Fresh Air Intake on the Return Duct and How Long to Run It

Under the 2021 and 2024 IRC, an outdoor air duct on the return can run as little as 25% of every 4-hour segment, as long as it moves the continuous ventilation rate times the Table M1505.4.3(2) factor while the damper is open. The factor is 4 at 25%, 3 at 33% and 2 at 50%, so a 1,800 sq ft house with 3 bedrooms that needs 48 CFM continuous by Equation 15-1 needs 144 CFM of outdoor air when the damper is open a third of the time.

Our free ASHRAE 62.2 ventilation calculator gives a continuous rate and says plainly that it skips intermittent operation. This article adds the run time and the CFM the duct has to carry while the damper is open, both from the IRC text. One limit up front: in Climate Zones 7 and 8 under the 2021 IRC, and 6, 7 and 8 under 2024, the energy chapter requires a heat or energy recovery ventilator.

Does an outdoor air duct on the return count as ventilation?

Yes. The 2018, 2021 and 2024 IRC all say an outdoor air duct connected to the return side of an air handler counts as supply ventilation. When it serves as the house's whole-house mechanical ventilation system under M1505.4, it has to meet the rate in M1505.4.3 and have a manual override control under M1505.4.2. The 2021 and 2024 energy chapters also want a damper that closes when it isn't running.

2021 IRC § M1505.4.1

"Outdoor air ducts connected to the return side of an air handler shall be considered as providing supply ventilation." - International Code Council, codes.iccsafe.org

The energy chapter line is N1103.6 in both the 2021 and 2024 IRC: "Outdoor air intakes and exhausts shall have automatic or gravity dampers that close when the ventilation system is not operating."

Whether the house needs mechanical ventilation at all is a separate question. In the 2021 IRC, R303.4 requires it, per M1505 or other approved means, for dwellings that comply with N1102.4.1, the building thermal envelope part of the air leakage section.

N1103.6.1 adds a climate limit. The 2021 IRC requires a heat or energy recovery ventilator in Climate Zones 7 and 8, and the 2024 IRC requires one in Zones 6, 7 and 8 as a balanced system. A return-side intake is supply ventilation, so by our reading it can't be the required system in those zones.

How much outdoor air does the code require?

Equation 15-1 gives 0.01 CFM per sq ft of floor plus 7.5 CFM per bedroom plus one. For 1,800 sq ft and 3 bedrooms that's 18 + 30 = 48 CFM continuous. Table M1505.4.3(1) gives 60 CFM for the same house, and by our reading the 2021 and 2024 text accepts either one. The ASHRAE 62.2 formula uses 0.03 per sq ft and lands at 84 CFM.

The 2021 and 2024 wording is a rate "not less than that determined in accordance with Table M1505.4.3(1) or not less than that determined by Equation 15-1." By our reading, that lets you use the lower of the two. The 2018 text says the rate is "as determined in accordance with Table M1505.4.3(1) or Equation 15-1."

Our 62.2 calculator runs the ASHRAE formula with the 0.03 coefficient and notes on the page that the IRC uses 0.01. If the job is permitted under the IRC, start from Equation 15-1 or the table. If a program or a jurisdiction asks for 62.2, use the page's number.

There's also a 30% credit in the 2021 and 2024 text, Exception 1, but it needs ducted supply to every bedroom and at least one living, dining or kitchen room, and a balanced system. A return-side intake is supply ventilation under M1505.4.1, so by our reading it doesn't qualify on its own.

How long does the damper have to stay open?

At least 25% of each 4-hour segment, which is 60 minutes, and the controls have to allow that much. While it's open, the duct has to carry the continuous rate times the Table M1505.4.3(2) factor, which is 4 at 25%, 3 at 33%, 2 at 50%, 1.5 at 66%, 1.3 at 75% and 1.0 at 100%. You can interpolate between columns anywhere from 25% to 100%.

2021 IRC § M1505.4.3, Exception 2

"The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25 percent of each 4-hour segment and the ventilation rate prescribed in Table M1505.4.3(1), by Equation 15-1 or by Exception 1 is multiplied by the factor determined in accordance with Table M1505.4.3(2)." - International Code Council, codes.iccsafe.org

Here's the table worked for the 48 CFM house. A 4-hour segment is 240 minutes.

The factors aren't exactly one over the run time. At 75% the code's 1.3 averages 46.8 CFM, a little under the continuous 48, and that's what the table prints. A timer set to 20 minutes every hour opens the damper 80 minutes per segment, which lands in the 33% column.

The 2018 IRC has the same 25% floor and the same factors, but its exception multiplies only "the ventilation rate prescribed in Table M1505.4.3(1)." By our reading, an intermittent system under 2018 starts from the table's 60 CFM for this house, which is 180 CFM at 33% and 120 CFM at 50%.

Which CFM has to meet the rate?

Only the outdoor air coming through the intake duct while the damper is open. The code asks the system to "provide outdoor air" at the rate, so the blower's airflow through the coil isn't the ventilation number. Under the 2021 IRC the flow is tested with a flow hood, flow grid or other device, which may sit in the connected ventilation duct, and the 2024 IRC points to ANSI/RESNET/ICC 380.

The air handler is the fan in this system, so the damper's open time only counts while the blower runs. If the thermostat calls don't keep the blower on for the minutes you picked, by our reading the controls have to run the fan for the rest.

