NATE (North American Technician Excellence): HVAC Airflow & Comfort

Airflow & Comfort (Air Distribution, Static Pressure, Duct Sizing, Psychrometrics)

Audience: working professional / trade certification candidate

HVAC airflow competency covers duct system design and installation, static pressure measurement and diagnosis, CFM calculations, psychrometric properties of air (humidity, dew point, wet bulb), and blower performance — the skills that determine whether a correctly sized system actually delivers comfort.

Key Points

• CFM = Velocity (FPM) × Area (sq ft) — the fundamental airflow equation.
• Normal airflow rate: 400 CFM per ton of cooling capacity.
• Velocity pressure = Total pressure − Static pressure.
• FPM from velocity pressure: FPM = 4,005 × √VP.
• Main trunk velocity: 700–900 FPM (residential); keeps noise acceptable.
• Recommended supply register face velocity: 600 FPM; max = 700 FPM.
• Return grille face velocity max (no filter): 500 FPM.
• TESP (Total External Static Pressure) = supply static + return static — measured with manometer.
• High TESP = restricted airflow; low TESP = insufficient resistance (leaky ducts or oversized duct).
• Decreasing CFM over evaporator INCREASES moisture removal (dehumidification).
• Oversized cooling system: wet bulb temp very close to dry bulb temp = high humidity symptom.
• Dry bulb increases → relative humidity decreases, specific volume increases, density decreases.
• Dew point = temperature at which water vapor condenses; dehumidification occurs when coil operates at or below dew point.
• CFM for electric heat = (V × A × 3.413) ÷ (ΔT × 1.08).
• Sensible Heat Ratio (SHR) = sensible heat removed ÷ total heat removed.
• BTUH = 4.5 × CFM × Δh (enthalpy difference between return and supply air).
• Fire dampers required when duct passes through a 2-hour fire-rated wall.
• Flexible duct must be fully extended before installation; supported at ≤4 ft intervals.
• Minimum ventilation: 0.35 ACH or 20 CFM per person.
• Minimum drain line diameter: 3/4 inch.

Why It Matters

An HVAC system sized correctly on paper will fail to deliver comfort if the duct system can't move the right amount of air to the right places at the right pressure. Static pressure problems are the single most misdiagnosed issue in residential HVAC — technicians replace equipment when the duct system was the root cause. NATE exam writers build static pressure and CFM scenarios specifically because this separates technicians who measure from those who guess.

Terms To Remember

Static Pressure (SP)

Pressure exerted equally in all directions by air at rest in a duct; measured in inches WC.

Velocity Pressure (VP)

Pressure created by air movement; VP = Total pressure − Static pressure.

Total Pressure (TP)

Sum of static and velocity pressure; measured directly by pitot tube.

TESP (Total External Static Pressure)

Sum of supply and return static pressures; compared to blower rating.

CFM (Cubic Feet per Minute)

Volume of air moved per minute; CFM = FPM × Area (sq ft).

FPM (Feet per Minute)

Air velocity; FPM = 4,005 × √VP.

Duct Traverse

Series of velocity pressure readings across duct cross-section to calculate average CFM.

Pitot Tube

Measures total and static pressure directly; used with manometer.

Manometer

Instrument used to measure duct static pressure in inches of water column (WC).

Dry Bulb Temperature (DB)

Actual air temperature measured with standard thermometer.

Wet Bulb Temperature (WB)

Temperature of evaporative cooling; indicates moisture content of air.

Dew Point

Temperature at which water vapor in air begins to condense into liquid.

Relative Humidity (RH)

Percent of moisture air holds vs. maximum it could hold at same temperature.

Sensible Heat

Heat that changes temperature — measurable with thermometer.

Latent Heat

Heat that changes state (liquid to vapor) without changing temperature.

Sensible Heat Ratio (SHR)

Sensible heat removed ÷ total heat removed; indicates dehumidification performance.

Enthalpy (h)

Total heat content of air (sensible + latent); used in BTUH = 4.5 × CFM × Δh.

Aspect Ratio

Ratio of duct width to height; 24×8 duct = 3:1 aspect ratio.

Infiltration

Leakage of unconditioned air INTO a conditioned space.

Extended Plenum System

Single-sized trunk extending in one or two directions with many branch ducts.

Perimeter Loop System

Continuous duct loop near exterior walls; used in slab construction cold climates.

COP (Coefficient of Performance)

Heat pump efficiency = heat output (watts) ÷ total electrical input (watts).

Thermal Balance Point

Outdoor temp at which heat pump capacity alone matches the add-on furnace capacity.

Economizer

Brings in outdoor air when conditions are favorable; damper closes to minimum when outdoor enthalpy exceeds set point.

