How to Estimate Blower Airflow From Static and the Fan Table

Why this matters

Almost every charging, dehumidification and heat-exchanger question on a ducted system starts with airflow, and most shops never measure it. The fan table in the furnace or air handler installation manual is the cheapest airflow estimate you have: one static reading, one row, one column. Done carelessly it is also a confident wrong number, because the table was published under conditions the maker states in small print, and the reading only means something when your measurement matches those conditions.

This article is reading the table correctly. Taking the static reading belongs to How to Measure Total External Static Pressure on a Furnace; the sensible-heat formula used to cross-check the result belongs to the Temperature Rise and the Sensible Heat Airflow Formula card. Neither is re-taught here.

What the table is, and what it is not

A fan table (also called a blower performance table or airflow chart) lists the airflow in cubic feet per minute a specific blower delivers at each speed tap or setting, across a range of external static pressures. The maker built it on a test stand, at a stated voltage, with stated components in or out of the airstream. It is the blower's curve, sliced.

It is not a measurement of your system. It converts your measured static into an estimate of airflow, and it is only as good as three things: the tap you think is running is the tap that is running, your static was measured across the same boundary the table assumes, and the operating conditions match the table's footnotes. Each of those is a step below.

Start state

• The installation manual for the exact model, including its fan table and every footnote under it. A table for a sister model with a different blower or cabinet width is not close enough.
• A TESP reading taken across the boundary the table assumes, in the mode you are evaluating, with the conditions recorded: filter as found or clean, coil wet or dry.
• A meter for supply voltage on a PSC motor, and for a constant-airflow ECM, access to the board's airflow settings.
• Power isolated before you open the control compartment to confirm leads (Step 1), and the door back on before any reading.

Step 1: confirm which tap or setting is actually running

Before opening the blower or control compartment, turn the furnace off at its service switch, lock and tag it, and prove the board dead with a tester checked on a known live source before and after, per NFPA 70E (current edition), with work practices under 29 CFR 1910.333(b)(2). The board's motor terminals and the PSC motor's run capacitor are line-voltage parts; the capacitor can hold a charge after power is off, so discharge it through a resistor before touching its terminals.

Then trace the motor lead from the board terminal that serves the call you are testing (COOL, HEAT or FAN on most boards) back to the motor, and identify the lead by its color against the motor's own label, not against a habit. Lead colors for speed taps vary between motor makers and even between production runs. On a constant-torque ECM with numbered tap inputs, trace the 24 V wire to its numbered tap. On a constant-airflow ECM, read the DIP switch or menu settings that select airflow.

What breaks if skipped: the most common bad airflow estimate is a correct static reading looked up on the wrong row. A previous tech moved the cooling lead to a different tap and nobody wrote it down.

Close the compartment, restore power, and confirm the door switch closes, before Step 2. Every reading is taken with the door on.

Step 2: read the table's footnotes before its numbers

Read every note under the table. They typically say some of the following, and each one changes what your static reading must include:

• "Airflow shown without filter" or "with filter." If the table excludes the filter and your filter sits in an external rack downstream of which you measured, your reading already counts the filter and you read the table directly. If the filter sits inside the cabinet, so your return port is upstream of it, and the table assumes no filter, add the filter's measured drop to your static before reading.
• "Coil not included" or "wet coil" / "dry coil." A furnace table normally excludes the add-on coil, so a TESP measured below the coil already includes it. An air handler table includes its internal coil and often states whether it is dry or wet, sometimes with a deduction for a wet coil. Follow the note.
• The test voltage, commonly the nominal 115 V or 230 V. A PSC motor slows and moves less air at low voltage.
• The static range. Readings beyond the last column are off the table. Do not extrapolate; the curve bends there.

Step 3: read the table, interpolating between columns

This is the artifact: the table from the manual in this example, with the measured static and the running tap circled, and the value read across.

 FAN TABLE, 70,000 BTU/H INPUT FURNACE, PSC BLOWER
 CFM vs external static (in w.c.), 115 V, filter
 and coil NOT included (both external)

 TAP     0.3   0.4   0.5   0.6   0.7   0.8
 HIGH   1550  1480  1400  1310  1210  1100
 MED-HI 1390  1330  1260 (1180)(1090)  990
 MED    1220  1170 (1110)(1040)  960   870
 MED-LO 1030   990   940   880   810   730
 LOW     840   810   770   720   660   590

 COOL tap = MED-HI, measured 0.62 in w.c.
   0.62 is 0.2 of the way from 0.6 to 0.7
   1180 - 0.2 x (1180 - 1090) = 1180 - 18
   = 1162, read as about 1160 CFM
 HEAT tap = MED, measured 0.55 in w.c.
   0.55 is 0.5 of the way from 0.5 to 0.6
   1110 - 0.5 x (1110 - 1040) = 1110 - 35
   = 1075 CFM

Linear interpolation between adjacent columns is close enough on a published blower curve over a 0.1 in w.c. step. Round the result; the table itself is not good to the last ten CFM.

