Jump Duct Size for a Bedroom With the Door Closed

A jump duct is sized from the closed bedroom's supply CFM so the room stays within 3 Pa of the hallway, and Building Science Corporation's table puts a 100 CFM bedroom at a 7.0 in. jump duct. That diameter is the bare opening, though. With BSC's 1/2 in. undercut on a 32 in. door and a Hart & Cooley 672 return grille at each end, our estimate on the maker's data comes to 12 x 8 grilles and a 9 in. duct for the same 3 Pa across the grilles.

A homeowner on r/hvacadvice asked whether a jump duct would help a master bedroom that sits at 79°F, door always shut, while the rest of the house holds 74°F. Below you'll find the 3 Pa math, the ENERGY STAR test the number comes from, grille and duct sizes from 50 to 200 CFM, and a field check to run before you cut two holes in the ceiling. Our free return air grille size calculator checks one grille at a time at its share of the airflow, which is how you'll use it here.

How big should a jump duct be for a closed bedroom?

Start from the room's supply CFM and BSC's orifice formula, Q = 1.07 x A x the square root of the pressure difference in Pa. At 3 Pa a 100 CFM bedroom needs 54.0 sq in. of free area. BSC credits a 1/2 in. undercut on a 32 in. door with 16 sq in., which leaves 38.0 sq in. and a 7.0 in. jump duct in its table.

Armin Rudd wrote the table for houses with a central return, where every closed bedroom needs a way back to the hall. Its first line states the goal.

Building Science Corporation RR-0005 § Transfer Grille Sizing Table

"Goal is to prevent pressurization of individual bedrooms when door is closed" - Armin Rudd, Building Science Corporation

The formula treats the opening as a square-edged orifice, with Q the room's supply CFM, A the free area in sq in. and the pressure difference in pascals. At 3 Pa the divisor is 1.07 x 1.732, or 1.853.

1. Get the room's supply CFM from the contractor's balancing report, or measure the register with the air handler on high, as the DOE's Building America Solution Center suggests.
2. For a master suite, add the bedroom, bath and closet supplies together. BSC sizes a suite's transfer path from that total.
3. Divide by 1.853 for the free area at 3 Pa.
4. Subtract the undercut, door width times gap height.
5. Size the jump duct for what's left, and then size its grilles, which the next sections cover.

Rudd's 2006 design guide for BSC uses the same 3 Pa and the same formula. It adds what it calls a rule of BSC's own, no jump duct under 6 in. for any room and usually 10 to 12 in. for master bedrooms. Above 250 CFM it suggests a dedicated return duct.

What pressure does the closed bedroom have to stay under?

ENERGY STAR's National Rater Field Checklist, Rev. 14, item 6.2, sets it at -3 to +3 Pa between each bedroom and the main body of the house with all air handlers running. Footnote 25 lets bedrooms with a design airflow of 150 CFM or more use -5 to +5 Pa. The limit applies to homes going for ENERGY STAR certification.

Three pascals is 0.012 in. wc, and 5 Pa is 0.020. Item 6.2 lists transfer grilles, jump ducts, dedicated return ducts and undercut doors as ways to get there, so the rater checks the result and leaves the method to you. Footnote 25 spells out the test.

• The fan runs at its highest design speed, and on a zoned system every zone calls at once.
• Doors between bedrooms and the main body of the house are closed. Closet and bathroom doors that open only from the bedroom stay open.
• The 5 Pa alternative is for bedrooms with a design airflow of 150 CFM or more.

ENERGY STAR National Rater Field Checklist Rev. 14 § Footnote 25

"The Rater-measured pressure shall be rounded to the nearest whole number to assess compliance." - EPA, energystar.gov

So a reading of 3.4 Pa rounds to 3 and passes. Don't design to that margin, though. BSC's table and Rudd's guide both size for 3 Pa.

Why is the 7 in. in the BSC table only the bare opening?

By our arithmetic, BSC's jump duct column is the round hole with the area left after the undercut. A 100 CFM room leaves 38.0 sq in., and a 7.0 in. circle has 38.5. A real jump duct passes air through two grilles in a row, and Hart & Cooley's 672/674 return grilles lose 0.017 to 0.020 in. wc each at 300 FPM, or 4.2 to 5.0 Pa.