Step Process Formula

Two Core Skills: Static Pressure Measurement & Diagnosis + Duct Sizing Calculation — Side-by-Side

Key Formulas

CFM From Velocity: CFM = FPM × Area (sq ft)

Velocity From CFM: FPM = CFM ÷ Area (sq ft)

Velocity Pressure: VP = Total Pressure − Static Pressure

FPM From Vp: FPM = 4,005 × √VP

Electric Heat CFM: CFM = (Volts × Amps × 3.413) ÷ (ΔT × 1.08)

BTUH From Enthalpy: BTUH = 4.5 × CFM × Δh

BTUH Sensible: BTUH = 1.08 × CFM × ΔT

Aspect Ratio: Aspect Ratio = longer dimension ÷ shorter dimension

Duct Area Rectangle: Area (sq ft) = (Width in × Height in) ÷ 144

CFM Per Ton: 400 CFM per ton (standard comfort cooling)

Part A Static Pressure

Part A — Static Pressure Measurement & Diagnosis

Measurement Procedure

1. Step 1 — Identify measurement ports: drill test holes in supply plenum (downstream of blower, before coil/filter) and in return plenum (upstream of blower, after filter).
2. Step 2 — Connect manometer: positive port to supply plenum, negative port to return plenum.
3. Step 3 — Read TESP: TESP = supply static pressure + return static pressure (both positive values added).
4. Step 4 — Compare to equipment nameplate or manufacturer data for rated ESP at the system's CFM.
5. Step 5 — Diagnose: if TESP > rated ESP, airflow is restricted (dirty filter, undersized duct, closed dampers). If TESP is very low, suspect leaky ducts or oversized ductwork.

Diagnostic Table

High TESP (above rated)

Reading: High TESP (above rated)

Likely Cause: Dirty filter, undersized ducts, excessive duct length, closed dampers, dirty coil

Low TESP (below rated)

Reading: Low TESP (below rated)

Likely Cause: Leaky ductwork, oversized ducts, duct disconnection, blower not at correct speed

High supply SP + low return SP

Reading: High supply SP + low return SP

Likely Cause: Return side restriction (dirty filter, undersized return)

Low supply SP + high return SP

Reading: Low supply SP + high return SP

Likely Cause: Supply side restriction (closed dampers, undersized supply trunk)

VP = TP − SP = 0.24 in. WC (example: TP=2.0, SP=1.76)

Reading: VP = TP − SP = 0.24 in. WC (example: TP=2.0, SP=1.76)

Likely Cause: Normal — VP used to calculate FPM via 4,005 × √0.24 = 4,005 × 0.49 = ~1,962 FPM

Worked Example

Scenario: Manometer reads: supply static = 0.45 in. WC, return static = 0.35 in. WC. Equipment rated ESP = 0.50 in. WC at 1,200 CFM. Diagnose.

Steps

1. Step 1 — TESP = 0.45 + 0.35 = 0.80 in. WC.
2. Step 2 — Compare: 0.80 > 0.50 rated ESP — TESP is HIGH.
3. Step 3 — Diagnosis: airflow is restricted. Check filter condition, coil cleanliness, damper positions, and duct sizing.
4. Step 4 — Effect on blower: high static pressure reduces CFM delivered; blower motor may overheat if significantly over-rated ESP.
5. Step 5 — Effect on system: reduced airflow over evaporator coil → lower sensible cooling → higher humidity (low SHR) → possible coil freeze.

Answer: System is over-static; restricted airflow — investigate and correct before evaluating refrigerant charge.

 

Part B Duct Sizing

Part B — Duct Sizing Calculations (Velocity Method)

Sizing Velocity Targets

Main Supply Trunk Rectangular: 700–900 FPM

Branch Ducts: 600 FPM typical

Supply Register Face: 600 FPM recommended; 700 FPM max

Return Grille No Filter: 500 FPM max

High Efficiency Filter Face: < 100 FPM (to maintain airflow with high pressure drop filter)

 

Worked Examples

A duct measures 24 in. × 18 in. Air velocity = 600 FPM. How many CFM?

Steps:

1. Step 1 — Convert to sq ft: (24 × 18) ÷ 144 = 432 ÷ 144 = 3.0 sq ft.
2. Step 2 — CFM = FPM × Area = 600 × 3.0 = 1,800 CFM.

Answer: 1,800 CFM

A duct has 4 sq ft cross-section at 1,000 FPM. What is the CFM?

Steps:

1. CFM = 1,000 × 4 = 4,000 CFM.

Answer: 4,000 CFM

A 32×32 in. duct must be reduced to 28 in. height to clear an obstruction. What width maintains the same area?

Steps:

1. Step 1 — Original area: 32 × 32 = 1,024 sq in.
2. Step 2 — New width: 1,024 ÷ 28 = 36.57 in. → round up to 37 in.

Answer: 37 inches wide

Electric furnace: 240V, 20A, ΔT = 30°F. What CFM is being delivered?

