What a Latent Load Does to Equipment Selection
Why this matters
The complaint that ends careers in humid climates is not "it will not cool." It is "the thermostat says 74 and the house feels awful." The equipment is holding the number it was told to hold and the customer is miserable, and the usual response, adding capacity, makes it worse. A selection that checked only the sensible load never checked humidity at all, and the arithmetic that shows why takes about five minutes. This article is that arithmetic, laid out as a record you fill in before you order anything.
Before you take the readings this selection depends on
The blower is a mechanical and stored-energy hazard, so it goes to 29 CFR 1910.147: open the disconnect, apply your own lock and tag, and confirm the wheel has stopped before a hand goes past a panel. Take airstream readings through test ports from outside the cabinet.
Only a technician certified under Section 608 may open a refrigerant circuit, under 40 CFR Part 82 Subpart F, and that applies to connecting for the purposes of this work as much as to a repair. Liquid refrigerant on skin or in an eye causes a cold burn, so wear splash-rated eye protection and gloves when you connect or disconnect. A refrigerant release in a closed mechanical room displaces oxygen; if a leak occurs, leave the space and ventilate it before returning, and do not rely on smell as a warning because the concentration that harms you arrives without one.
Where a reading has to be taken energized, that exposed live work sits under the troubleshooting exception at 29 CFR 1910.333(a)(1), and the meter and leads must be rated for the circuit category, CAT III for branch-circuit and distribution work.
The equation that decides humidity
A conventional thermostat senses dry bulb. It runs the equipment until the sensible load is satisfied and then it stops, regardless of what the moisture in the room is doing. That single fact determines everything downstream.
While the equipment runs, it removes sensible and latent heat in the fixed proportion set by the coil's sensible heat ratio at that entering condition. So:
Runtime fraction = sensible load divided by sensible capacity.
Latent removed = runtime fraction x latent capacity.
Substitute the first into the second and the capacities cancel into a ratio:
Latent removed = sensible load x (1 minus SHR) divided by SHR.
Read that again, because it is the load-bearing claim of this article. Under a dry bulb thermostat, the moisture the equipment removes depends on the sensible load and on the coil's sensible heat ratio. It does not depend on the tonnage. A bigger machine runs proportionally less and removes the same amount of water.
Why tonnage is not the lever people think it is
The common advice, "it is oversized, that is why it is humid," gets the right answer through the wrong mechanism, and the wrong mechanism leads to wrong repairs.
Oversizing does not reduce latent capacity per hour. What it does is shorten the cycle, and cycle length is where the equation above stops being an upper limit and starts being a fiction.
The first minutes of a cycle are nearly all sensible. The coil starts at room temperature. It has to fall below the entering air's dew point before a single drop condenses, and getting there takes minutes, not seconds. A five-minute cycle spends most of itself producing no dehumidification at all.
The end of a cycle can give moisture back. The coil finishes a cycle wet. If the blower keeps running after the compressor stops, air moves over that wet coil and re-evaporates the condensate straight back into the space. A generous fan-off delay is worth real efficiency in a dry climate and is actively harmful in a humid one.
So the honest version is: latent removal equals sensible load times the latent-to-sensible ratio, degraded by however much of the runtime is spent getting the coil cold and however much condensate goes back into the air at the end. Short cycles get you a fraction of the calculated number. That is what oversizing does.
The selection record
Nine fields. If you cannot fill them, you have not selected equipment, you have picked a tonnage.
- Design conditions: outdoor dry bulb and wet bulb, indoor target dry bulb, and indoor target expressed as a dew point rather than a relative humidity.
- Block sensible load and block latent load, from the load calculation, and the required sensible heat ratio they imply.
- Candidate total capacity and SHR read at the actual entering condition, not at the nameplate rating point.
- Latent removed at design, from the equation above.
- Latent removed at the humid part-load hour, which is where the complaints come from.
- Airflow in cubic feet per minute per ton, and the SHR the manufacturer's table gives at that airflow.
- Expected cycle length at part load, and the staging or modulation that produces it.
- Blower off-delay strategy.
- Supplemental dehumidification decision, with its own capacity if the answer is yes.
The record, filled in
Design conditions. Indoor target 75 F dry bulb at a 55 F dew point, which is 50 percent relative humidity at that temperature. Entering coil condition therefore about 75 F dry bulb and 62.5 F wet bulb.
Loads. Block sensible 24,000 Btu/hr, block latent 8,000 Btu/hr, total 32,000 Btu/hr. Required sensible heat ratio: 24,000 divided by 32,000, or 0.75.
Candidate at the actual entering condition. The nameplate rating point is a higher entering wet bulb than this house will run, and a coil's SHR rises as entering wet bulb falls, so the rated SHR always flatters the selection. Read the expanded table at 75 F dry bulb and 62.5 F wet bulb entering with the design outdoor temperature. Say it returns 33,800 Btu/hr total at an SHR of 0.82, giving sensible 27,716 and latent 6,084. Use your own table; these are the values this example runs on.
