Zoning Bypass Damper Size for a Single-Stage System

Honeywell's zoning selection guide sizes a bypass at total system CFM minus the CFM of the smallest zone. On a 5-ton system moving 2,000 CFM with a 500 CFM smallest zone, that's 1,500 CFM and the top size on Honeywell's bypass chart, while Arzel's 2002 chart for the same system with branch dampers bypasses 500 CFM through a 7 in. duct.

That's a 3 to 1 spread on the same system. The zoning makers agree on the worst case, the smallest zone calling alone, and disagree on how much air that zone can take. Below, one system goes through five published methods, then through the rules that say to skip the bypass entirely. Total airflow comes first, and our free HVAC CFM calculator turns tonnage and CFM per ton into that number.

How do you size a bypass damper for a zoned system?

Start with the worst case, the smallest zone calling by itself. Take the blower's total CFM and subtract what that zone accepts, and the rest has to go somewhere. Honeywell subtracts the zone's design CFM. For 2,000 CFM and a 500 CFM zone that leaves 1,500 CFM, which lands on Honeywell's CPRD14, the size its chart lists for over 1,400 CFM.

We'll run one example system of our own through the whole page:

1. A 5-ton single-stage AC at 400 CFM per ton moves 2,000 CFM.
2. Two zones. The living zone takes 1,500 CFM at design and the bedroom zone takes 500 CFM.
3. The 500 CFM zone is the smallest, so it sets the worst case.

Our post on room CFM from room loads walks through ACCA's method for the room airflows that add up to each zone's design CFM.

Honeywell Zoning Selection Guide § Step 7

"SYSTEM CFM minus CFM of the smallest zone = Bypass CFM required" - Honeywell, crescentparts.com

Honeywell's chart, dated January 1, 2016, runs CPRD8 up to 600 CFM, CPRD10 up to 1,000, CPRD12 up to 1,400 and CPRD14 over 1,400. It adds that a bypass falling between sizes should go up a size. The same guide says zones with fairly close CFM work best, because a small zone means bypassing a larger share of the air.

Resideo's data sheet for its modulating round damper (MARD) uses the same subtraction. Its Table 1 rates a 12 in. MARD at 1,200 CFM and a 14 in. at 1,800, so 1,500 CFM needs the 14 in. Resideo recommends the MARD for systems over 2,000 CFM, so our system sits right at its lower edge.

Why do zoning makers give different bypass sizes?

They disagree on how much air the smallest zone takes once its dampers are the only ones open. Honeywell assumes design CFM. EWC also credits leakage past the closed dampers. With branch dampers, Arzel assumes that at 5 tons and under the blower drops to 300 CFM per ton, and the zone takes double its design flow. On our 2,000 CFM system that moves the bypass from 1,500 CFM down to 500 CFM.

EWC's own worked example is a 5-ton, 2,000 CFM system too. It subtracts a 500 CFM smallest zone, 300 CFM of leakage (20% of each of two closed 750 CFM zones) and 200 CFM of open runs, and gets a 1,000 CFM bypass. EWC lets the installer set the zone dampers' travel stops to leak up to 20% on large zones, and the 20% in the table comes from that setting. EWC says to match the bypass CFM to the closest size in its table and go down one size if that won't fit, and its own example puts a 1,000 CFM bypass on a 20 x 8 rated 900 CFM. The PRD sizes in the table follow that closest-size rule.

Arzel's case against the subtraction came out in ACHR News in 2002. Joe Ramunni of Arzel said the old formula "sidesteps the dynamics of zoning and, in most cases, drastically oversizes the bypass." His 3-ton example took 900 CFM at 300 per ton, minus 400 CFM for two 6 in. runs at double their design flow, for a 500 CFM bypass. The 300 CFM per ton start is for branch dampers. With trunk dampers, Arzel's 2002 chart says to start from 400 CFM per ton, because static pressure in the open supply duct rises faster. Arzel's chart also gives a screening rule: if the smallest zone moves less than 40% of total CFM at design static, "you might need a bypass duct."

How fast should air move in a bypass duct?

