s supposed to. A newer variable-speed blower does the opposite, it senses the restriction and works harder, drawing more electricity to push against a duct system you just narrowed.

Neither one saves you anything. Bailes cites research by Iain Walker at Lawrence Berkeley National Laboratory finding that closing registers led to increased energy use, and notes the effect holds across every climate zone.

So the trade you think you’re making, less air to a room you don’t use in exchange for a smaller bill, isn’t the trade that happens.

What happens instead

Your ducts leak more. Raise the pressure inside a duct system and more air escapes through every seam and gap it has. Bailes points out that most homes don’t have sealed ducts to begin with, so the higher pressure means more leakage. That air isn’t going to the closed room or the open one. It’s going into your crawl space. ENERGY STAR already estimates that about 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes, and poor connections. Closing vents pushes on that number in the wrong direction.

Your indoor coil gets too cold. Airflow across the coil is what carries heat into it. Starve the airflow and, in Bailes’ words, “it’ll dump less heat into the coil in summer, and the coil will get colder.” Cold enough and the condensation on it turns to ice, and an iced coil moves no air at all. That’s how a vent you closed in March becomes a system that quits in July.

The room doesn’t stay out of it anyway. A closed door and a closed vent don’t make a sealed box. That room still shares walls, gaps, and a ceiling with the rest of your house, and in an Alabama August it gets hot and gives that heat back to the rooms you do use.

Go look at your outdoor unit

The other half of the airflow problem lives outside.

Your condenser coil has to reject the heat your house is dumping into it, and it does that by pulling air through a set of fins. Pack those fins with grass clippings, cottonwood, dryer lint, or dog hair and the whole system starts fighting itself.

Bryan Orr of HVAC School measured a single condenser before and after cleaning it. Head pressure dropped from 278.6 psi to 216.6. Capacity rose about 8 percent. Efficiency, measured as EER, went from 7.32 to 8.86, a jump of roughly 21 percent. Power draw fell 10.5 percent and the compressor pulled about an amp less.

That’s one cleaning on one unit, so treat it as an illustration rather than a promise about your house. But it’s a useful illustration, because it shows a dirty coil isn’t a cosmetic problem. High head pressure means the compressor works harder and runs hotter, and compressors that run hot don’t last as long as ones that don’t.

You can see this one yourself. Walk outside and look at the fins on the outdoor unit. If you can’t see through them, that’s the problem.

If you’re closing vents to even out a hot room

This is the part most articles skip, and it’s the part that actually matters.

People don’t close vents at random. They close them because one room is freezing and another never gets comfortable, and shutting a register is the only lever they’ve been handed. That’s a real problem and it deserves a real answer instead of a lecture.

The real answer is usually duct design or airflow balance. Undersized returns, a run that’s too long, a room that’s fighting the sun through three windows. Those things get measured, and once they’re measured they get fixed, sometimes with dampers, sometimes with a duct correction, sometimes with a zoning setup that closes vents properly instead of improvising.

The version where you guess with a register lever is the version that ices a coil.

If one room in your house has never been comfortable

Get the airflow measured before you spend money on anything else.

That means static pressure, temperature split, and a look at where the ducts actually run, written down with the numbers on the page. Once you know what the system is really doing, the fix is usually smaller and cheaper than people expect.