Does Closing Vents Hurt Your HVAC System

By our estimate, closing one or two of ten equal supply registers on a 0.50 in. w.c. system raises external static pressure 0.03 to 0.14 in. In Lawrence Berkeley National Laboratory simulations of a California house with attic ducts, closing 1 to 3 registers never saved energy and raised design-day gas use by 0% to 39%. Close 3 of 10, and by the same estimate a blower that holds its airflow has to push 0.66 to 0.76 in. to do it.

The question keeps coming up from homeowners who shut the vents in a guest room or two to "save on heating." The answer depends on two things you can check on the job: how much of the static budget sits in the branch runs, and whether the blower lets airflow fall or holds it. Our free fan law calculator does the branch math below, and the static pressure calculator subtracts the component drops you enter from the blower's ESP.

Does closing one or two vents hurt the system?

It depends on how much room the system has. In LBNL's lab system with 10 registers and about 1,200 CFM, closing the far registers first barely moved airflow until the last couple were shut. With tight ducts and the near ones closed first, airflow fell 20% at six closed. Leakage is the cost: supply boot leakage rose from 5% to 55% of air handler flow as all ten closed.

The lab work is LBNL report 54005 by Iain Walker, November 2003. Its test duct system had ten supply registers, a single return, flex duct in a crawlspace, and register flows from about 50 to 250 CFM. The team closed registers one at a time, farthest from the air handler first and then nearest first, and measured flows, pressures and leakage at every step.

LBNL set about 20% as the most airflow loss it would accept, because lower airflow cuts heat exchanger efficiency, "particularly for cooling systems." With no added duct leaks and the near registers closed first, the system hit that line at six closed. With the far registers closed first, "only closing the last couple of registers has any significant effect."

LBNL-54005 § Executive Summary

"Closing too many registers (more than 60%) is not recommended because the added flow resistance severely restricts the air flow though the system leading to safety concerns." - I.S. Walker, Lawrence Berkeley National Laboratory, escholarship.org

The report gives two examples of what can happen past that point: furnaces may run on the high-limit switch, and cooling coils may freeze.

What happens to static pressure as registers close?

Each open branch picks up the closed ones' air, and its pressure drop rises with the square of its new flow. In our example with ten equal branches and 0.15 in. of a 0.50 in. budget in them, closing 3 of 10 at a held 1,200 CFM takes ESP to 0.66 in. With 0.25 in. in the branches, it's 0.76 in. Four closed reaches 0.77 to 0.94 in.

A fixed duct system follows S.P. = K x CFM squared, as CaptiveAire puts it, and "once any element of the system changes" the curve changes. Closing a register is that kind of change for the whole system. Each open branch, though, is still the same duct with the same boot and register. Only its airflow went up, so the square law still applies branch by branch.

Here's the worked example. It's our own estimate, not a measurement:

1. Ten equal branches share 1,200 CFM, so each carries 120 CFM.
2. The external static budget is 0.50 in. w.c. Say 0.15 in. of it is in the branch run, boot and register, and 0.35 in. is in the coil, filter, trunks and return. We also ran 0.25 in. in the branches.
3. Close 3 registers. Seven branches now share 1,200 CFM, which is 1,200 / 7 = 171.4 CFM each.
4. The branch drop goes from 0.15 to 0.15 x (171.4 / 120) squared = 0.31 in. The shared part stays at 0.35 in. at 1,200 CFM.
5. New ESP is 0.35 + 0.31 = 0.66 in. With 0.25 in. in the branches, 0.25 + 0.25 x (171.4 / 120) squared = 0.76 in.

We treat a closed register as fully shut and the ducts as tight. LBNL notes that without leaks, closing registers "would just increase system pressures and reduce total air flow." In the lab, the leakiest duct setup lost the least airflow, because the leaks gave the air another way out. With ducts in an attic or crawlspace, that air ends up outside the conditioned space. A register that doesn't seal fully still sends some air into its own room.

The last column moves the system curve against Goodman's published blower table for that furnace, so airflow drops a little and ESP rises less. Our calculation script interpolates between the table's 0.1 in. steps.

Why does the blower type change the answer?

A blower that lets airflow fall, like the multi-speed ECM in Goodman's GR9S80, gives up some CFM and settles at a lower pressure. A constant-airflow ECM, like the variable-speed motor in Goodman's GMVC96, holds its CFM and takes the full pressure rise. Goodman lists the same cooling CFM from 0.1 to 0.8 in. and tells you not to run the GMVC96 above 0.5 in. w.c. in heating, so 0.50 in. leaves no room.

Look at the GR9S800603B, a 60,000 Btu/h input furnace, on tap F02, its default heating speed. Goodman lists 1,161 CFM at 0.5 in., 1,125 at 0.6, 1,087 at 0.7 and 1,073 at 0.8. That's why the table shows only 1.9% less airflow with two of ten closed and 5.8% less with four. In Goodman's fan and cooling table, the same tap draws 257 W at 0.5 in. and 286 W at 0.8 in.

