11.4 Airflow Verification, Static Pressure Testing & Commissioning

Alabama HVAC Contractor License Exam

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1 Alabama HACR Laws, Rules & Licensing

1.1 Alabama HACR Board Authority, Licensing Pathways & Experience Requirements 1.2 Contractor Bonding, Insurance, Fees & Annual Renewal 1.3 Permits, Local Jurisdictions, Inspections & Disciplinary Actions

2 Contractor Business, Contracts, Estimating & Alabama Lien Law

2.1 Business Organization, Risk Management & Project Administration 2.2 Construction Contracts, Change Orders & Warranties 2.3 Alabama Mechanics' Lien Law & Financial Management 2.4 Construction Estimating, Markup & Bid Preparation 2.5 Project Management, Scheduling & Field Supervision 2.6 Risk Management, Contractor Insurance & Surety Bonds

3 Safety Standards, OSHA & EPA Section 608

3.1 OSHA Construction Safety (29 CFR 1926) for HVAC Contractors 3.2 EPA Section 608 Refrigerant Regulations & Recovery Requirements 3.3 Refrigerant Management, Leak Repair Rules & A2L Transition

4 General Technical Knowledge & Thermodynamics

4.1 Principles of Heat Transfer, Temperature & Pressure 4.2 The Mechanical Refrigeration Cycle & Refrigerant Properties 4.3 Psychrometrics & Air Properties for Humid Climates 4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC

5 Piping Systems, Refrigerant Lines & Fuel Gas Piping

5.1 Refrigerant Piping Sizing, Materials & Installation 5.2 Fuel Gas Piping Sizing & Installation (IRC Chapter 24) 5.3 Condensate Drain Systems & Trapping

6 System Sizing & ACCA Manual J Load Calculations

6.1 ACCA Manual J Fundamentals & Design Conditions 6.2 Envelope Loads: Conduction, Radiation & Fenestration 6.3 Infiltration, Ventilation & Internal Heat Gains

7 Duct System Design & ACCA Manual D

7.1 Airflow Principles, Static Pressure & Fan Performance 7.2 ACCA Manual D Duct Sizing & Friction Rate 7.3 Duct Materials, Installation Standards & Air Distribution

8 Heating & Cooling Equipment Operation

8.1 Heat Pump Systems & Supplemental Heating 8.2 Gas Furnaces & Combustion Systems 8.3 Air Conditioning Units, Compressors & Metering Devices

9 Mechanical Codes, Electrical Applications & Maintenance Diagnostics

9.1 Combustion Air & Venting Systems (IRC Chapter 24) 9.2 Electrical Applications for HVAC (NFPA 70 / NEC Article 440) 9.3 Preventative Maintenance, Troubleshooting & Plan Reading

10 Employment Law, Payroll Taxes & Contractor Finance

10.1 Federal & Alabama Labor Laws for HVAC Employers 10.2 Payroll, Employment Taxes & Worker Classification 10.3 Job Costing, Break-Even Analysis & Cash Flow Forecasting

11 Preventative Maintenance, Service Procedures & Commissioning

11.1 Preventative Maintenance Programs & Service Agreements 11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate 11.3 Evacuation, Dehydration & Refrigerant Charging Procedures 11.4 Airflow Verification, Static Pressure Testing & Commissioning

12 Electrical Applications & Construction Documents

12.1 Electric Motors, Compressors & Motor Protection 12.2 Transformers, Control Circuits & Low-Voltage Wiring 12.3 Branch Circuits, Conductors, Grounding & Disconnects 12.4 Construction Documents, Specifications & Submittals

11.4 Airflow Verification, Static Pressure Testing & Commissioning

Key Takeaways Explain This Topic AI

Total external static pressure is measured across the air handler with the filter and coil in place, and most residential equipment is rated at only 0.50 in. w.c.

Airflow can be computed from a furnace temperature rise using CFM = Btu/h output divided by (1.08 times the temperature rise).

