How to Measure Delivered Cooling Capacity With Wet Bulb and Airflow

Why this matters

Superheat and subcooling tell you the refrigerant side is where the maker wants it. They do not tell you how much heat the system is actually pulling out of the house. A unit can sit dead on its subcooling target and still deliver three quarters of its rated capacity because air is sneaking around the coil, the coil face is matted, or the compressor is tired. The customer only experiences the second number, and when they say "it runs all afternoon and never catches up," a charge reading is not an answer.

Measured capacity is the answer. You take the airflow, take the heat content of the air going in and coming out, multiply, and compare with what the maker says this matched pair should do at today's conditions. It takes about twenty minutes once the system is stable, and it ends arguments in both directions: it proves a healthy system is healthy, and it proves a "correctly charged" one is not.

This is the measurement. How to charge first is in HVAC Superheat and Subcooling Method Reference and the two charging HowTos on this shelf; do not run a capacity test on a system whose charge you have not verified, because you will not know which problem you are measuring.

The relationship, and the condition it holds under

Total capacity in BTU/h = 4.5 x CFM x (h return - h supply), where h is enthalpy, the total heat content of the air in BTU per pound of dry air.

The 4.5 is 60 minutes per hour times 0.075 lb per cubic foot, the density of standard air at sea level. At 5,000 feet air is roughly a sixth lighter, and the same CFM carries roughly a sixth less heat; use a constant corrected for elevation and a psychrometric chart drawn for that pressure, or the test overstates capacity by that amount.

Enthalpy is the right quantity because it carries both kinds of heat removal: the sensible part (the air got colder) and the latent part (water came out of it and went down the drain). Enthalpy of moist air is very nearly constant along a line of constant wet bulb, which is why the test uses wet bulb: one reading in each airstream gives you h straight off a table, without needing dry bulb and humidity separately. Dry bulb still gets recorded, because you want the sensible and latent split.

Start state

• Charge verified by the maker's method, superheat for a fixed orifice, subcooling for a TXV or EEV.
• Steady operation. At least 15 minutes of continuous run with doors and windows shut, or longer if the maker's procedure says so. Capacity in the first minutes includes cooling down the coil and the duct.
• Filter as you want to judge it. If the question is "what does this house get," test with the filter in it. If the question is "is the equipment right," test with a clean one, and write down which.
• The maker's expanded performance data for this outdoor model with this indoor coil, not the nominal tonnage. Expanded data gives total and sensible capacity by entering wet bulb, outdoor dry bulb and indoor airflow.
• One psychrometer used for both airstreams if you can. A fixed offset in a single instrument largely cancels when you subtract h supply from h return; two instruments each bring their own offset, and those do not cancel.

The procedure

1. Measure airflow, never assume it. Use total external static and the fan table (How to Estimate Blower Airflow From Static and the Fan Table), a flow hood at the return, or a duct traverse. "400 CFM per ton" is a design target, not a measurement, and airflow is the multiplier on everything else. A system at 320 CFM per ton entered as 400 overstates capacity by a quarter before a single wet bulb is read.

2. Read return conditions where the coil sees them. Dry bulb and wet bulb at the air handler, downstream of the filter and upstream of the coil, through a test port. A grille reading misses anything that joins the return between the grille and the unit, and a return duct in a hot attic with an open seam can lift the coil's entering wet bulb by several degrees. If you want to know whether that is happening, read both places; the difference is its own finding.

3. Read supply conditions in a mixed airstream. Air leaving a coil is stratified: colder where it passed through the fins, warmer where it slipped by a gap or crossed a bare section. Read the supply far enough downstream to be mixed, ideally past an elbow or several duct diameters along the trunk, and take three or four points across the duct. If the points spread by more than about 1 F of wet bulb, the air is not mixed yet; move downstream. A reading right above the coil in the cold core is how a system with bypass reads perfect.

4. Give the wick or sensor time. A wet bulb wick needs air moving across it and needs to come to equilibrium; an RH-based digital probe needs time to settle after moving from a 78 F room into a 58 F duct. Watch the reading stop moving before you write it.

5. Look up enthalpy at each wet bulb. Use a psychrometric table or chart at your elevation; the tables in the ASHRAE Handbook, Fundamentals are the usual source at standard pressure. Interpolate between whole degrees; at these temperatures one degree of wet bulb moves h by roughly 0.6 to 0.8 BTU/lb.

6. Calculate total, then sensible. Total from step 5. Sensible = 1.08 x CFM x (DB return - DB supply), where 1.08 is 0.075 lb/ft3 x 60 x 0.24 BTU/lb-F, again for standard air. Latent is total minus sensible. The sensible heat ratio is sensible divided by total.

7. Compare with expanded data at the measured conditions. Enter the table at the measured return wet bulb, the outdoor dry bulb at the condenser coil inlet, and the measured CFM, interpolating as needed. Do not compare with nominal: a "3-ton" unit delivers 36,000 BTU/h only at the rating conditions, and on a 95 F afternoon with a 63 F entering wet bulb it is not supposed to.

Worked example: a 3-ton heat pump that passes

A 3-ton matched heat pump, TXV, charged that morning to the maker's subcooling target. Outdoor air at the condenser inlet 85 F. System has run 25 minutes, house shut, filter one month old and left in.

