Should You Lower CFM per Ton in a Humid Climate

Yes, and expect the latent gain to be small. On Goodman's published table for a 3-ton GSX16, dropping from 1,200 to 1,050 CFM (400 to 350 CFM per ton) at 95°F outdoor and 63°F entering wet bulb raises latent capacity by about 460 Btu/h and cuts total capacity by 2,400 Btu/h. That closes a latent gap of a few hundred Btu/h. Against a 2,000 Btu/h gap it covers less than a quarter.

The ACCA HVAC Blog puts the rule plainly: 350 CFM per ton in hot, humid climates and 450 where the air is dry. Below, the same rule runs through a real expanded performance table and a published Manual J load, so you can see what the 50 CFM per ton buys. The free HVAC CFM calculator converts tons and CFM per ton to total airflow without signup.

Does lowering CFM per ton really remove more moisture?

Slower air spends more time on the coil and leaves it colder, so more of the capacity goes to condensing water and less to dropping temperature. FSEC modeled the trade in 1997. Going from 400 to 300 cfm per ton added 720 Btu/h of latent capacity (8%) and cost 1,730 Btu/h of sensible (10%), in a simulation at 75°F, 60% RH indoors and 95°F outdoors.

The same FSEC paper by Parker, Sherwin, Raustad and Shirey explains why the trade stops at 350.

FSEC-PF-321-97 § Laboratory Test of Impact of Low Air Flow

"Generally, the data show the wisdom of the recommendation not to allow air flow to drop below 350 cfm/ton ... Ninety-five percent of potential latent capacity is achieved at this point at a cost of only about 10% in sensible capacity." - Parker, Sherwin, Raustad and Shirey, fsec.ucf.edu

ACCA's quality installation standard frames the range the same way. Footnote 7 of ANSI/ACCA 5 QI-2015 says airflow across the coil "is typically between 350 to 450 CFM per ton," and section 4.1.1 requires it to sit "within the CFM range listed in the OEM product data." For a specific unit, pull its expanded performance table and read the 350 and 400 CFM per ton rows at your design conditions.

What changes between 350 and 400 CFM per ton on a real coil?

On the Goodman GSX16 3-ton table at 95°F outdoor, 75°F entering dry bulb and 63°F entering wet bulb, 1,050 CFM gives 29,800 Btu/h total at a 0.79 sensible-to-total ratio. 1,200 CFM gives 32,200 at 0.82. That works out to 6,258 Btu/h latent at 350 CFM per ton against 5,796 at 400, a gain of 462 Btu/h.

These are the three airflow rows from page 17 of the GSX16 spec sheet, for the 3-ton condenser matched with the CA*F4860 coil and TXV. Goodman shades the 1,200 CFM cell at these conditions as the ACCA (TVA) rating point. Sensible and latent columns are our arithmetic from Goodman's total and S/T.

Goodman prints S/T to two decimals, so each sensible and latent figure carries roughly 150 Btu/h of rounding either way. The direction is clear anyway. Each step down in airflow trades total and sensible capacity for latent.

Three things in that table matter on the job:

1. The latent gain is small next to the total loss. You give up 2,400 Btu/h of total (7.5%) and 2,862 Btu/h of sensible to pick up 462 Btu/h of latent.
2. Capacity per kW drops. Total output per kW goes from 11.5 at 1,200 CFM to 10.9 at 1,050, about 5% less. Sensible output per kW, which sets energy use under a thermostat, falls further, from 9.5 to 8.6, about 9% less.
3. The temperature split barely moves, 20°F to 21°F. You can't confirm a 50 CFM per ton change with a thermometer.

Latent Btu/h alone undersells the change. A thermostat runs the unit until the sensible load is met, so the water removed per cycle follows latent divided by sensible. At 63°F wet bulb that ratio is 0.27 at 1,050 CFM against 0.22 at 1,200, about a fifth more moisture for the same cooling. In the 67°F wet bulb column of the same 75°F block (about 67% RH by our psychrometric math), latent comes out 12,880 Btu/h at 1,050 CFM and 13,262 at 1,200, about 380 Btu/h less at the lower airflow and only slightly more than the table's rounding, but latent per unit of sensible still rises from 0.61 to 0.67, about 9%. So the lower setting also helps in a damp house, and any latent gap it leaves goes to equipment selection or a dehumidifier.

