29.2 Air Volumes, Enthalpy, Wet-Bulb & Dry-Bulb
Key Takeaways
- Typical comfort-cooling airflow is about 400 cfm per ton (often a 350–450 band on expanded data). A 5-ton coil that is actually delivering 2,000 cfm is in family; 1,200 cfm is a low-air problem, not a refrigerant problem to charge through.
- CFM = FPM × free area (ft²). A velometer or pitot traverse gives velocity; a flow hood reads cfm at the grille. For standard air, pitot velocity is V ≈ 4005 × √VP with VP in inches of water.
- Total cooling Qt = 4.5 × CFM × Δh (enthalpy in Btu/lb of dry air). Sensible cooling is 1.08 × CFM × ΔT. Dry-bulb is the ordinary thermometer; wet-bulb is the wetted-wick reading that locates enthalpy on the chart.
- Working (design) pressure is the nameplate/piping rating; FBC Mechanical 1110 tests field refrigerant pipe at not less than the lower of design or relief. Operating pressure is what the gauges show while the machine runs — they are not the same number.
- Non-pressure tanks (open cooling-tower basins, atmospheric condensate receivers, open expansion tanks) are vented to atmosphere. They are not diagnosed with a refrigerant manifold and are not ASME unfired pressure vessels (Class A scope for boilers/unfired vessels; Class B does not install those).
29.2 Air Volumes, Enthalpy, Wet-Bulb & Dry-Bulb
The same Trade E Maintenance Analysis cluster (Class A 9 percent, Class B 10 percent) now asks for the air side of the diagnosis and two pressure-vessel distinctions the outline lists with it: air volumes (velometer, pitot tube, manometer, flow hood), enthalpy changes, wet-bulb and dry-bulb, working pressure in pipes, and non-pressure tanks. Chapter 17 designed CFM with (1.08 \times \Delta T). This section measures whether that air is actually moving and whether the coil is actually changing enthalpy. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the instrument book. FBC Mechanical 2023 still owns design pressure and tanks. Class B still stops at 25 tons / 500,000 Btu per system.
Quick Answer: Typical comfort-cooling air is about 400 cfm/ton. CFM = FPM × area. Read velocity with a velometer or pitot + manometer; read grille cfm with a flow hood. Dry-bulb and wet-bulb locate enthalpy; (Q_t = 4.5 \times \text{CFM} \times \Delta h). Working pressure is the pipe/vessel rating. Non-pressure tanks are atmospheric.
Why air volume is a refrigeration diagnosis
Low airflow looks like a refrigerant problem if you only read superheat. A dirty filter, a wet coil, a backward blower, or a crushed flex run drops evaporator load. Superheat falls, the coil ices, suction drops, and someone adds charge. Trade E scores whether you measure cfm before you open a cylinder. High airflow that bypasses the coil (missing filter rack, open return) raises superheat and cuts dehumidification — another Florida no-cool that is not a TXV.
About 400 cfm per ton is the typical comfort-cooling check on a DX coil (a manufacturer’s expanded-data point often lands in a 350–450 cfm/ton band). It is not a Florida code-required design formula — design still uses CFM = sensible Btu/h ÷ (1.08 × ΔT) (Chapter 17) and the coil’s rated cfm. On a service call, a 3-ton air handler that should be near 1,200 cfm and is delivering 700 is a blower/duct/filter problem. A 5-ton that should be near 2,000 cfm and is at 2,050 with a 20°F ΔT is in family; then you go to P-h.
Worked 400 cfm/ton check. Nameplate 4 tons. Expected air ≈ (4 \times 400 = 1{,}600) cfm. A flow hood on all supplies totals 1,120 cfm (280 cfm/ton). Do not add 410A. Pull the 90-day filter, check the 230-volt blower tap, and measure external static. Class B is legal on 4 tons. The same math on a 30-ton VAV box is Class A contracting if it is one system.
