How to Proportionally Balance Branch Duct Runs

Why this matters

Most residential balancing is done by closing the dampers to the rooms that are too cold and hoping the warm rooms pick it up. It usually takes three visits and ends with half the dampers half-shut, static pressure up, and the far bedroom still short. The proportional method fixes that with arithmetic: you never chase a room's design CFM directly. You make every run deliver the same fraction of its design, leaving the weakest run wide open, and then set the total with the blower. Done that way it takes two or three measurement passes, it uses the least damper throttling the system allows, and it tells you on the first pass whether a run is a balancing problem or a duct problem.

Before you start

What you need: design CFM for every supply register (from the room-by-room load and duct design), a flow hood with its back-pressure correction, access to every branch damper, and the system's fan table and rating plate.

Start state: clean filter, clean coil, every register fully open, every branch damper fully open, and the system running in the mode and blower speed you will leave it in (usually high cooling). Balancing against a loaded filter sets every damper for a static that disappears at the next filter change.

Hazards on this job are where the dampers are. Attic branch dampers mean attic heat, ceiling drywall between joists, and fiberglass: in summer, work the attic in the morning, carry water and a partner who knows where you are, and step only on joists or a walk board, since a foot through the drywall falls into the room below. Wear long sleeves, gloves and eye protection for the insulation, and a dust mask at minimum (a respirator under a written program per 29 CFR 1910.134 if your shop requires one for the work). In homes built before the 1980s, old duct wrap, tape or mastic may contain asbestos: do not cut, scrape or disturb it, and route it to an abatement contractor. Changing a blower tap means opening the blower compartment: turn the unit off at its switch or breaker and verify dead at the motor leads with a tester proven on a known live source before and after (NFPA 70E Article 120, current edition).

The sheet

Eight supply registers on a 3-ton system, design total 1,000 CFM. Ratio is measured divided by design. P1 is all dampers open; P2 is after damper adjustment; P3 is after the blower trim.

 ROOM       DES   P1  R1    P2  R2    P3  R3
 Kitchen    120  140 1.17  100 0.83  115 0.96
 Living     180  190 1.06  150 0.83  173 0.96
 Bed 1      140  105 0.75  112 0.80  129 0.92
 Bed 2      110  120 1.09   92 0.84  106 0.96
 Bed 3      100   70 0.70   82 0.82   95 0.95
 Bath        50   65 1.30   43 0.86   50 1.00
 Primary    200  210 1.05  168 0.84  194 0.97
 Dining     100  120 1.20   85 0.85   98 0.98
 TOTAL    1,000 1,020 1.02  832 0.83  960 0.96

The steps

1. Measure every outlet with everything open, and compute every ratio. Do not adjust anything until the whole sheet's first pass is filled in. The ratio column, not the CFM column, is what you balance on, because it puts a 50 CFM bath and a 200 CFM primary bedroom on the same scale. Seal the hood fully around each grille and take the reading the hood maker's back-pressure correction gives; a hood that does not seal on a textured ceiling reads low on exactly the registers you are trying to judge.

2. Find the index run: the lowest ratio. On P1 that is Bed 3 at 0.70 (70 against 100 CFM), with Bed 1 close behind at 0.75. The index run stays fully open for the whole job. Everything else gets throttled down to match it, because the index is the run with the most resistance per unit of design flow, and opening its own damper cannot help it: that damper is already wide open. The only lever left is throttling the others.

Check the index before you trust it. If the lowest ratio is far below the rest, say 0.4 against a field of 1.1, the index run has a duct fault, not a balancing problem: crushed or kinked flex, a disconnected inner liner, a closed damper you did not know about, a register boot sealed into the attic. Fix it first, then restart at step 1. Balancing a system around a crushed run throttles seven healthy runs to match one broken one, and the blower cannot make up the total. See related: 'Flex Duct Inspection and Correction'. Bed 3 at 0.70 with Bed 1 at 0.75 is not that pattern; both are long runs to the far end of the house, which is what an index normally looks like.

3. Throttle every other run toward the index ratio, highest ratio first. Start with the run furthest above the index (Bath at 1.30, then Dining 1.20, Kitchen 1.17) and set its branch damper so its ratio reads at or slightly above the index. Two effects make this converge rather than chase:

• Closing one damper raises the static in the trunk, so every other run, including the index, gains a little flow. The index ratio creeps up as you work, which is why you set each run to the index's current value, re-measuring the index after every two or three adjustments rather than holding to the 0.70 you started with.
• Working from the highest ratio down means the big moves happen first and the small corrections last.