The GMVC96 sheet lists the same cooling CFM for each tonnage "@ .1" - .8" w.c. ESP." The GMVC960603BNB moves 1,200 CFM on its 3-ton cooling setting across that whole range. Above 0.8 in. the sheet gives no airflow, so every cell of the table above 0.8 in. is outside anything Goodman publishes. That's four closed with 0.25 in. in the branches, and everything at five and six closed.

Goodman SS-GMVC96 § Airflow Data

"Do not operate above .5" w.c. ESP in heating mode. Operating CFM between .5" and .8" w.c. is tabulated for cooling purposes only." - Goodman Manufacturing, goodmanmfg.com

By our estimate, a GMVC96 that already runs at 0.50 in. in heating goes past that limit with one register closed. All three ESP columns of the table rise above 0.50 in. at one closed. Those limits belong to those two Goodman series. For any other furnace or air handler, open its own blower table, find the column for the ESP you expect, and check whether the CFM holds or falls.

Does closing vents save energy?

No, not in the LBNL work. The building load does drop when rooms go unconditioned, but in the lab higher duct pressure pushed more air out of leaks. Supply boot leakage climbed from 5% to 55% of air handler flow as registers closed, up to all ten. In a simulated California house with attic ducts, closing 1 to 3 registers raised gas use 0% to 39% across three climate zones and two leakage levels.

The lab numbers show where the energy goes. As registers closed, leakage at the supply boots rose almost tenfold, from about 50 CFM to 500 CFM, and leakage at the supply plenum went from 10% to 35% of air handler flow. Return plenum leakage stayed the same share of flow, because supply registers don't change the return's resistance.

LBNL then ran its REGCAP model on a two-story, 1,800 sq ft California Title 24 house with R-4 flex ducts in the attic, heating at 68°F on a design day. Each closed register took about 10% of the house out of the conditioned space. The result held "even for the low leakage configuration."

LBNL's Table 11 gives these as 24-hour averages, with low leakage at about 5% of air handler flow and high at about 23%. The ducts were in the attic. With ducts inside conditioned space, leaked air stays in the house, and the study didn't model that case.

How many registers can you close, and which ones?

If a customer insists, close the registers farthest from the air handler, close as few as you can, and stay well under LBNL's 60% line. With four closed, LBNL measured average boot pressure 3 to 4 times higher when the near registers were shut than the far ones, and about double the leakage. Then measure ESP and compare it with the blower table.

Far registers matter less because closing one mostly changes pressure in its own branch. Closing a register next to the plenum raises pressure across the whole system, and every leaky boot and seam on the supply side feels it.

Partly closing a register sits between open and fully shut: some flow stays in that branch, so the others pick up less. It's still a change to the system curve, so the only way to know where the system landed is to measure.

How to check a system before you close registers

Measure total external static pressure with everything open, find the branch share of it, and run the new airflow per open register through the fan law. Add the shared part back and compare the result with the blower table for that unit. If ESP passes the manufacturer's limit or airflow drops toward LBNL's 20% limit, leave the registers open.

1. Measure ESP at the furnace or air handler with all registers open. The static pressure calculator has a budget mode that subtracts the filter, coil, register and grille drops you enter from the blower's ESP.
2. Estimate the drop in one branch run, boot and register at their design flow. Whatever is left belongs to the coil, filter, trunks and return.
3. Work out how much each open branch grows. With the blower holding total airflow, our simplification for branches fed from one plenum scales every open branch by the same factor, total CFM divided by the sum of the open registers' design CFM. With ten equal 120 CFM branches and three closed, that's 1,200 / 840 = 1.43, so each open branch goes to 171.4 CFM.
4. In the fan law calculator, choose "Speed, pressure and power for a target airflow," enter a branch's design flow as the known airflow, that flow times the factor as the target, and the branch drop as the known static pressure. Clear the speed and brake horsepower fields, which this mode fills with example values. The calculator takes a target up to 2 times the known airflow, which covers closing 5 of 10 equal registers.
5. Add the shared part back and read the blower table at that ESP. A blower that holds CFM shows you the pressure it has to make; one that lets CFM fall shows you the airflow the coil gets.
6. After closing, measure ESP again. The fan law calculator itself warns that a closed register changes the system, so the measurement is the real answer.

On a furnace, lower airflow also raises the temperature rise. Check it against the furnace's listed rise range with our furnace temperature rise calculator before you leave.

Sources

• I.S. Walker, Register Closing Effects on Forced Air Heating System Performance, LBNL-54005, Lawrence Berkeley National Laboratory, November 2003, escholarship.org
• CaptiveAire, Designing Air Flow Systems, captiveaire.com
• Goodman, SS-GR9S80_GD9S80-R32 specification sheet, fan, cooling and heating airflow data, goodmanmfg.com
• Goodman, SS-GMVC96 specification sheet, airflow data, goodmanmfg.com
• r/hvacadvice thread with the question, "Should you close vents to save energy?", reddit.com