Faster than any other duct in the house. Arzel's 2002 chart sizes bypass ducts for 1,500 to 2,000 FPM, and Johnson Controls' Verasys note for commercial rooftop units, now marked discontinued, selects dampers at a maximum of 1,750 to 2,250 FPM. Resideo sizes by friction instead, at 0.25 in. per 100 ft. For 1,500 CFM in galvanized round, that gives 14 in. at about 1,400 FPM.

In our duct velocity and friction calculator, choose "Round size for an airflow and a friction rate," set the duct to "Galvanized round (rigid sheet metal)" and where it sits to "Supply trunk," then enter 1,500 CFM and 0.25 in. w.c. per 100 ft. It returns 14 in., 1,403 FPM and 0.188 in. per 100 ft. A 12 in. duct would run at 0.404 in. per 100 ft. The page will also warn that 1,403 FPM is over the 900 FPM Manual D maximum for a rigid supply trunk. That's Manual D's limit for supply ducts, and the bypass sources above size past it.

The charts don't agree on size for a given airflow either. Arzel's 2002 chart puts 1,150 to 1,600 CFM on a 12 in. round, which at 1,500 CFM runs about 1,910 FPM. Resideo's 0.25 in. friction rule puts the same airflow on a 14 in. Arzel's 7 in. bypass for 500 CFM runs at 1,871 FPM by the same calculator.

A few placement rules come with the charts:

• Arzel's 2015 chart says to go down one size in flex and keep the bypass at least 8 ft from the return drop when you can.
• EWC wants the bypass tap on the return at least 6 ft from the equipment where there's room, so the air mixes before the coil, and a balancing hand damper in the bypass duct.
• Arzel's 2002 chart calls for two bypass ducts above 2,100 CFM.

What does bypass air do to the coil?

It sends supply air straight back into the return, so the coil sees colder air in cooling and hotter air in heating. By our estimate, with 55°F supply air and a 75°F return, a 1,500 CFM bypass around a 500 CFM zone drops the air entering the coil to about 60°F on the first pass. Arzel's 500 CFM branch-damper bypass leaves it near 68°F.

Those two numbers are a dry-bulb mix only. Honeywell's sizing has 1,500 CFM at 55°F joining 500 CFM of 75°F return air, which is 59.9°F. With Arzel's branch-damper numbers, 500 CFM at 55°F joins 1,000 CFM at 75°F, which is 68.2°F. Our mixed air temperature calculator gives the same results in its "Mix outdoor air and return air" mode if you put the bypass stream in the outdoor air fields and clear the outdoor air wet-bulb and return air relative humidity fields, which come filled in. It's only the first pass. The coil cools that colder air further, the next pass comes back colder still, and we didn't model where that ends.

The California Energy Commission's 2025 compliance manual names the problem directly: a bypass sends "excessively hot air (heating mode) or excessively cold air (cooling mode) back into the system." Arzel's chart says bypass air "does not provide BTU exchange at equipment." EWC makes a supply air temperature sensor mandatory on its zone systems, to protect the heat exchanger from overheating and the DX coil from freeze-up.

When should a zoned system have no bypass at all?

When the controls or the code say so. Carrier's Infinity zoning instructions say a bypass must not be used, and each zone has to carry the system's minimum airflow instead. California's 2025 Energy Code, Title 24 Part 6, bars bypass ducts on the single-family prescriptive path unless the certificate of compliance allows one, and requires more than 350 CFM per ton in every zonal mode.

Carrier UIZ01-5SI § Design Considerations

"The Infinity Zone system is unique because a bypass damper must not be used." - Carrier, shareddocs.com

Carrier's instructions for the Infinity Zone Control add three design rules. Each zone should deliver at least the system's minimum airflow in heating and in cooling, ductwork should be oversized by 25% to keep noise down at minimum zone airflow, and heating airflow may be higher than cooling. Carrier credits "the intelligence of the system and variable speed motor technology" for working without one, so the no-bypass rule is specific to Infinity systems.