Instruments — velometer, pitot, manometer, flow hood
| Instrument | What it reads | How you turn it into cfm |
|---|---|---|
| Velometer (vane anemometer) | Air speed, feet per minute (fpm) | Traverse the duct or grille; average FPM × free area (ft²) |
| Pitot-static tube | Total and static pressure; velocity pressure VP = TP − SP | (V \approx 4005 \sqrt{\text{VP}}) fpm for standard air (VP in inches of water); then × area |
| Manometer (U-tube, digital, or Magnehelic) | Pressure difference, inches of water column (in. w.c.) | Hooked to the pitot, to a coil ΔP, or to duct static |
| Flow hood (capture hood) | Direct cfm at a supply or return opening | Best for registers; still needs a tight seal to the grille |
Velometer traverse. Do not take one center reading and call it average. A duct traverse uses a grid (often 6 or more points in a small trunk, more in a large one). Average the velocities.
Worked velometer. A 16 × 20 inch supply grille, manufacturer free-area factor 0.70, nine-point average 650 fpm.
Gross area = ((16/12) \times (20/12) = 2.22) ft². Free area = (2.22 \times 0.70 = 1.55) ft².
(\text{CFM} = 650 \times 1.55 \approx 1{,}008\ \text{cfm})
Using gross area without the free-area factor invents 1,443 cfm and you walk away from a low-air coil.
Worked pitot. A pitot in a 12 × 12 inch internal duct (1.0 ft²) reads VP = 0.16 in. w.c. on the manometer.
(V = 4005 \times \sqrt{0.16} = 4005 \times 0.40 = 1{,}602\ \text{fpm})
(\text{CFM} = 1{,}602 \times 1.0 = 1{,}602\ \text{cfm})
The 4005 constant is for standard air (~0.075 lb/ft³). Hot attic air is lighter; a precise TAB uses density correction. On CBT, 4005 × √VP is the expected move. Point the pitot into the flow; a backwards tube reads junk. The manometer must be zeroed and on the in. w.c. scale, not psig.
A flow hood is the fast register tool. Add the supplies and compare to return; a large mismatch is duct leakage (Trade H blower-door/duct tests go deeper). Do not use a flow hood as a refrigerant scale.
Wet-bulb, dry-bulb, and enthalpy change
Dry-bulb (DB) is the ordinary airstream thermometer. Wet-bulb (WB) is a thermometer with a wetted wick in moving air — it approaches the adiabatic-saturation temperature. A sling or digital psychrometer that is not aspirated reads a fake WB. Enthalpy (h) is total heat, Btu per pound of dry air. On the sea-level psychrometric chart, enthalpy lines run nearly with wet-bulb. Two independent properties (commonly DB and WB) locate the state; then you read (h).
Approximate moist-air enthalpy is (h \approx 0.24,t_{DB} + W(1061 + 0.445,t_{DB})), with (W) in lb moisture per lb dry air. On the exam you will usually read (h) at the DB/WB intersection rather than derive (W).
| Load | Standard-air equation | Diagnostic use |
|---|---|---|
| Sensible (Q_s) | (1.08 \times \text{CFM} \times \Delta T_{DB}) | Supply-to-return dry-bulb split |
| Total (Q_t) | (4.5 \times \text{CFM} \times \Delta h) | Capacity check across the coil |
| Latent (Q_l) | (0.68 \times \text{CFM} \times \Delta\text{grains}) | Whether the coil is wringing Florida air |
The 4.5 factor is (60 \times 0.075). AHRI indoor coil air is often near 80°F DB / 67°F WB, enthalpy about 31.5 Btu/lb. Leaving air on a healthy DX coil is often near saturation (WB ≈ DB), enthalpy in the low-20s Btu/lb.
Worked capacity. Measured 1,600 cfm. Return 80/67, (h_{in} \approx 31.5) Btu/lb. Supply 58/57, (h_{out} \approx 24.5) Btu/lb. (\Delta h = 7.0) Btu/lb.
(Q_t = 4.5 \times 1{,}600 \times 7.0 = 50{,}400\ \text{Btu/h} \approx 4.2\ \text{tons})
On a 4-ton nameplate that is in family once you allow for rating conditions. If the same 1,600 cfm only drops enthalpy 4.0 Btu/lb, (Q_t = 28{,}800) Btu/h — 2.4 tons on a 4-ton box. Now the P-h diagnosis in 29.1 earns its keep (undercharge, restriction, bad compressor valves, or a reversing valve leaking through).
Worked split that lies. Return 75°F DB, supply 55°F DB looks like a pretty 20°F drop. WB only fell 2°F and the house is still clammy: high sensible-heat ratio, short-cycling, or bypass. Florida latent load is not optional. A coil leaving at 55°F DB and 54°F WB is actually dehumidifying; a coil leaving at 55°F DB and 52°F WB with a 70°F wet return is not the same process.