Use the branch damper at the takeoff, not the register's own louvers. Throttling at the face makes noise right in the room and changes the throw; a damper at the trunk drops the pressure where nobody hears it. A run with no branch damper needs one before it can be balanced; see 'How to Install a Takeoff and Flex Branch to a New Register'.

4. Re-measure everything: the second pass. On P2 the ratios run from 0.80 to 0.86, a spread of 0.06. The index has moved: Bed 3 rose from 0.70 to 0.82 as the trunk static came up, and Bed 1 is now the lowest at 0.80. That is normal and it is not a reason to start over. Shop default: when every ratio sits within 0.05 of the sheet's total ratio, stop adjusting dampers. P2's total ratio is 0.83 (832 over 1,000), and every room falls between 0.80 and 0.86, inside 0.78 to 0.88. If one run falls outside, adjust only that run and re-measure it plus the index.

5. Trim the total with the blower, not the dampers. Proportional balance has a property that makes the last step easy: with the dampers fixed, the duct system's resistance curve is fixed, so changing blower speed changes every run's flow by roughly the same factor. P2 delivers 83 percent of design everywhere; the dampers have done their job and the blower now has to supply the difference. Raise the blower one setting and re-measure. On this system the next tap up gave 960 CFM.

Before you accept a higher blower setting, recheck what it costs: total external static against the rating plate, per 'Static Pressure Budget Across a Residential System', and the airflow per ton per 'Airflow Per Ton Targets and What Moves Them'. A blower speed that brings the rooms to design while pushing static past the plate is a duct-sizing problem the dampers were masking.

6. Accept against the design. P3 runs 0.92 to 1.00, total 0.96. Shop default acceptance, unless a program or a design specifies otherwise: every room within 10 percent of design and the total within 10 percent. The lowest room is Bed 1 at 129 against 140 CFM, 8 percent low; the total is 960 against 1,000, 4 percent low. Both pass.

Check that the scaling held, because it is only approximately true: P2 total 832 times 960/832 reproduces P3 by definition, but the room-by-room check is the real one. Bed 3 went from 82 to 95, a factor of 1.16, and Bath from 43 to 50, a factor of 1.16; every room on the sheet scaled between 1.15 and 1.16. A room that scales noticeably differently from the rest is on a damper that is near its fully closed position, where the resistance does not behave linearly, and it needs one more pass.

7. Lock and record. Tighten each damper's wing nut or quadrant lock, mark the position on the damper with a paint pen, and put the sheet in the job record with the blower setting and static. A customer who later "opens the dampers up" can be put back from the marks in one attic trip.

What the sheet just told you, and what it would have told you

The index was found, not guessed. Bed 3 is the room the customer complained about, and it is the room most techs would have tried to help first by opening its damper. Its damper was already open; the job was closing everything else.

Total airflow went down before it came back up. P1 delivered 1,020 CFM and P3 delivered 960 against the 1,000 design, a real 6 percent drop in total airflow (1,020 minus 960 is 60 CFM) in exchange for every room landing within 8 percent of design. Had the blower already been on its top setting at P2, the system would have finished at 83 percent of design everywhere, and the honest result is a ticket that says the duct system is undersized for this blower, with the balanced sheet as the evidence.

What would have changed the method. A zoned system is balanced zone by zone, each with its zone damper fully open and the bypass closed if there is one, because the trunk static is different for every zone combination. A system with returns in every room benefits from checking the returns too, but return balancing is a pressure question, not a ratio one, and belongs to a separate visit.

How to verify you got it right

• Every room on the final pass within 10 percent of design, printed on the sheet, and the total within 10 percent.
• The index run's damper fully open, and visibly so.
• Static and airflow per ton at the final blower setting recorded and inside the rating plate's limits.
• Damper positions marked and locked.
• The blower compartment door back on and the unit restarted and running before you leave, with the thermostat returned to the customer's settings.

The visit around this method, including how to schedule it and what to tell the customer about rooms that will still feel different, is 'Register and Grille Balancing Visit'.

References

• See related: 'Register and Grille Balancing Visit'; 'Flex Duct Inspection and Correction'; 'Static Pressure Budget Across a Residential System'; 'Airflow Per Ton Targets and What Moves Them'.
• Flow hood maker's instructions for back-pressure correction and grille sealing.
• Equipment rating plate and blower fan table.
• 29 CFR 1910.134 for respirator use where the shop's program requires it in attic work.