Every row of the table fits the same way. At 150 CFM, 64.9 sq in. is the area of a 9.09 in. circle, and the table prints 9.1. BSC does size the grilles. RR-0005 assumes 75% net free area, so its table gives a 100 CFM room a grille 10 in. wide and 5.1 in. high, about 51 sq in. Rudd's guide also has you divide the transfer area by at least 0.8 for the finished grille size. Neither document adds up the losses of the two grilles a jump duct puts in a row. In the attic the air enters a ceiling grille in the bedroom, runs through the duct and leaves through a second grille in the hall, and each grille takes its own pressure drop.

Hart & Cooley's engineering data for its 672 return grille starts at 300 FPM face velocity. At that lowest tested speed, one grille already uses more than the whole 3 Pa budget. To share 3 Pa between two grilles, each has to run much slower.

Here's where the two methods part. A 10 x 6 grille is 60 sq in., more than the 10 x 5.1 in RR-0005's table, and the 10 x 6 in BSC's own list of grille free areas, an Ameri-Flow return grille from a 2001 Grainger catalog, has 51 sq in. free. Put an H&C 10 x 6 (Ak 0.28 sq ft) at each end, push 100 CFM through the pair and BSC's 16 sq in. undercut, and our estimate puts the room near 5.1 Pa, over the 3 Pa limit. It takes a pair of 12 x 8s to get under 3 Pa.

Our estimate is simple. It splits the room's pressure evenly between the two grilles and lets the undercut carry its share by BSC's formula. Below 300 FPM it scales H&C's pressure loss by the square of the face velocity, the same way our grille calculator does, and it leaves out duct friction and the boots. H&C tested these grilles as returns, so using the same loss for the hall grille, where the air leaves the duct, is an approximation.

Jump duct and grille sizes from 50 to 200 CFM

For each supply CFM, the table gives the smallest Hart & Cooley 672 grille for each end and a duct at least as large as that grille's net area. With BSC's 1/2 in. undercut, the grille pair alone holds the room under 3 Pa in our estimate. A 100 CFM bedroom takes 12 x 8 grilles and a 9 in. duct, and a 150 CFM suite takes 16 x 10 grilles and a 12 in. duct.

The 200 CFM row sits at 2.96 Pa, right at the line. Each grille in the table runs at about 154 to 168 FPM, well under the 300 FPM where H&C's tests begin. That's why the grilles look oversized next to a central return grille, which Rudd's guide designs at 350 FPM.

We sized each duct so its inside area is at least the grille's Ak, which keeps the duct from becoming the tightest point. The estimate leaves the duct and its boots out. If they cost one velocity pressure, about 0.2 to 0.4 Pa at these flows, the rows from 75 CFM up land at about 3.0 to 3.3 Pa, so on a long run or one with bends, go a size up. The small end lands on Rudd's 6 in. minimum and the 150 CFM row on his 12 in. The 175 and 200 CFM rows, at 14 in., go past the 10 to 12 in. he says master bedrooms usually get.

The undercut matters more than it looks. With no undercut to count on, the jump duct carries everything: 100 CFM takes 14 x 10 grilles and a 12 in. duct, and 150 CFM takes 14 x 14 grilles and a 14 in. duct, by the same estimate. BASC is warier of undercuts than BSC's table and puts it plainly.

DOE Building America Solution Center § Undercut Doors

"In most applications, the door undercut would need to be 1.5 inches or greater to allow enough air to pass." - U.S. DOE, basc.pnnl.gov

The same page notes that new carpet can obstruct that return path. If the gap under the door is small, carpeted or likely to be, use the no-undercut sizes.

Worked example for a 150 CFM master bedroom

Take a master bedroom with a design airflow of 150 CFM. With BSC's undercut, 16 x 10 grilles at each end and a 12 in. duct hold the room near 2.9 Pa in our estimate. The 5 Pa option for rooms of 150 CFM or more allows 14 x 8 grilles and a 10 in. duct, but that version lands at about 5.0 Pa with no margin left.

Say the room is designed for 150 CFM and its register measures 150 on high, all of it leaving through one hall door.

