How to Calculate Room CFM From Manual J Room Loads

Each room gets the blower's CFM times its share of the load: room CFM = blower CFM x room load / total load, worked once with the heating loads and once with the sensible cooling loads. On ACCA's own 800 CFM training house each of the eight supply runs is sized for the larger of its two numbers, which puts the design CFMs between 64 and 144.

A tech who had just learned Manual J and D asked on r/HVAC whether room CFM gets assigned before or after the furnace or air handler is picked. After, because the blower CFM comes off the selected unit's blower table. Below is ACCA's example worked through, a totals check on its numbers, and what the equal split in our free duct sizing calculator does differently.

How do you calculate room CFM from the room load?

Divide the blower CFM by the whole-house load to get a factor, then multiply each room's load by it. ACCA's lesson plan uses two factors. The heating factor is blower CFM over the Manual J heat loss, and the cooling factor is blower CFM over the Manual J sensible heat gain. Each room ends up with a heating CFM and a cooling CFM, and the larger one sizes its run.

ACCA spells the method out in the Manual D part of its 2015 instructors' lesson plan, right after the formulas HF = Blower Cfm / Manual J Heat Loss and CF = Blower Cfm / Manual J Sensible Heat Gain:

ACCA Instructors' Lesson Plans § Duct Sizing, Manual D

"These two factors help us determine how many Cfm of heating or cooling each room needs based on the room's contribution to the total heating and (sensible) cooling load." - ACCA, Instructors' Lesson Plans, 2015

The cooling factor divides by sensible gain, so the latent part of the load stays out of the split. An ICC training deck on the same ACCA process writes the formula as room CFM = blower CFM x MJ room load / MJ total load, heating or cooling, and says "the designer must use the larger of the two cfm values for sizing the duct runs." ACCA's own duct design checklist asks the same question of every design: does each room have a heating and a cooling CFM, proportioned from the Manual J room-by-room loads?

1. Read the blower CFM at the design external static pressure from the unit's blower table.
2. Divide it by the whole-house heat loss for the heating factor, and by the whole-house sensible gain for the cooling factor.
3. Multiply each room's heating load by the heating factor and its cooling load by the cooling factor.
4. Take the larger result as the room's design CFM and size its run at the system friction rate.

Do you assign room CFM before or after picking the equipment?

After. The formula needs blower CFM, and that number comes from the blower table of the unit you selected, read at the external static pressure the duct system will see. ACCA's example reads 800 CFM at 0.36 in. wc on the medium setting. The ICC deck starts from an air handler that delivers 1,000 CFM at 0.23 in. wc net.

Manual J gives the room loads, Manual S picks the unit, and that unit's blower table sets the CFM the rooms share. Work it the other way, with a guessed 400 CFM per ton, and every room number shifts in proportion when the real blower table arrives. For a first estimate before the unit is chosen, our HVAC CFM calculator turns tons or sensible load into one system CFM, and the split below scales with it.

The same blower table feeds the friction rate. In ACCA's example the 0.36 in. wc of ESP minus 0.20 of component losses (an electronic filter at 0.11, plus 0.03 each for the supply outlet, return and hand damper) leaves 0.16 available. Over a 221 ft total effective length that's 0.07 in. wc per 100 ft. Manual J vs Manual S vs Manual D covers how the three steps hand off.

Worked example on ACCA's 800 CFM house

ACCA's house has eight radial supply runs, a 29,910 BTU/h heat loss and a 21,120 BTU/h sensible block gain. The factors come out to 0.0267 CFM per BTU/h for heating and 0.0379 for cooling. Run 4 carries 3,800 BTU/h of cooling load, so it gets 144 CFM for cooling against 123 for heating, and ACCA sizes that run at 144 CFM and 0.07.

HF = 800 / 29,910 = 0.0267 and CF = 800 / 21,120 = 0.0379. Every row below is ACCA's, from page 35 of the lesson plan. The totals row is ours.

Heating sets the design CFM in runs 1, 3 and 8, cooling in the other five. ACCA works run 1 as 4,250 x 0.0267 = 113.5 CFM and lists it as 114. By our arithmetic, the unrounded factor 800 / 29,910 = 0.02675 reproduces every heating CFM in the table (run 1 is 113.7). With the rounded 0.0267 and a single rounding, runs 1 and 3 come out one CFM lower, at 113 and 101. ACCA then sizes each run on a duct wheel at its design CFM and the 0.07 friction rate.

Why do the design CFMs add up to more than the blower?