Working pressure in pipes vs non-pressure tanks
Operating pressure is the running (or standing) gauge reading from 29.1. Working pressure (design pressure, MAWP on a vessel nameplate) is the rating the pipe or vessel is built and listed to carry in service. FBC Mechanical 1110.5 pressure-tests field refrigerant piping at not less than the lower of design pressure or the relief setting — that design number is the working/design rating, not yesterday’s suction gauge. A healthy R-410A high side at 400 psig on a 95°F day is operating pressure. The condensing-unit nameplate high-side design (often in the 600 psig neighborhood on 410A — read the plate, do not memorize a fake CILB key) is working/design pressure. Relief must be set at or below that rating. If operating pressure is climbing toward relief, stop diagnosing with “add gas” and find the dirty condenser or noncondensables.
Hydronic and condenser-water pipe also have a working pressure (a 150 psi class main is not a 15 psi toy). Pump head of 30 psig on a tower loop is not R-410A. Do not hydro-test a 150 psi chilled-water coil at refrigerant test pressure, and do not nitrogen-bomb a water loop to 600 psi because the RTU next to it is 410A.
Non-pressure tanks are vessels open to atmosphere (or designed only as atmospheric storage): cooling-tower basins, open expansion tanks, atmospheric condensate receivers, some chemical feed pots. They have vent, overflow, and makeup, not a 400 psig relief. Diagnose them with level, overflow path, and FBC 908 tower rules (Chapter 23) — not with a refrigerant manifold. Do not hydro-test an open basin as if it were an ASME unfired pressure vessel. F.S. 489.105: boilers and unfired pressure vessels are Class A statutory scope; Class B does not install them. Class B still has to recognize a closed hydronic expansion tank (bladder, precharge) versus an open tank, and still has to keep a tower basin from overflowing into a Miami return well.
Worked distinction. A Jacksonville Class A shop has (1) an R-410A liquid line at 418 psig operating, nameplate high-side design 610 psig, relief at that listing — working pressure is the 610 design, test per 1110 with nitrogen, never oxygen; (2) a condenser-water loop at 35 psig pump pressure in 150 psi pipe — different working pressure; (3) an open tower basin — non-pressure tank, check makeup and overflow, no manifold. Treating all three as “the pressure in the pipes” is how a helper puts 410A gauges on a basin and calls it a leak test.
Florida scenario
Gulf Breeze Mechanical, Class B in Hillsborough, has a 5-ton Tampa split that “doesn’t cool.” Suction is a little low and superheat is 4°F. The helper wants to recover two pounds. A flow-hood total is 1,150 cfm on a coil that should be near 2,000 (400 cfm/ton × 5). Return 80/67, supply 48°F and icing. That is low air, which lowers superheat — the 29.1 table, not a TXV. After a caked filter and a blower wheel packed with drywall dust, cfm is 1,980, supply 57/56, (\Delta h \approx 7.5), (Q_t = 4.5 \times 1{,}980 \times 7.5 \approx 66{,}825) Btu/h — about 5.6 tons at that condition. The same ticket notes an open plastic condensate tank the homeowner added in the garage: atmospheric, overflow to the slab, not a pressure vessel, and not a direct sanitary tie (FBC 307 / 489.105 condensate grant). Class B is legal on 5 tons.
Traps: (1) Charging by superheat when cfm is 280/ton. (2) One-point velometer reading × gross grille area. (3) Using 1.08 with total Btu/h on a capacity check that needs 4.5 × CFM × Δh. (4) Calling 400 cfm/ton a code-maximum. (5) Manifold-gauging a tower basin. (6) Treating operating psig as the pipe’s working-pressure rating.
A 5-ton comfort-cooling air handler is being checked for air volume. Which measurement and target match Trade E practice?
Return air is 80°F DB / 67°F WB (enthalpy about 31.5 Btu/lb). Supply is 58°F DB / 57°F WB (enthalpy about 24.5 Btu/lb). Measured airflow is 1,600 cfm. What is the approximate total cooling capacity using the standard-air enthalpy equation?
Which statement correctly distinguishes refrigerant operating pressure, pipe working/design pressure, and a non-pressure tank during diagnostic analysis?