A cooling temperature split of roughly 16 to 22 degrees Fahrenheit at 400 cfm per ton indicates normal operation; a wide split points to low airflow and a narrow split to low capacity.

The fan laws state that airflow varies directly with fan speed, static pressure with the square of speed, and brake horsepower with the cube of speed.

Commissioning is a measured handover: design airflow at every outlet, verified charge, verified safeties, a balance report, and owner training - not simply a system that turns on.

Last updated: September 2026

11.4 Airflow Verification, Static Pressure Testing & Commissioning

Important

Airflow first, refrigerant second. Most systems diagnosed as "low on charge" in the field are actually short of air. Every measurement in this section should be taken before a set of gauges is connected.

Total External Static Pressure

Total external static pressure (TESP) is the sum of the pressure the blower must overcome outside the equipment cabinet - supply duct, return duct, filter, and coil.

How to Measure

Drill or use existing test ports and insert a static pressure tip pointing into the airflow on the return side, between the filter and the blower.

Insert a second tip on the supply side, downstream of the coil and any electric heat.

Connect both to a manometer and read the sum of the two magnitudes (return is negative, supply is positive).

Take the readings with the system in its highest-airflow mode, with a clean filter in place and all registers open.

Interpreting the Result

• Measured TESP Meaning
• At or below the rated value, commonly 0.50 in. w.c. Duct system is within the equipment's design
• 0.60 to 0.80 in. w.c. Restriction present; airflow is already below nominal
• Above 0.80 in. w.c. Serious restriction; capacity and equipment life are both compromised

Once TESP is known, break it into components - filter drop, coil drop, supply drop, return drop - to identify which part of the system is restrictive. A 0.75 in. w.c. total made up of 0.35 in. w.c. of filter drop is a filter problem, not a duct problem.

• Four Ways to Determine Actual Airflow
• 1. Blower Table Lookup

Enter the manufacturer's blower performance table with the measured TESP and the selected speed tap. Fast, and accurate enough for most service work, provided the table matches the exact model.

2. Temperature Rise on a Gas Furnace CFM = Btu/h Output 1.08 × Δ T \text{CFM} = \frac{\text{Btu/h Output}}{1.08 \times \Delta T} CFM = 1.08 × Δ T Btu/h Output ​

Worked example : an 80 percent furnace with an 80,000 Btu/h input produces 64,000 Btu/h of output. Measured supply-air temperature is 130 F and return is 70 F, a rise of 60 F. CFM = 64,000 1.08 × 60 = 64,000 64.8 = 988 cfm \text{CFM} = \frac{64{,}000}{1.08 \times 60} = \frac{64{,}000}{64.8} = 988\text{ cfm} CFM = 1.08 × 60 64 , 000 ​ = 64.8 64 , 000 ​ = 988 cfm

Compare the measured rise against the nameplate temperature rise range (commonly 35 to 65 F). A rise above the range means airflow is too low; below the range means airflow is too high or the furnace is underfiring.

3. Temperature Rise on Electric Heat CFM = kW × 3,412 1.08 × Δ T \text{CFM} = \frac{\text{kW} \times 3{,}412}{1.08 \times \Delta T} CFM = 1.08 × Δ T kW × 3 , 412 ​

Electric strip heat is a precise heat source, which makes this the most accurate field method on an air handler. Use measured voltage and amperage to compute actual kW rather than the nameplate rating.

4. Flow Hood or Anemometer Traverse

A balancing hood reads each outlet directly; the sum approximates system airflow, though duct leakage downstream of the equipment will make the sum read low. A rotating-vane or hot-wire anemometer traverse in a straight duct run is more accurate but requires a proper traverse grid.