In both worked examples they do, by our sums. ACCA's design CFMs add to 877 against an 800 CFM blower, and the ICC example's add to 1,020 against 1,000 (100, 420 and 500 CFM for three rooms). Taking each run's larger value adds air, and in ACCA's example so does dividing cooling by a block gain below the room sum. Neither example scales the runs back.

The cooling column has its own gap. It adds to 834 because the cooling factor divides by the block sensible gain of 21,120 BTU/h, while the room cooling loads in the same table add to 22,020, which is 900 more. 800 x 22,020 / 21,120 = 834. The heating loads add up exactly to the 29,910 block, so that column lands on 801 with rounding.

The lesson plan doesn't say why its block gain sits below the room sum, and its example divides by the block. By our arithmetic, dividing by the room sum instead would bring the cooling CFMs to 800 before rounding, about 4% lower in every room. Know which total you divided by when the room numbers don't add up. The sample design in ACCA's duct checklist lists room CFMs that add up to its 1,200 CFM blower, so the overshoot isn't in every design.

Is this the same as the 1.08 x CFM x delta T method?

By our arithmetic it's the same equation, with the supply temperature difference set by the blower CFM and the block load together. Room CFM = room sensible load / (1.08 x dT), so a cooling factor equals 1 / (1.08 x dT). By our arithmetic, ACCA's 0.0379 matches a 24.4°F room-to-supply difference at sea-level standard air, and its heating factor 0.0267 matches 34.6°F. ACCA doesn't state those numbers.

HVAC School writes room CFM as the room sensible load divided by 1.08 x dT, where 1.08 = 60 x 0.075 x 0.24 for dry air at sea level (HVAC School). Choose the dT yourself and the rooms add up to whatever total that dT implies, which may differ from what your blower moves. The factor method starts from the blower CFM, so each room number is a share of air the unit delivers.

Run 4 at a 20°F split shows the difference: 3,800 / (1.08 x 20) = 176 CFM. That's 32 more than the 144 the cooling factor gives, because ACCA's blower and block gain work out to 24.4°F.

What does an equal split do to the branch sizes?

It gives every room the same CFM. Our free duct sizing calculator divides total airflow by the number of branches, so ACCA's house at 800 CFM and 8 branches gets 100 CFM per run. By our sizing in fully stretched flex at ACCA's 0.07 friction rate, 100 CFM takes a 7 in. duct, while the load split puts runs 2, 4 and 8 at 8 in. and run 5 at 6 in.

We solved these sizes with the ASHRAE Darcy and Colebrook equations at 0.07 in. wc per 100 ft for straight round duct, fully stretched flex and galvanized, the same math our duct velocity and friction calculator runs in its size mode. Fittings are already in the 0.07 through ACCA's 221 ft TEL, so these are straight-duct sizes at that rate. In galvanized only run 4 moves up and run 5 down. In flex, three of the eight runs come out one size small under the equal split. Run 8 is the close call, at 7.05 in. needed, and a 7 in. flex at 120 CFM runs about 4% over 0.07.

The duct sizing page states the trade in its limitations, where it says a large room "gets the same branch size as a closet." It also takes available static pressure from 0.20 in. wc up, so ACCA's 0.16 can't be entered there. Room-by-room Manual J and full Manual D are in the paid Pro and Team plans. Free whole-house Manual J gives you the totals for the two factors but not the room loads. This is preliminary sizing for a quote, and plan review needs an ACCA-approved calculation, covered in Manual J for permit.

For a quick whole-system pass, the duct sizing calculator works out its own friction rate from available static pressure, the longest supply run and a fittings factor, then splits the airflow evenly. With room loads and the blower CFM in hand, work both factors and size each run at its design CFM and your friction rate in the size mode of the duct velocity and friction calculator.

Sources

• ACCA, Instructors' Lesson Plans for Manual J, Manual S and Manual D (EDU-1, September 2015), Duct Sizing, pages 33 to 35: higherlogicdownload.s3.amazonaws.com
• ACCA, Verifying ACCA Manual D Procedures, duct design checklist (copy hosted by AHRI): ahrinet.org
• John D. Sedine, An Overview on ACCA's Residential HVAC System Design Process, 2018 ICC Annual Conference PMG Educational Program, slides Finding Each Room Cfm and Example: iccsafe.org
• HVAC School, Room Sensible Heat and CFM, Advanced Psychrometrics Part 1 (May 13, 2020): hvacrschool.com

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