CAPACITY WORKSHEET - unit A, 3 ton, TXV

airflow (TESP 0.52 in w.c., fan table)  1,140 CFM
return at coil inlet    78.0 F DB / 65.0 F WB
supply, 4 pt avg        58.0 F DB / 55.0 F WB
supply WB spread        0.4 F  (mixed, ok)

h return @ 65.0 WB      30.06 BTU/lb
h supply @ 55.0 WB      23.22 BTU/lb
delta h  30.06 - 23.22  =  6.84 BTU/lb

total     4.5 x 1,140 x 6.84    = 35,089 BTU/h
sensible  1.08 x 1,140 x 20.0   = 24,624 BTU/h
latent    35,089 - 24,624       = 10,465 BTU/h
SHR       24,624 / 35,089       = 0.70

expanded data, 85 F OD, 65 WB,
interpolated to 1,140 CFM       = 35,500 BTU/h
measured / table  35,089/35,500 = 0.99

Check the multiplication from the other side: 35,089 divided by 1,140 CFM is 30.78, and 30.78 divided by 6.84 is 4.50. The expanded-data figure is illustrative; read your own maker's table.

Measured total is 99 percent of the table value. That is well inside the measurement's own uncertainty (next section), so the conclusion is "delivers what it was sold to deliver," and that goes on the ticket with the numbers. The 0.70 sensible heat ratio says it is also wringing out water at a normal rate for a humid afternoon.

How good the number is, and the gate to apply

Wet bulb error. Take a psychrometer good to plus or minus 0.5 F of wet bulb, a random reading-to-reading spread rather than a fixed offset. At 55 to 65 F WB that is about 0.3 to 0.4 BTU/lb on each reading. On a 6.84 BTU/lb difference, each reading's spread is roughly 5 percent of the delta, and two independent spreads combine in quadrature: about 5 x 1.4, or 7 percent. A fixed offset from one instrument used in both ducts mostly cancels in the subtraction; that is why the start state asks for one instrument.

Airflow error. A fan-table airflow is commonly good to perhaps 5 to 10 percent of reading, and it multiplies straight through.

So a shop default gate is: measured total within 10 percent of the expanded-data value, read at the same entering wet bulb, outdoor dry bulb and CFM, passes. Below 90 percent is a finding to chase. Do not chase 4 percent; it is inside the instrument.

Same gate, a unit that fails

Unit B is the same model in a different house. The tech charged it to the subcooling target, the temperature split came out at 17 F, and the customer still complains. The capacity test:

• Airflow by fan table: 1,150 CFM.
• Return at coil inlet: 77.0 F DB / 64.0 F WB, h = 29.31 BTU/lb.
• Supply, first reading right above the coil: 55 F DB, looks great. Moved 6 feet down the trunk past an elbow, four points, spread 0.5 F: 60.0 F DB / 57.0 F WB, h = 24.48 BTU/lb.
• Delta h: 29.31 - 24.48 = 4.83 BTU/lb.
• Total: 4.5 x 1,150 x 4.83 = 24,995 BTU/h.
• Expanded data at 64 F WB, the same 85 F outdoor and 1,150 CFM, illustrative: 34,000 BTU/h.
• Ratio: 24,995 / 34,000 = 0.74.

At 74 percent of the table value it fails the 90 percent gate by a margin no instrument error explains. The 17 F split was inside the band many techs accept, and it hid the problem because a split says nothing about how much of the air went through the coil.

The difference between the readings above the coil and down the trunk is the clue: cold air above the coil, warmer mixed air downstream, means some air is not touching the fins. Pulling the coil access panel showed the factory block-off missing on one side of a cased coil, an open path around the coil's slab. The refrigerant side was right, which is exactly why subcooling read correctly: the coil was doing its job on the air it saw. The repair is a sheet metal fix and a retest, not refrigerant.

What else produces a low ratio with a good charge

• Air-side fouling. A coil matted on its entering face moves less air through the fins and drops airflow; re-measure CFM after cleaning, since both terms in the formula move.
• A weak compressor. On a TXV system it can hold subcooling while capacity falls. Suction runs high and amps run low against the maker's chart at that condition; route it to compressor diagnosis rather than adding charge.
• Return leakage from an unconditioned space. If the coil-inlet test passes and the grille reading is several degrees of wet bulb lower, the equipment is fine and the house is paying to cool attic air. That result fails the house, not the unit, and it is a duct sealing conversation.

Recording it

Write every input, not just the verdict: CFM and how it was found, both dry bulbs and wet bulbs with their locations, the elevation basis, the outdoor dry bulb, the table value and its conditions, and the ratio. A capacity test with only "75 percent" on the ticket cannot be rechecked after the repair, and the retest is the proof the repair worked.

References

• Equipment manufacturer's expanded performance data and installation instructions for the matched outdoor and indoor units
• ASHRAE Handbook, Fundamentals, psychrometric tables (enthalpy of moist air at standard pressure)
• See related: HVAC Superheat and Subcooling Method Reference
• See related: How to Estimate Blower Airflow From Static and the Fan Table; Airflow Per Ton Targets and What Moves Them
• See related: Temperature Rise and the Sensible Heat Airflow Formula