Low / High Duct CFM Troubleshooting

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Measured CFM Below Design

Undersized duct

The run cannot carry the CFM at a legal velocity without huge friction. TESP is often high on that side. Size a replacement with the ductulator using the design CFM, not the starved measurement. Velocity maxima (residential branch ~600 FPM, main ~900) are already in the duct sizing guide — do not invent a second chart.

Restrictions — kinks, dampers, dirty coil

Flex crushed, a balancing damper left at 20%, an evaporator mat. Supply-side static high, CFM low. Straighten, open, clean, re-traverse.

Filter

Return CFM collapses with a loaded media cabinet. Replace, re-measure both CFM and return static. If CFM recovers, stop talking about a new furnace.

Static pressure too high

The blower is working against more external static than the nameplate. PSC airflow drops; ECM may scream and still miss. Measure TESP with the ESP calculator. High ESP plus low CFM is the same story told by two meters. Causes: high/low ESP troubleshooting.

Blower issues

Wrong tap, failed capacitor, wheel caked, ECM in a fault. TESP can look low or odd while CFM is dead. Amp draw and the OEM fan table beat guessing.

Measured CFM Above Design

Oversized duct

Real extra area drops velocity for the same CFM — or the fan dumps extra air into a lazy trunk. Confirm with a hood at the rooms; oversized metal plus leaks can still starve registers.

Leaks feeding a false reading

If your plane is downstream of a return leak (or you are in an unsealed supply plenum sucking attic air around the probe port), FPM is not “the system.” Seal the port, pick a better plane, or hood the grille. Duct CFM can read high while delivered CFM is low — that is the whole point of anemometer vs flow hood.

Center-point only

The most common “high CFM” on a phone photo. Re-traverse. The calculator will not save a bad average.

Before You Cut or Condemn

1. Confirm inside dimensions and liner.
2. Confirm the average is a real grid, entered in FPM (not in. WC, not m/s unless you toggled units).
3. Split: low CFM + high TESP → path. Low CFM + low TESP → leaks or fan. That pairing is the static vs CFM rule — use both meters.
4. If the rooms disagree with the trunk, stop averaging more holes in the same bad plane. Hood the outlet.

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