Step 4: apply the corrections the footnotes ask for

Each correction gets its own line on the ticket, so the next tech can see it was applied.

Filter location, this example: filter in a rack in the return drop; the return port was downstream of it. Measured static already includes the filter. Correction: none.

Coil, this example: a cased 3-ton coil on top of the furnace; the supply port was in the plenum above the coil. Measured static already includes the wet coil. Correction: none.

Voltage, this example: measured 118 V at the furnace with the blower running, against a table at 115 V. That is within a few percent of the table voltage, so the table applies as published. A reading well below 115 V would mean the motor is turning slower than the table assumes and the airflow is below the number you just read, by an amount the table does not give you.

Range: 0.62 and 0.55 in w.c. both sit inside the published 0.3 to 0.8 range.

Step 5: cross-check against something independent

A table estimate should be checked against a second method once per system. On a gas furnace the easiest is heating temperature rise, because the furnace's output is on its rating plate.

This example's plate: 70,000 BTU/h input, 56,000 BTU/h output. With the gas input confirmed at the plate (clocked, see related: How to Clock a Gas Meter to Check Furnace Input), the furnace on the MED heating tap ran a measured temperature rise of 47 F.

Using the sensible-heat relationship for standard air, CFM = output BTU/h / (1.08 x rise):

• 1.08 x 47 = 50.76
• 56,000 / 50.76 = 1,103 CFM

Against the table: the table gave 1,075 CFM on the same tap. 1,103 minus 1,075 is 28 CFM, which is about 2.6 percent of the table value (28 divided by 1,075). Two independent methods agree within a few percent, so the table, the tap identification and the static reading are behaving on this unit.

What that lets you conclude about cooling: the cross-check ran on the MED tap in heating, not the MED-HI tap in cooling. It does not measure cooling airflow. What it does is confirm the table matches this blower on this voltage and that the static method is sound, which is the reason to trust the MED-HI read of about 1,160 CFM. On a 3-ton system, 1,160 divided by 3 is about 387 CFM per ton, inside the range most condenser makers publish for nominal airflow (see related: Airflow Per Ton Targets and What Moves Them).

When the method does not apply as written

• A constant-airflow ECM. Its table shows nearly the same airflow across a wide range of static, because the motor raises torque to hold its programmed airflow. Your static reading then tells you whether the system is inside the range where the motor can still hold that airflow, not what the airflow is. Above the table's limit, airflow falls off and the motor is working at its ceiling. Configuring one belongs to ECM Blower Motor Replacement and Configuration.
• A constant-torque ECM with numbered taps. Its table looks like a PSC table, airflow falling with static, and is read the same way.
• High altitude. A blower moves roughly the same volume in thinner air, so the table's CFM still reads, but the 1.08 constant in the cross-check assumes sea-level air and overstates heat carried at altitude; the cross-check needs the altitude-corrected constant, which the formula card covers.
• A disagreement over about 10 percent between the table and the rise cross-check. Something in the chain is wrong: a tap misidentified, a port in turbulence, a filter counted on the wrong side, or gas input not at the plate. Find it before writing a number on the ticket.

What a wrong reading costs

Read the wrong row and you may charge by subcooling on a coil with too little air, where the subcooling reads high and you pull refrigerant the system needs, or tell a customer the ductwork is fine when the blower is barely moving 300 CFM per ton. Read the right row with the filter on the wrong side of the line and you will overstate airflow on exactly the systems with loaded filters, which are the ones that needed the right answer. The fix for both is the same habit: write the tap, the static, the boundary it was measured across and the table's conditions on the ticket, every time, so the number can be checked.

References

• Furnace or air handler installation manual: fan table, its footnotes, and the motor's lead or tap identification.
• NFPA 70E (current edition), Article 120, and 29 CFR 1910.333(b)(2), for de-energizing and proving the control compartment dead before confirming leads.
• See related: How to Measure Total External Static Pressure on a Furnace; Temperature Rise and the Sensible Heat Airflow Formula; Airflow Per Ton Targets and What Moves Them; ECM Blower Motor Replacement and Configuration.