How to Set Blower Speed Taps for Cooling and Heating Airflow

Why this matters

Most furnaces leave the factory with the cooling call on the blower's highest speed and the heating call on a lower one, because the factory does not know your ductwork, your condenser or your climate. Left that way, a lot of systems move far more air in cooling than the condenser maker wants, which raises the coil temperature, wrings less water out of the air, and leaves a house that is cool and clammy. The same system may run heat at a rise near the top of the plate range, or a duct system may be pushed past the furnace's rated external static on the high tap.

Setting the taps is a fifteen-minute job with the panel off. Doing it from measurements instead of from the factory sticker is what turns "installed" into "commissioned."

This article is the tap change on an existing or new blower. Replacing and configuring an ECM motor is ECM Blower Motor Replacement and Configuration. Reading the fan table is How to Estimate Blower Airflow From Static and the Fan Table, and the rise math is Temperature Rise and the Sensible Heat Airflow Formula.

Start state

• The furnace rating plate and the blower's fan table from the installation manual. The plate gives the temperature rise range and the maximum external static; see related: Furnace Rating Plate Read Before Start-Up.
• The condenser maker's airflow requirement for the matched coil, usually stated per ton or as a nominal CFM.
• A manometer, two temperature probes, a meter, and a CO monitor worn for any heat-call measurement.
• The wiring diagram on the blower door. Lead colors and board terminals vary by maker; the diagram governs over anything you remember.

Know which motor you are looking at

PSC (permanent split capacitor). A run capacitor on the blower housing and three to five colored speed leads at line voltage. The board switches line voltage to whichever lead is on its HEAT or COOL terminal. Airflow on each tap falls noticeably as static rises, which is why the fan table lists CFM by static column.

Constant-torque ECM. A line-voltage plug plus a set of low-voltage tap inputs, commonly numbered 1 through 5, switched by 24 V from the board or thermostat. Each tap is a torque setting; airflow still falls with static, but less steeply than a PSC. The motor's label says which tap takes priority if two are energized at once.

Variable-speed (constant-airflow) ECM. Airflow is selected with DIP switches or board settings, not by moving wires, and the motor holds that airflow across a range of static. Setting it is a configuration task covered by the ECM article.

Steps

1. Measure before changing anything. Run each call (cooling, heating, continuous fan) and read total external static across the furnace and coil, as in How to Measure Total External Static Pressure on a Furnace. Read the fan table at that static for the tap actually wired to that call. This is the as-found record, and it often explains the complaint by itself.

2. Set the targets. Cooling: the condenser maker's airflow for this tonnage, adjusted for climate; humid climates usually sit at the lower end of the maker's range for moisture removal, dry climates higher (see related: Airflow Per Ton Targets and What Moves Them). Heating: a rise inside the plate range, in practice somewhere around the middle of it, which you can predict from output and airflow and confirm by measurement.

3. Pick the taps from the table. Find the tap whose airflow at the expected static comes closest to each target. Expect the static to change when you change speed: more air means more static on the same ducts, roughly with the square of airflow, so the CFM you read at the old tap's static is not quite what the new tap will deliver. You confirm it by measuring afterward, not by trusting the first read.

4. Shut off and lock out the furnace before touching the board. Turn off the service switch and the breaker, lock and tag, and prove absence of voltage at the board's line terminals with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices under 29 CFR 1910.333(b)(2). The door interlock switch is not an isolation device. Leave the run capacitor's terminals alone; you are moving leads at the board, not at the motor.

5. Move the leads, and park the rest. On a PSC, move the chosen lead to the HEAT or COOL terminal and put every unused speed lead on the board's PARK terminals. Unused PSC leads carry voltage when the motor runs; a lead cut off, taped and stuffed in the cabinet is a short waiting for vibration to find it. On a constant-torque ECM, move the low-voltage tap wires to the chosen tap numbers.

6. Confirm what the board does with each call. Read the board's sequence in the manual. Many boards run the HEAT terminal after a blower-on delay once the burner lights, the COOL terminal on Y, and something specific on G alone (a separate FAN terminal, or one of the other taps). Two-stage furnaces, heat pump add-ons and dehumidification inputs add their own rules. A tap set correctly on a terminal the board never energizes for that call does nothing.

7. Restore power and measure every call again. Remove your lock and tag, close the breaker standing to the side of the panel, turn the service switch on, refit the blower door so the interlock closes, and run each call. On the heat call, the burner is firing and the flue is hot: keep the CO monitor on, keep hands off the vent connector, and confirm the inducer, ignition and flame sequence are normal before you start reading. Read static and the fan table on cooling and heating, and measure temperature rise on heating per How to Measure Temperature Rise on a Gas Furnace.