1. The undercut is 32 x 0.5 = 16 sq in. At 3 Pa it carries about 30 CFM (1.07 x 16 x 1.732 = 29.7), so the jump duct takes the other 120.
2. Each grille gets half the 3 Pa, 1.5 Pa or 0.006 in. wc. An H&C grille that loses 0.019 in. wc at 300 FPM hits 0.006 at about 169 FPM (300 x the square root of 0.006 / 0.019).
3. 120 CFM at 169 FPM needs 0.71 sq ft of Ak. The 16 x 10 has 0.73, the smallest size in the H&C table that's big enough.
4. 0.73 sq ft is 105 sq in., the area of an 11.6 in. circle, so the duct is 12 in. flex. That's the top of Rudd's 10 to 12 in. range for master bedrooms.
5. Check one grille. On the return air grille size calculator, choose the check for a listed Hart & Cooley size, pick 16 x 10 and enter 120 CFM. It reads about 164 FPM and 0.0057 in. wc, which is about 1.4 Pa, and labels that loss an estimate because it's below the tested 300 FPM. Two grilles at 1.4 Pa keep the room under 3 Pa.

Now the 5 Pa option. At 5 Pa the undercut carries about 38 CFM and each grille gets 2.5 Pa, so a 14 x 8 (Ak 0.51) with a 10 in. duct just makes it, at about 5.0 Pa. That leaves nothing for the duct loss the estimate ignores. We'd still build to the 3 Pa sizes. Footnote 25's 5 Pa alternative is a pass limit, and designing right to it leaves nothing in hand.

Will a jump duct cool a bedroom that runs 5 degrees hot?

A jump duct fixes room pressure. Measure that first. With the air handler on high and the door shut, read the bedroom against the hall with a manometer, the way BASC describes. If the room reads within 3 Pa, a jump duct has little pressure to relieve, so measure the supply register and compare it with the room's design CFM.

The thread doesn't give a CFM or a pressure reading, only the 5°F gap. BASC notes that a bedroom "will typically be pressurized (positive) when the doors are closed," and its test needs only a manometer and some tubing.

1. Set the air handler to its highest speed, close the interior doors and open any closet or bath door inside the bedroom.
2. Run the manometer tube under the bedroom door. BASC says the reference port can stay open.
3. Read the room against the hall. Above +3 Pa after rounding, the room needs a return path, sized in the table above from its design CFM or the balancing report's figure.
4. Within 3 Pa, measure the supply register on high and compare it with the room's design CFM.

The design CFM comes from the room loads, worked step by step in our guide to room CFM from Manual J room loads. Room-by-room Manual J is in the paid Pro plan, and the free whole-house Manual J gives the total load only. If the room reads within 3 Pa and its register still delivers less than the design CFM, the shortfall is on the supply side.

When the manometer does call for a return path, take the room's design CFM to the table, pick the grille pair, and check each grille on the return air grille size calculator with the jump duct's share of the airflow. The page doesn't size the jump duct itself, only one grille at a time. It takes 25 CFM and up, so the 50 CFM row, where the jump duct carries about 20 to 23 CFM, is read from the table alone. The supply branch feeding the room is a separate job. The duct sizing calculator sizes the supply trunk and gives every branch the same CFM, and the duct velocity and friction calculator sizes one branch at the room's own CFM.

Sources

• Armin Rudd, Building Science Corporation, RR-0005 Transfer Grille Sizing (2000), Transfer Grille Sizing Table: buildingscience.com
• Armin Rudd, Building Science Corporation, Design Process for Sizing Cooling and Heating System Capacity, Room Air Flows, Trunk and Runout Ducts, and Transfer Air Ducts (revised December 19, 2006), section 11: buildingscience.com
• ENERGY STAR Single-Family New Homes, National Rater Field Checklist, Version 3.1/3.2/3.3, Rev. 14 (revised January 15, 2025), item 6.2 and footnote 25: energystar.gov
• Hart & Cooley, Engineering Data, 672/674 Return Air Grille and 673 Return Air Filter Grille: hartandcooley.com
• U.S. DOE Building America Solution Center, Jump Ducts (updated September 30, 2022): basc.pnnl.gov
• U.S. DOE Building America Solution Center, Undercut Doors (updated September 30, 2022): basc.pnnl.gov

For estimation purposes only. Not a substitute for a licensed engineer, and not ACCA-approved for permit submission.