Title 24 Part 6, 2025 § 150.1(c)13

"Unless otherwise specified on the Certificate of Compliance, bypass ducts that deliver conditioned supply air directly to the space conditioning system return duct airflow shall not be used." - California Energy Commission, energy.ca.gov

Title 24 section 150.0(m)13C is a separate mandatory requirement on airflow. In a single-family home, a zoned central cooling system has to deliver more than 350 CFM per ton "in every zonal control mode," from the house, through the air handler, and back to the house. For 5 tons, that's over 1,750 CFM with only one zone calling. By our reading, air that goes from supply to return through a bypass never reaches the house and doesn't count. Our 500 CFM bedroom zone would have to take 3.5 times its design flow by itself. Multispeed and variable-speed compressor systems are tested at full capacity with every zone calling instead. On the performance path, a bypass is allowed only if it's modeled and shown on the CF1R.

Outside California, check what your jurisdiction adopted. EWC, which recommends a bypass damper on every zoning install, also says to specify modulating or multistage equipment when you zone, if you can.

What to check on a single-stage system zoned without a bypass

Find out where the extra air goes when only the smallest zone calls. In the source thread, a 5-ton single-stage system was split into two zones with no bypass, and the family room howls no matter which zone calls. The owner suspects a dump zone. California's manual says dump zones are usually unoccupied spaces like hallways, and that they get overconditioned.

The homeowner's installer skipped the bypass because the closet had no room for one. The post doesn't say which zone panel was used or how the zones split the airflow, so here's how we'd check it, step by step:

1. Get total CFM from tonnage and the blower setting. At 400 CFM per ton, the HVAC CFM calculator gives 2,000 CFM for 5 tons. Use the actual blower setting if you can read it.
2. Add up each zone's design CFM. Against Arzel's 40% screen, a zone under 800 CFM on a 2,000 CFM system may need a bypass or another place for the air.
3. Borrow the airflow check from EWC's bypass set-up procedure. Call every zone with the fan on its highest speed, then shut off all but the smallest zone, wait about 45 seconds, and check airflow and noise in that zone.
4. If a bypass won't fit, look at the other tools the sources list. Resideo's MARD has a range stop that keeps the blade from closing fully, which it calls useful "where installing a bypass damper is not possible." EWC suggests letting closed zone dampers leak 10% to 20%, more on large zones than on small ones.
5. If the system dumps air on purpose, put the dump somewhere nobody sits. The CEC describes dump zones as hallways or vaulted ceiling areas above the occupied zones, and says this design "may lose some of the comfort benefits of a zonally controlled system."
6. Confirm the panel has a supply air sensor and that it's wired. That's the protection EWC names against coil freeze-up.

Then run the numbers for the house you're working on. Get total airflow from the HVAC CFM calculator, size any bypass duct in the duct velocity and friction calculator, and check the coil's entering air temperature in the mixed air temperature calculator.

Sources

• Honeywell, Zoning Application and Selection Guide, January 1, 2016, crescentparts.com
• Resideo, Modulating Automatic Round Damper (MARD) product data 33-00647, resideo
• EWC Controls, Model PRD Barometric Bypass Damper, TB-224, gemaire.com
• EWC Controls, Ultra-Zone Guidance, ewccontrols.com
• Arzel Zoning Technology, Bypass Sizing Chart, 2002, crescentparts.com
• Arzel Zoning Technology, Bypass Sizing Chart, branch and trunk dampers, file dated January 22, 2015, meiersupply.com
• James J. Siegel, Static Pressure, Bypass Sizing Keys To Zoning, ACHR News, May 23, 2002, achrnews.com
• Johnson Controls, Verasys changeover bypass zoning application note, Sizing the Bypass Dampers, johnsoncontrols.com
• Carrier, Infinity Zone Control UIZ01 installation instructions, shareddocs.com
• California Energy Commission, 2025 Single-Family Residential Compliance Manual, Appendix F, Field Verification of Zonally Controlled Systems, energy.ca.gov
• r/hvacadvice thread with the question, "HVAC tech zoned my 5-ton single stage with NO bypass", reddit.com