8. Re-check the charge after a cooling airflow change. Less air across the coil lowers suction and changes superheat and subcooling. A system charged at the old airflow is no longer charged at the new one. Re-verify by the maker's method once the new airflow is stable.

Worked example: a 2.5-ton system on a single-stage furnace

A 60,000 BTU/h input, 80 percent single-stage furnace (output 48,000 BTU/h on the plate) with a 2.5-ton air conditioner. Four-speed PSC blower. Plate (as read on this unit): temperature rise 35 to 65 F, maximum external static 0.50 in w.c. The condenser maker asks for 350 to 450 CFM per ton. The house is in a humid climate and the complaint is "cold but sticky."

As found. COOL on HIGH, HEAT on LOW, MED-HI and MED-LO parked. Board runs the HEAT tap on G alone.

• Cooling: TESP 0.85 in w.c. Fan table, HIGH at 0.85: 1,300 CFM. That is 1,300 / 2.5 = 520 CFM per ton, over the maker's 450, and the static is 0.85 against a 0.50 plate maximum.
• Heating: TESP 0.34 in w.c. Fan table, LOW at 0.34: 820 CFM.

Targets. Cooling at the low end of the maker's range for a humid house: 375 CFM per ton x 2.5 = about 940 CFM. Heating: predicted rise on LOW is 48,000 / (1.08 x 820) = 48,000 / 885.6 = 54.2 F, inside 35 to 65 and a little above the middle (50 F). Heat stays on LOW.

Tap choice. MED-LO's column in the table, near where static should fall to, reads about 940 CFM. Square-law estimate of the new static: 0.85 x (940 / 1,300) squared = 0.85 x 0.523 = 0.44 in w.c.

 FURNACE BOARD, 4-speed PSC blower, as left

   HEAT     COOL     PARK     PARK    NEUTRAL
    |        |        |        |        |
   LOW     MED-LO    HIGH    MED-HI   motor
   lead    lead      lead    lead     common
                     (unused leads parked,
                      never cut and taped)

 G alone runs the HEAT tap on this board

After the change, measured.

• Cooling: TESP 0.44 in w.c., matching the estimate. Fan table, MED-LO at 0.44: 940 CFM, which is 940 / 2.5 = 376 CFM per ton, inside 350 to 450. Static is now under the 0.50 plate maximum.
• Heating, unchanged tap: measured rise 53 F against the predicted 54.2 F. Inside the plate range; the 1.2 F gap is well within what two probes and a fan-table read can resolve.
• Charge re-checked by subcooling at the new airflow: it had come up slightly with less air across the coil; adjusted to the maker's target.

Why both numbers moved. The tap change did two things at once: it brought airflow into the condenser maker's range and it pulled the static back under the furnace's rating. Note what it did not do: the ductwork is still restrictive. At 520 CFM per ton it measured 0.85; a 3-ton condenser on these same ducts would need about 1,125 CFM and would be back over 0.50. That belongs in the notes as a duct conversation for the next equipment change, not something a speed tap can fix.

What changes the choice

• A constant-airflow ECM. There are no leads to move; the same targets are set in the motor's configuration, and static changes do not move airflow much until the motor runs out of torque.
• A heat pump with an air handler. Heating airflow is set for the heat pump, usually close to the cooling airflow, with the heat strips' own minimum airflow as a floor; the strip kit's label states it.
• A dry climate. The same 2.5-ton system might run at 425 to 450 CFM per ton for sensible capacity, and the factory HIGH tap might even be close to right once static is checked.

Recording it

Write the tap on each terminal, the static and table CFM on each call, the CFM per ton in cooling, the measured rise in heating, and the charge re-check. Put a label inside the blower door with the taps as left. The next tech who opens the door will otherwise assume the factory settings and start from the wrong place.

References

• Furnace installation manual: fan table, board sequence, wiring diagram; outdoor unit manual for airflow per ton
• NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2)
• See related: How to Estimate Blower Airflow From Static and the Fan Table; How to Measure Total External Static Pressure on a Furnace
• See related: Airflow Per Ton Targets and What Moves Them; Temperature Rise and the Sensible Heat Airflow Formula
• See related: ECM Blower Motor Replacement and Configuration; Furnace Rating Plate Read Before Start-Up