11.4 Airflow Verification, Static Pressure Testing & Commissioning

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.

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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

1. 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.
2. Insert a second tip on the supply side, downstream of the coil and any electric heat.
3. Connect both to a manometer and read the sum of the two magnitudes (return is negative, supply is positive).
4. Take the readings with the system in its highest-airflow mode, with a clean filter in place and all registers open.

Interpreting the Result

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.

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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

$\text{CFM} = \frac{\text{Btu/h Output}}{1.08 \times \Delta T}$

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.

$\text{CFM} = \frac{64{,}000}{1.08 \times 60} = \frac{64{,}000}{64.8} = 988\text{ cfm}$

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

$\text{CFM} = \frac{\text{kW} \times 3{,}412}{1.08 \times \Delta T}$

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.

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Cooling Temperature Split

$\Delta T = \text{Return Air Dry Bulb} - \text{Supply Air Dry Bulb}$

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:

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.

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The Fan Laws

$\frac{\text{CFM}_2}{\text{CFM}_1} = \frac{\text{RPM}_2}{\text{RPM}_1} \qquad \frac{\text{SP}_2}{\text{SP}_1} = \left(\frac{\text{RPM}_2}{\text{RPM}_1}\right)^2 \qquad \frac{\text{BHP}_2}{\text{BHP}_1} = \left(\frac{\text{RPM}_2}{\text{RPM}_1}\right)^3$

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: $4{,}000 \times 1.25 = 5{,}000\text{ cfm}$
• Static pressure: $1.0 \times 1.25^2 = 1.56\text{ in. w.c.}$
• Brake horsepower: $2.0 \times 1.25^3 = 3.91\text{ BHP}$

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.

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Test, Adjust and Balance

1. Verify the design. Balancing cannot fix an undersized duct; confirm the Manual D design and required cfm per room before adjusting anything.
2. Set the fan. Adjust the sheave or the speed tap to deliver total design airflow at the measured TESP.
3. 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.
4. Re-measure the total after branch adjustments, since branch dampering raises system static.
5. Record. The balance report lists design cfm, measured cfm, and percent of design at each outlet.

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Commissioning: What a Finished Job Looks Like

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