House Humid Despite Running AC - Latent vs Sensible Decision Tree

Why this matters

Indoor relative humidity above 55 percent at the same time the AC is hitting setpoint is one of the most common customer complaints in a humid climate. The system is satisfying sensible load and ignoring latent load, and the cause is almost always upstream of the equipment - oversize, blower CFM too high, undersize evap, or short cycling driven by a setpoint that no longer matches the load profile. Adding a portable dehumidifier hides the symptom and adds load. Use this tree to identify which lever moves the latent fraction without throwing parts at the call.

Symptom presentation

Customer reports the thermostat reads 72 F but "it feels sticky." Hygrometer reads 58 to 65 percent RH at the same time. Symptom is worst on humid days with mild outdoor dry bulb (70 to 82 F outdoor, 75 to 80 percent outdoor RH) - shoulder season and overnight. On the hottest dry days the symptom partially clears because runtime is high enough to do meaningful dehumidification.

Variants:

• Sticky in shoulder season only: short-cycling, oversize, no latent-dominant mode.
• Sticky during long runtime at peak: blower CFM too high, evap face velocity too high, no real dehumidification per pass.
• Sticky after install of a "more efficient" replacement system: oversize relative to actual load or the new system was set up for sensible-only commissioning.
• Sticky with a working whole-house dehumidifier on the rack: it is not draining or the humidistat is mis-set.

Quick checks before gauges

Capture indoor dry bulb and RH at the thermostat, outdoor dry bulb and RH, supply temperature at the plenum, and return temperature at the air handler. Calculate enthalpy or use a psychrometric chart - the goal is the supply-to-return enthalpy drop, not just temperature drop. A unit doing 18 F sensible split and 4 Btu/lb enthalpy drop has a sensible heat ratio of about 0.85 (sensible dominant) - cold air, not dry air.

Read indoor blower CFM. ACCA Manual S latent design point is 350 CFM per ton for a humid climate, not the 400 CFM per ton sensible-design default. A system set to 400 CFM per ton on a humid-climate install gives up roughly 6 percent latent capacity to gain 4 percent sensible.

Measure cycle length on the thermostat history. Cycles under 12 minutes do not lower indoor RH meaningfully - the coil never reaches steady-state surface temperature before shutdown and the moisture on the coil evaporates back into the supply on the off cycle.

Isolation tree

Branch 1: blower CFM.

• ECM commissioned at 400 CFM per ton in a humid climate - re-commission to 350 CFM per ton. Confirm supply temperature drops 2 to 3 F and RH falls by 5 to 8 percent over 24 hours of steady weather.
• PSC blower locked at high speed for "more airflow" - move to medium-low cooling tap. Confirm static, CFM, and supply temperature.
• Variable-speed blower with no humidity sensor input - check thermostat or zone panel for humidity offset programming and enable.

Branch 2: thermostat strategy.

• Single-stage thermostat with a single-stage or non-modulating system - enable "dehumidify on demand" if equipment supports overcool with reheat, OR install a humidity-aware thermostat that reduces blower CFM on a humidity call.
• Communicating thermostat with humidity setpoint left at default 60 percent - set to 50 percent for occupant comfort, monitor.
• Setpoint at 74 F with night setup to 78 F - the deeper setup extends off cycles overnight when latent load is highest. Reduce or remove the night setup in humid season.

Branch 3: equipment sizing and short cycling.

• Calculate runtime over a 24-hour humid-mild day (75 F outdoor average, 80 percent RH). Total runtime under 6 hours indicates oversize. Recommend variable-capacity, two-stage, or a whole-house dehumidifier rather than replacing a properly sized system with an even larger one.
• Cycle count over 8 per hour in shoulder season indicates differential is too tight, thermostat is in a bad location, or system is oversized. Widen differential, relocate sensor, or correct sizing.
• Verify Manual J was done; oversize on replacement is the most common cause of post-install humidity complaints.

Branch 4: equipment latent performance.

• Coil face velocity above 450 fpm - latent capacity falls and reentrainment of condensate increases. Drop CFM or quote a larger evap face area on next replacement.
• Reheat circuit or hot-gas reheat not energizing on humidity call (equipment that has it) - check control wiring and OEM commissioning bulletin.
• Evap coil hydrophilic coating gone (older coil) or coil is dirty - condensate sheets poorly and reentrains. Wash and re-evaluate; quote replacement if performance does not recover.

Branch 5: separate dehumidifier integration.

• Whole-house dehumidifier on the return - confirm condensate drain is clear, humidistat is set 50 to 55 percent, ducted return is not bypassing the unit, and the dehu interlocks correctly with the air handler.
• Dehumidifier rated 70 pints per day in a house with a documented latent load of 110 pints per day at design - undersized, will run continuously and never satisfy.

Confirming the diagnosis

After remediation, log indoor RH and dry bulb at 30-minute intervals across a 24-hour humid-mild period. RH should stabilize at 50 to 53 percent at the thermostat with the equipment cycling normally (not continuous run). Supply temperature should be 2 to 4 F lower than baseline if CFM was reduced. Cycle length should rise to 18 to 30 minutes per cycle. If RH does not respond to CFM and thermostat strategy changes within 48 hours, the equipment is oversized for the load and the long-term fix is a system change or a dedicated dehumidifier.

Remediation summary

References

• ACCA Manual S, Residential Equipment Selection - latent vs sensible sizing methodology and 350 vs 400 CFM per ton humid-climate guidance.
• ACCA Manual J, Residential Load Calculation, 8th Edition - latent load calculation methodology including infiltration and internal moisture gain.
• ASHRAE Standard 55-2020 - thermal environmental conditions for human occupancy including RH bands for comfort.
• ASHRAE 62.2-2022 - residential ventilation and the latent load it imposes.
• AHRI Standard 210/240-2023 - test conditions for cooling sensible and latent capacity rating.