Latent removed at design. 24,000 x (0.18 / 0.82) = about 5,270 Btu/hr, against 8,000 required. Short by about 2,730 Btu/hr, which is 34 percent of the latent load.
Note what happens in the space when that gap exists. Humidity rises until the entering wet bulb climbs enough to drag the coil's SHR down, which does eventually restore some balance, but at an indoor dew point well above the 55 F target. The house finds a wetter equilibrium and stays there. That is precisely the "74 and awful" complaint.
Latent removed at the humid part-load hour. An evening at moderate outdoor temperature with high moisture: sensible load falls to roughly 35 percent of design, about 8,400 Btu/hr, while latent load barely moves because occupancy, infiltration, and ventilation moisture are all still running. Call it 6,500 Btu/hr.
8,400 x (0.18 / 0.82) = about 1,840 Btu/hr removed, against 6,500 needed. Short by 72 percent.
This is the hour that generates the callback, and no amount of capacity fixes it, because capacity is not in the equation.
The levers, and what each is worth
Lower the airflow. Less air over the same coil means colder coil surface, more condensation, lower SHR. Read the table at 350 cubic feet per minute per ton instead of 400: say it gives 32,800 total at an SHR of 0.77, so sensible 25,256 and latent 7,544.
Latent removed at design: 24,000 x (0.23 / 0.77) = about 7,170 Btu/hr against 8,000 needed, short by 10 percent. Compare that against the 34 percent shortfall computed above, which is the same calculation at the same design hour, so the two are directly comparable.
That is a large improvement from a blower tap. Its limit is the manufacturer's minimum airflow, below which the coil approaches freezing at low load and the compressor sees liquid return. Do not go under the stated minimum, and verify with a coil temperature check and superheat after the change.
At the part-load hour the same lever gives 8,400 x (0.23 / 0.77) = about 2,510 Btu/hr against 6,500, still 61 percent short. Better than 72 percent, and still not a solution. Airflow rescues the design hour and does not rescue the shoulder-season evening.
Lengthen the cycle. Two-stage or variable capacity does not change the equation's answer, since capacity cancels out, but it changes how much of the answer you actually get, by keeping the coil cold and wet for longer stretches instead of restarting from room temperature every few minutes. This is the lever that recovers the degradation, not the lever that changes the arithmetic, and it is worth being precise about that distinction when you explain it to a customer.
Kill the fan-off delay, or reverse it. In a humid climate, running the blower after the compressor stops evaporates condensate off the coil and returns it to the space. Set it to the manufacturer's minimum or off.
Reduce the latent load itself. Ventilation air is frequently the largest single latent term. Bringing it in without any treatment loads the coil with outdoor moisture at every hour the fan runs, including the part-load hours where the coil has the least ability to deal with it.
When the record says you need a separate machine
Run the part-load line honestly and it will often show that no combination of the levers above closes the gap. That is a finding, not a failure.
A dedicated dehumidifier removes moisture on its own control, indexed on humidity rather than on dry bulb, which means it runs during the hours when the cooling equipment is barely running at all. Size it against the part-load gap, not the design gap, because the design hour is the one the cooling equipment handles best. In the example above, the gap to close at the shoulder hour is roughly 4,000 Btu/hr of latent capacity after the airflow change, and that is the number the selection is made against.
The argument to make to the customer is a sequencing argument, not a comfort adjective. The cooling equipment is controlled by a thermostat that cannot see moisture, so on the exact days moisture is worst, the thermostat is asking for the least runtime. Something has to be controlled by a humidity sensor or nothing is.
Verifying after commissioning
Do not verify this on the design day. Verify it on a humid part-load evening, because that is the condition the selection was actually at risk on.
Measure the space dew point, not relative humidity, on a probe that has equilibrated, and compare it against the 55 F target the record was built around. Record the outdoor conditions alongside it, or the reading is not comparable to anything.
Then confirm the two things the record assumed. Measure airflow rather than trusting the blower tap, because everything in the table read is proportional to it. And time a few cycles at part load: if the compressor is running for a handful of minutes at a time, the calculated latent removal is an overstatement and the staging or the load calculation needs another look.
Write the entering wet bulb, the measured airflow, the space dew point, and the outdoor conditions on the commissioning sheet. Those four fields are what let a tech two years from now tell the difference between equipment that changed and a house that did.
References
- ASHRAE Handbook, Fundamentals and Systems and Equipment volumes, for sensible heat ratio, coil performance, and load calculation methods
- Manufacturer expanded performance tables, indexed on entering wet bulb, entering dry bulb, outdoor temperature, and airflow, which govern the values used in any real selection
- 40 CFR Part 82 Subpart F, EPA refrigerant handling and Section 608 technician certification requirements
- 29 CFR 1910.147, OSHA general industry, for lockout of blower mechanical and stored energy
- See related: What Wet Bulb Tells You That Dry Bulb Cannot; Psychrometrics Fundamentals; Whole-House Dehumidification Reference