Does 350 CFM per ton cover a humid-climate Manual J load?

Not on this unit and this load. Wes Davis's published ACCA example is 30,000 Btu/h total, 22,000 sensible and 8,000 latent at 75°F, 50% RH and 95°F outdoor, the same 63°F wet bulb. The GSX16 at 350 CFM per ton covers 78% of the latent load, up from 72% at 400. Manual S-2023 wants 100%.

The loads come from Davis's 2009 Manual S review article in the ICC Building Safety Journal. That's a load sensible heat ratio of 22,000 / 30,000 = 0.73. Run both airflow rows against it:

1. Total at 1,050 CFM. 29,800 / 30,000 = 0.993, or 99.3%. That clears the Manual S-2023 total minimum of 0.90 and sits well under the 1.15 cap of 34,500 Btu/h.
2. Sensible at 1,050 CFM. 23,542 / 22,000 = 1.07, over the 0.90 minimum.
3. Latent at 1,050 CFM. 6,258 / 8,000 = 0.782, under the 1.00 minimum. The shortfall is 1,742 Btu/h.
4. Same checks at 1,200 CFM. Total 32,200 / 30,000 = 1.073, sensible 26,404 / 22,000 = 1.20, latent 5,796 / 8,000 = 0.725, short by 2,204 Btu/h.

Dropping to 350 CFM per ton closes 462 of the 2,204 Btu/h gap, about 21%. The equipment SHR goes from 0.82 to 0.79 and is still above the 0.73 load SHR. The Manual S-2023 limits used here (0.90 total and sensible, 1.00 latent, 1.15 cap) are the ones worked through in our Manual S oversizing limit guide.

Davis's own example ran 1,050 CFM, "the duct design value," on a fictional "XYZ" unit. His footnote is why you read the real table: "If the airflow value changes, the equipment capacity and performance also change." On a real table, run the latent check at the 350 row first, then at the 400 row for comparison.

When the latent check still fails at 350, the fix is a different coil or condenser match with more latent at your conditions, variable-capacity equipment with a dehumidify mode, or a separate dehumidifier sized with our dehumidifier calculator.

When is 350 CFM per ton the wrong setting?

Leave it at 400 or higher if the load SHR is high, sensible is already tight, the ducts already starve the coil, or the OEM table has no 350 row. A house with a load SHR near 0.95 gets little from extra latent and, per the FSEC data above, gives up about 10% of sensible. Push airflow much lower and FSEC warns the evaporator coil can ice, and the Florida homes FSEC measured already averaged 320 cfm per ton.

Skip the lower setting in these cases:

• Dry climates. ACCA's blog says 450 CFM per ton "lets the same system deliver more sensible capacity" where humidity is low. On the GSX16 table that's 28,552 Btu/h sensible at 450 against 23,542 at 350, about 5,000 Btu/h more.
• Sensible is already tight. At 350 the GSX16 lost 2,862 Btu/h of sensible. If your selection only clears the 0.90 sensible minimum by a little at 400, check the 350 row before you switch.
• The duct system already starves the coil. FSEC measured 27 Florida homes at a mean of 320 cfm per ton, with a range of 130 to 510. Setting the blower to 350 on a restrictive duct system can land the coil well below that.
• The OEM doesn't list it. QI-2015 section 4.1.1 ties airflow to "the CFM range listed in the OEM product data."

Rheem's installation instructions put both sides in two sentences. "Excessive indoor airflow increases the possibility of high humidity problems. Low indoor airflow reduces total capacity, and causes coil icing."

How do you set and verify the lower airflow?

Use the blower setting the manufacturer provides for it, then measure. Carrier's 58TN furnace delivers a nominal 350 CFM per ton with its two airflow switches off, and the switches select 400 or 325. Trane's humid-climate guide recommends 350 to 370. Confirm the delivered CFM against the blower table at your measured static pressure, since the temperature split moves only about 1°F.

Manufacturers build this into the equipment. The Carrier 58TN product data reads: "Nominal 350 CFM/ton cooling airflow is delivered with SW1-5 and SW4-3 set to OFF. Set SW1-5 to ON for nominal 400 CFM/ton (+15% airflow). Set SW4-3 to ON for nominal 325 CFM/ton (-7% airflow)." Trane's humid-climate installer guide for its dehumidification package says the cooling airflow setting "is recommended to be 400 CFM/ton or lower. Optimal airflow selections are 350 - 370 CFM/ton." The same guide advises against continuous fan when indoor humidity is high.