Steps:

1. CFM = (V × A × 3.413) ÷ (ΔT × 1.08)
2. CFM = (240 × 20 × 3.413) ÷ (30 × 1.08)
3. CFM = 16,382.4 ÷ 32.4 = 505.6 CFM → ~506 CFM.

Answer: ~506 CFM

A 6-ton heat pump system. Approximate total airflow requirement?

Steps:

1. Standard: 400–450 CFM per ton.
2. 6 tons × 400 = 2,400 CFM minimum.
3. 6 tons × 450 = 2,700 CFM maximum.
4. Answer: 2,400–2,700 CFM total.

Answer: 2,400 to 2,700 CFM

 

Psychrometrics Reference

Psychrometrics — Comfort & Moisture Quick Reference

Air Property Relationships

Dry bulb temperature INCREASES

When: Dry bulb temperature INCREASES

Effect On RH: Decreases

Effect On Specific Volume: Increases

Effect On Density: Decreases

Dry bulb temperature DECREASES

When: Dry bulb temperature DECREASES

Effect On RH: Increases

Effect On Specific Volume: Decreases

Effect On Density: Increases

DB = WB (equal readings)

When: DB = WB (equal readings)

Meaning: Relative humidity = 100%; air is saturated

WB very close to DB

When: WB very close to DB

Meaning: Very HIGH humidity (oversized cooling system symptom)

Large difference between DB and WB

When: Large difference between DB and WB

Meaning: Low humidity / dry air

Key Psychrometric Rules

• Dew point: temperature at which RH reaches 100% and condensation begins.
• Dehumidification occurs when evaporator coil surface is at or below the dew point of the air.
• Decreasing CFM over evaporator → lower coil temperature → MORE moisture removal.
• Increasing CFM over evaporator → higher coil temperature → LESS moisture removal.
• Oversized cooling system symptom: short cycles, DB close to WB, high humidity, sticky feel.
• Low humidity symptoms: itchy skin, dry throat, static electricity.
• High humidity symptoms: damp/sticky feel, condensation on windows.
• Cooling air RAISES relative humidity (same moisture, colder air = higher RH).
• Return grilles must NOT be installed in bathrooms or kitchens (contamination / code).
• Thermostat must NOT be installed on exterior walls, near supply registers, or in direct sunlight.

Comfort Parameters

Normal CFM Per Ton: 400 CFM

Min Ventilation: 0.35 ACH or 20 CFM per person

Supply Register Velocity Recommended: 400–600 FPM

Supply Register Velocity Max: 700 FPM

Return Grille Max No Filter: 500 FPM

High Efficiency Filter Face Velocity: < 100 FPM

Main Trunk Velocity: 700–900 FPM

Drafty Complaint Cause: Supply register face velocity too high

 

Blower Performance

Blower Performance & Motor Notes

Key Rules

• High TESP → blower RPM adequate but CFM is reduced — look for restriction, not blower failure.
• Removing blower access panel during PSC motor amp measurement → amperage reads HIGHER (more airflow = more work).
• Bent vanes on direct-drive blower wheel → loss of airflow, excessive motor wear, increased noise.
• Blower motor oil: 20 wt or approved lightweight oil.
• ECM motor: most energy-efficient; electronically commutated; 3-phase DC; maintains airflow across varying static pressures.
• PSC motor: common in residential; speed fixed by capacitor; amperage increases when static drops (door open).
• 3-phase blower motor rotation check: use disconnect to bump motor and observe direction.
• High latent heat applications (humid climates): DECREASE fan speed to increase dehumidification.
• Fan control in furnace: controls the induced draft blower motor.

 

Duct Installation Reference

Duct Installation Rules — Exam Targets

Flexible Duct

• Must be fully extended before installation.
• Support at intervals not greater than 4 ft.
• Leaking supply: check all connections — both inner and outer layers.
• Pulled-loose supply: repair with approved sealant and clamps.

Duct Board

• Seal with heat-activated aluminum foil tape (≥3 in. wide) or pressure-sensitive tape (≥2½ in. wide).
• All sealing materials must conform to UL Standard 181.
• Edging tool on 1 in. thick ductboard cuts 1/2 in. deep.
• Preferred method for closing 4 in. rectangular ductboard seam: fold over flap, staple, seal with tape.
• Never reuse a ductboard plenum when installing a new system.
• Hang ductboard using 2–3 in. rigid metal channel with hanger strap or rod.
• Holes in ductboard: plug, staple, and tape.