At 144 CFM a 6 in. round duct runs about 733 FPM, since a 6 in. circle is 0.196 sq ft. In the friction and velocity mode of the duct velocity and friction calculator, with the duct set as a return branch, 6 in. at 144 CFM is flagged as above the page's 700 FPM Manual D maximum, and a 7 in. duct brings it to about 539 FPM. A longer run time also drops the target, to 96 CFM at 50%, where the 6 in. duct runs about 489 FPM.

Where the energy chapter's N1103.6 applies, the 2021 and 2024 IRC also set fan efficacy in Table N1103.6.2. An air handler that is integrated to tested and listed HVAC equipment needs at least 1.2 CFM of design outdoor airflow per watt of fan power. That's 120 W or less at 144 CFM and 80 W or less at 96 CFM, so a longer run time lowers the watt limit too. Check the blower's watts at its ventilation speed against it.

The outdoor air the intake pulls changes with blower speed, so measure and balance at the speed the blower runs during ventilation-only periods.

2021 IRC § N1103.6.3

"Mechanical ventilation systems shall be tested and verified to provide the minimum ventilation flow rates required by Section N1103.6." - International Code Council, codes.iccsafe.org

A ranch house from r/hvacadvice, worked through

The owner wants a roof intake into the return on a damper that shuts in very hot or cold weather. The thread gives no floor area or bedrooms, so take 1,800 sq ft and 3. That's 48 CFM continuous and 144 CFM at 20 minutes an hour. By our reading, at 144 CFM a lockout can't cut the damper below 80 minutes in any 4-hour segment, if the intake is the house's M1505.4 system.

The owner has a 1950s ranch over an encapsulated crawl space, already air sealed and insulated, and is buying a new system. The equipment closet is too small for an HRV or ERV, so the plan is an outdoor air duct through the roof into the return. A commenter runs the same setup, a 6 in. flex from the roof to a damper and on to the furnace return.

This is an existing house with new equipment going in. R303.4 ties mechanical ventilation to dwellings that comply with N1102.4.1, so whether M1505 applies here is the code official's call. If the intake is installed as the house's whole-house ventilation system under M1505.4, here is what it has to do.

1. Rate. Equation 15-1 at 1,800 sq ft and 3 bedrooms gives 48 CFM.
2. Run time. A timer at 20 minutes an hour gives 80 minutes per 4-hour segment, the 33% column.
3. Target while open. 48 x 3 = 144 CFM of outdoor air in the intake duct.
4. Duct. A 6 in. duct at 144 CFM is over the calculator's 700 FPM limit, so go to 7 in., or to the 50% column at 96 CFM, then set the balancing damper so the measured outdoor air reaches the target. Before cutting the roof, check the intake against R303.5.1, which puts outdoor air intake openings at least 10 ft from vents and chimneys or 3 ft or more below them, and sends vents of fuel-burning appliances to the termination rules of chapters 18 and 24.
5. Lockout. Exception 2 asks for controls that enable operation for at least 25% of each 4-hour segment, and with the duct balanced at 144 CFM the 33% column is the floor for every segment. By our reading, a lockout that keeps the damper closed through a whole hot afternoon segment fails Exception 2 when the intake is the house's M1505.4 system. Keep the 80 minutes per segment through the heat, or ask the inspector how they read the exception.
6. Override. M1505.4.2 asks for a manual override control, labeled with text or a symbol in the 2021 and 2024 IRC. The 2018 text asks only for the manual override.

Setting the run time before you quote the job

Get the continuous rate for the adopted edition, pick a run-time column, multiply, and size and balance the intake duct for that CFM. Then measure the outdoor air in the intake duct with the blower running and the damper open, and keep the report for the inspector. In all three editions the controls must allow at least 25% of every 4-hour segment.

1. Find the edition. The 2018 exception names only the table rate, and the 2021 and 2024 exceptions name the table, Equation 15-1 and the 30% credit.
2. Work out the continuous rate from Equation 15-1 or Table M1505.4.3(1). If the job runs on ASHRAE 62.2 instead, take the continuous rate from the ASHRAE 62.2 ventilation calculator and apply that standard's own rules.
3. Pick the run time and multiply by its factor from the table above.
4. Size the intake duct for that CFM and add a balancing damper.
5. Measure in the intake duct and adjust until the outdoor air meets the target while the damper is open.

Start with Equation 15-1 or Table M1505.4.3(1) for the adopted IRC edition, use the factor table above to set the timer, then run the CFM while open through the duct velocity and friction calculator, set as a return branch, to size the intake before you price the job.

Sources

• International Code Council, 2021 International Residential Code, Sections M1505.4.1 through M1505.4.3 and Tables M1505.4.3(1) and M1505.4.3(2), codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Sections M1505.4.1 through M1505.4.3, codes.iccsafe.org
• International Code Council, 2018 International Residential Code, Sections M1505.4.1 through M1505.4.3, codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Sections R303.4 and R303.5.1, codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Sections N1103.6 through N1103.6.3, codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Sections N1103.6 through N1103.6.3, codes.iccsafe.org
• Source thread on r/hvacadvice, an owner adding a fresh air intake with a damper to the return duct, reddit.com/r/hvacadvice