1. Read the latent check at the 350 row of the OEM table, as above, before you change anything.
2. Set the tap, DIP switch or control airflow per the installation instructions.
3. Measure total external static pressure and read delivered CFM off the blower table at that static. QI-2015 asks for airflow within 15% of design, and also inside the OEM range. At a 350 design, 15% low is about 300 CFM per ton, below the lowest row of the GSX16 table, so keep measured CFM at or above the OEM table's lowest row even when it sits inside the 15% band.
4. Size or check the ducts at the CFM you actually set. At 350 CFM per ton a 3-ton system moves 1,050 CFM, so size the trunk for 1,050 and rebalance the room CFMs so the branches add up to 1,050.

Sensible Btu/h equals 1.08 x CFM x temperature difference, per the ACCA HVAC Blog. Cut airflow 12.5% while the split rises about 1°F and that equation puts sensible about 8% lower. The GSX16 table, which rounds the split to whole degrees, shows 11%. Only 462 of the 2,862 Btu/h of lost sensible capacity comes back as latent. The other 2,400 Btu/h is lost total capacity, so recheck the total and sensible minimums at the row you set.

Running the latent check before you quote

Start with the whole-house Manual J to get sensible and latent loads, divide sensible by total for the SHR, then read the OEM table at 350 and 400 CFM per ton and compare latent to the load. If neither row reaches 100% of latent, move to a coil or condenser match with more latent at your conditions, variable-capacity equipment with a dehumidify mode, or a dehumidifier. Our CFM calculator converts tons to total airflow and flags values under 300 or over 450 CFM per ton.

Here's the sequence:

1. Run the whole-house load in the free Manual J calculator and note sensible and latent separately.
2. Pull the candidate unit's expanded table at your outdoor design temperature, 63°F entering wet bulb for 75°F and 50% RH, and both airflow rows.
3. Compute latent at each row and compare it with the latent load, as in the worked example.
4. Convert the chosen CFM per ton to total CFM with the HVAC CFM calculator, which warns below 300 (coil icing risk) and above 450 (weaker moisture removal).

On the Pro and Team plans, the Manual S equipment selection tool compares the load SHR with the equipment SHR. It has no airflow input and assumes a typical 0.78 equipment SHR unless you enter the unit's sensible capacity, so read the airflow row yourself and enter both total and sensible from that row. Its total check uses a 100% floor, so the 350 row's 99.3% shows as short there even though it clears the Manual S-2023 0.90 minimum used above. This is preliminary sizing for a quote. For a permit, the AHJ may want a load report from ACCA-approved software, covered in Manual J for permit.

Sources

• Goodman, GSX16 Split System Air Conditioner spec sheet SS-GSX16, Expanded Cooling Data, page 17: goodmanmfg.com
• Matt Akins, ACCA, 400 CFM Per Ton, Or Is It?, February 4, 2021: hvac-blog.acca.org
• ACCA, ANSI/ACCA 5 QI-2015, HVAC Quality Installation Specification, section 4.1.1 and footnote 7: acca.org
• Wes Davis, ACCA, Reviewing HVAC Designs for Compliance with ACCA Manual S, ICC Building Safety Journal, 2009: media.iccsafe.org
• Parker, Sherwin, Raustad and Shirey, Impact of Evaporator Coil Air Flow in Residential Air Conditioning Systems, FSEC-PF-321-97: fsec.ucf.edu
• Trane, Integrated Dehumidification Solution Installer's Guide 18-HD94D1-1B-EN, section 3.2: tranetechnologies.com
• Carrier, 58TN0A/58TN1A Product Data, Catalog No. 58TN0A-03PD (edition 03/21), pages 9 and 13
• ACCA, ANSI/ACCA 3 Manual S-2023, Addendum c, Table N2.3.1 (0.90 total and sensible minimums, 1.00 latent minimum, 1.15 cap)
• Rheem, 13 SEER Condensing Units Installation Instructions 92-21354-47-01, page 12: bakerdist.com
• Allison Bailes, ACCA HVAC Blog, The Physics of Heat and Air Flow, May 2, 2018: hvac-blog.acca.org

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