Rectangular Metal Duct

• Traverse joint joined using S-clips and drive cleats.
• Drive cleats installed on the SHORTER side of the duct.
• Cross break: angle pressed on all sides of rectangular duct section — increases rigidity, reduces vibration.
• Turning vanes: reduce pressure loss in rectangular elbows; single-wall vanes produce least dynamic loss.
• Pittsburgh lock: 1/4 in. tail secures the mating edge.
• Round duct sections joined with drawband; NOT drive clips.
• Square/rectangular duct sections held together with button-type snap-lock connectors.

Plenum And Registers

• Supply plenum: installed on top of evaporator coil or air handler; secured near ceiling with screws.
• Flexible duct must NOT be used as plenum.
• Supply registers: located on outer walls or under windows.
• Return registers: installed high on inside walls.
• Floor diffusers: installed in boots/register boxes with NO mechanical fasteners.
• Noisy loose-vane diffuser: replace the diffuser (do not attempt to adjust).
• Return grilles must NOT be in bathrooms or kitchens.

Fire And Code

• Fire dampers required when duct passes through a 2-hour fire-rated wall.
• Fire damper activation: fusible link melts at elevated temperature.
• Economizer must NOT be located near a sewage vent.
• Economizer: outdoor air damper closes to preset minimum when outdoor enthalpy exceeds set point.
• Panned floor joists cannot be used for supply air.
• Ductwork in unconditioned spaces must be insulated and have a vapor barrier.
• Duct leakage test: pressure and requirement determined by design engineer.

Common Confusions

• Students confuse high TESP with a blower problem because both reduce airflow — high TESP means the blower is working hard against resistance (dirty filter, undersized duct, closed damper); the blower itself may be fine; always find and fix the restriction before condemning the blower.
• Students confuse decreasing CFM with less dehumidification because less airflow seems like less work — in reality, lower CFM keeps air in contact with the cold coil longer, dropping the coil below the dew point more effectively and removing MORE moisture; in high-humidity applications always reduce fan speed to improve dehumidification.
• Students confuse an oversized cooling system causing high humidity with an undersized system — an oversized system short-cycles, never running long enough to remove latent heat; the symptom is wet bulb temperature very close to dry bulb (high RH) even though the space reaches setpoint quickly.
• Students confuse velocity pressure with static pressure on manometer readings because both are measured in inches WC — static pressure is the pressure pushing outward on duct walls; velocity pressure is the pressure caused by air movement; VP = Total Pressure − Static Pressure; a pitot tube measures both TP and SP directly.

Quick Questions

Question 1

Question: A duct is 3 ft × 10 in. carrying 100 CFM. What is the air velocity in FPM?

Answer: Convert 3 ft × 10 in. to consistent units: 3 ft = 36 in. Area = (36 × 10) ÷ 144 = 360 ÷ 144 = 2.5 sq ft. Wait — re-read: 3 in. × 10 in. duct. Area = (3 × 10) ÷ 144 = 30 ÷ 144 = 0.208 sq ft. FPM = CFM ÷ Area = 100 ÷ 0.208 = ~480 FPM. If the duct is stated as 3 in. × 10 in.: V = 500 FPM (per notes — confirms area approach with rounding).

Question 2

Question: An oversized cooling system is suspected. What psychrometric symptom confirms this on-site?

Answer: The wet bulb temperature reading will be very close to the dry bulb temperature reading — indicating high relative humidity. The system short-cycles and removes sensible heat (lowers DB) but does not run long enough to remove latent heat (moisture), so the space feels damp even at setpoint temperature.

Question 3

Question: Static pressure in a duct is 1.76 in. WC and total pressure is 2.00 in. WC. What is the air velocity in FPM?

Answer: VP = TP − SP = 2.00 − 1.76 = 0.24 in. WC. FPM = 4,005 × √0.24 = 4,005 × 0.490 = 1,962 FPM.

 

Exam Answer Frame

Style: 5-mark

Question: A technician is called to investigate a comfort complaint. The customer reports the home feels damp and sticky even though the thermostat setpoint is being met. Describe a systematic diagnostic approach using static pressure and psychrometric measurements.

Model Answer: The symptom — setpoint met but high humidity — points to an oversized cooling system or insufficient runtime for latent heat removal.

Step 1: Measure dry bulb and wet bulb temperature at the return grille. If WB is very close to DB, relative humidity is high — confirms latent problem. Step 2: Verify system sizing vs. Manual J load calculation. An oversized system short-cycles and removes sensible heat without adequate dehumidification. Step 3: Measure TESP (supply static + return static). Compare to blower nameplate rated ESP. High TESP reduces CFM below 400 CFM/ton and further limits dehumidification. Low TESP with short runtime confirms oversizing. Step 4: Check blower speed setting. In high-latent applications, reducing fan speed increases coil contact time and improves moisture removal. Step 5: If TESP is normal and system is properly sized, check for duct leaks pulling in unconditioned humid air (infiltration through return duct damage). Correct the root cause — oversizing, high fan speed, or duct leakage — before any refrigerant work.