The supply vents in your home probably have dampers on them. But should you close them, even if only part of the way? In multi-story homes, some people adjust these dampers during the hottest and coldest times of the year.

During summer, they’ll partially or completely close the downstairs dampers so that warmer upstairs rooms get more attention from the air conditioner. When they’re running the furnace in winter, they’ll do the reverse. By closing the upstairs dampers, they reduce the amount of air going to the upper stories and direct additional warm air to colder rooms downstairs.

At least, that’s how the thinking goes. But it’s not always that simple.

There’s this little problem called “static pressure.”

And most HVAC systems already have too much of it.

To effectively cool or heat your home, your HVAC system contains a large blower fan that pulls air from return grilles and blows the same (now conditioned) air through supply ducts and, ultimately, through your supply registers. That’s a lot of air moving through a lot of ductworks! Static pressure is a quantitative measurement that refers to the amount of resistance that air encounters as it moves through the system.

Some resistance is good. Otherwise, you’d have very little airflow and conditioned air wouldn’t circulate well throughout your home.

However, it’s common to have too much static pressure. That is, the air encounters excessive resistance in the system. When this happens, the system is trying to move a large volume of air through an undersized or restricted pathway. Potential negative effects include:

  • Poor airflow to certain areas
  • Hot and cold spots throughout your home
  • Lots of noise from your air ducts
  • A frozen AC coils
  • Higher utility bills
  • Overworked equipment that fails prematurely
  • A cracked heat exchanger: it’s a rare occurrence, but low airflow can cause the heat exchanger to become too hot and crack. This is actually a very dangerous problem to have!

Why do many systems experience high static pressure? Common reasons include:

  • Dirty air filters that block airflow
  • Undersized ductwork that inhibits air circulation
  • Damaged, bent, or sagging flexible ducts that prevent air from getting to or from the system

If you’re thinking, “Hey, closing off my air supply dampers might also increase static pressure,” you’d be right! A fully or partially closed damper pushes back against the air being forced through the system by the blower fan. It increases air resistance inside your ductwork.

Closing your supply dampers might get the results you’re after… for a while. Just know that it might backfire.

At the time of writing, it’s August here in Atlanta. It’s hot. Many people in two-story homes are partially closing downstairs supply registers. In seldom-used rooms, they might be closing them completely.

In some cases, these folks might get what they’re looking for: a cooler, more comfortable environment upstairs. They might even be able to adjust the dampers without impacting HVAC performance or putting undue strain on their air conditioner… even though they’re increasing static pressure.

But this tactic can backfire. Many systems have undersized ductwork and deal with excessive static pressure even with all the registers open! Closing them can compound an existing static pressure problem, leading to one or more of the issues described above.

Instead of closing your supply register dampers, do this:

Measure static pressure in your system.

If it isn’t time to replace your HVAC system and you’re not in a position to add zones, have a professional measure static pressure the system. Doing so will tell you:

  • Whether you’ve got too much static pressure; if you do, your system may require ductwork or blower speed adjustments to sort out the problem.
  • Whether closing supply air dampers will cause a problem; if there’s not too much static pressure in the system, closing off supply dampers might be ok! The only way to know is to measure static pressure and make sure it aligns with HVAC system manufacturer specifications, even when the dampers are closed.

Make sure your HVAC system and your ductwork are sized for your home.

When you home has proper HVAC design, you shouldn’t have to close dampers. The system will keep you comfortable with all the registers open.

If you’re on the cusp of an HVAC system replacement, have an experienced HVAC or home performance contractor perform a Manual J load calculation to determine the right size for your new system. Most HVAC contractors fail to complete this step and end up installing equipment that is oversized for the home.

The contractor should also perform a Manual D calculation to determine the right size for ductwork. By changing our existing (and often undersized) ducts, plenum boxes, grilles, and registers with properly sized replacements, you can reduce static pressure inside the system and make your house more comfortable. No supply dampers are needed.

Add zones to your HVAC system.

Multi-story homes are more comfortable with zones. In a typical zoned system, you’ve got two thermostats: one for upstairs and one for downstairs. You won’t have to fiddle with supply dampers because you can just set different temperatures on each thermostat.

When adding HVAC zones to an existing system, here’s something to keep in mind: Zones can work well when you have a variable speed HVAC system. Standard, single-speed systems? Not so much.

We actually wrote an article about why zones don’t pair nicely with single-speed equipment. Long story short: you have to be careful about how you set up the zones; otherwise, you might create a static pressure problem.

Bottom line: You probably shouldn’t close your air supply vents.

Excessive static pressure inside an HVAC system is never a good thing. Closing too many air supply dampers could put you in the static pressure “danger zone.”

Instead, have a pro measure the static pressure to see if you can safely adjust your dampers. If you can’t, consider one of the alternative solutions. In the long run, you’ll be a lot more comfortable!

company icon