Cooling Temperature Split Δ T = Return Air Dry Bulb − Supply Air Dry Bulb \Delta T = \text{Return Air Dry Bulb} - \text{Supply Air Dry Bulb} Δ T = Return Air Dry Bulb − Supply Air Dry Bulb

At roughly 400 cfm per ton and typical indoor conditions, a healthy split runs about 16 to 22 F . The split is a function of both airflow and entering wet bulb:

• Observation Likely cause
• Split well above 22 F Low airflow - dirty filter, dirty coil, loaded blower wheel, crushed flex
• Split well below 16 F Low capacity - undercharge, failing compressor valves, non-condensables
• Split normal but house not cooling Undersized system, duct losses in an unconditioned attic, high infiltration

In Alabama's humidity the split is also read alongside indoor relative humidity. A system holding an acceptable dry-bulb temperature at 62 percent indoor relative humidity is moving too much air for the latent load, and reducing airflow toward 350 cfm per ton often restores comfort.

The Fan Laws CFM 2 CFM 1 = RPM 2 RPM 1 SP 2 SP 1 = ( RPM 2 RPM 1 ) 2 BHP 2 BHP 1 = ( RPM 2 RPM 1 ) 3 \frac{\text{CFM}_2}{\text{CFM}_1} = \frac{\text{RPM}_2}{\text{RPM}_1} \qquad \frac{\text{SP}_2}{\text{SP}_1} = \left(\frac{\text{RPM}_2}{\text{RPM}_1}\right)^2 \qquad \frac{\text{BHP}_2}{\text{BHP}_1} = \left(\frac{\text{RPM}_2}{\text{RPM}_1}\right)^3 CFM 1 ​ CFM 2 ​ ​ = RPM 1 ​ RPM 2 ​ ​ SP 1 ​ SP 2 ​ ​ = ( RPM 1 ​ RPM 2 ​ ​ ) 2 BHP 1 ​ BHP 2 ​ ​ = ( RPM 1 ​ RPM 2 ​ ​ ) 3

Worked example : a belt-drive blower delivers 4,000 cfm at 600 rpm against 1.0 in. w.c. and draws 2.0 brake horsepower. Increase speed to 750 rpm - a factor of 1.25.

Airflow: 4,000 × 1.25 = 5,000 cfm 4{,}000 \times 1.25 = 5{,}000\text{ cfm} 4 , 000 × 1.25 = 5 , 000 cfm

Static pressure: 1.0 × 1.25 2 = 1.56 in. w.c. 1.0 \times 1.25^2 = 1.56\text{ in. w.c.} 1.0 × 1.2 5 2 = 1.56 in. w.c.

Brake horsepower: 2.0 × 1.25 3 = 3.91 BHP 2.0 \times 1.25^3 = 3.91\text{ BHP} 2.0 × 1.2 5 3 = 3.91 BHP

A 25 percent airflow increase nearly doubles the horsepower. This is the calculation that decides whether the existing motor can take a sheave change, and it is a recurring exam item.

Test, Adjust and Balance

Verify the design. Balancing cannot fix an undersized duct; confirm the Manual D design and required cfm per room before adjusting anything.

Set the fan. Adjust the sheave or the speed tap to deliver total design airflow at the measured TESP.

Proportionally balance branches. Open the branch with the greatest restriction fully, then dampers on the remaining branches down to proportion. Dampers are set at the takeoff , not at the register, so throw and noise are preserved.

Re-measure the total after branch adjustments, since branch dampering raises system static.

Record. The balance report lists design cfm, measured cfm, and percent of design at each outlet.

Commissioning: What a Finished Job Looks Like

• Item Evidence
• Airflow TESP reading plus measured cfm and the method used
• Charge Weigh-in amount, or superheat/subcooling readings with the target and conditions
• Electrical Voltage, compressor and fan amperage against nameplate, capacitor microfarads
• Combustion Manifold pressure, temperature rise, carbon monoxide, draft
• Safeties Verified operation of the high limit, rollout, pressure switch, and float switch
• Controls Thermostat configured for equipment type and staging; heat pump balance point set
• Balance Outlet-by-outlet report against design
• Owner Filter size and location, thermostat operation, signed training record

A job is not finished when the equipment runs. It is finished when the numbers are on paper.