AC Running but Humidity High? Portland Solutions
Sticky rooms, clammy sheets, and fogged-up windows mean your cooling system is not removing moisture effectively. Use this guide to restore dehumidification, protect your home from mold, and breathe easier every day.
Serving Portland, Beaverton, Lake Oswego, West Linn, and the greater metro area since 2008.
Owner & Lead HVAC Technician
Quick Answer
How to Quickly Lower Indoor Humidity
AC systems should keep humidity below 55%. If yours isn’t, complete the checklist below to address airflow, drainage, and thermostat settings. Persistent issues require professional diagnostics to evaluate refrigerant charge, equipment sizing, and ductwork integrity.
Confirm the thermostat is set to COOL with fan on AUTO and that the system is running long enough to dehumidify.
Replace the air filter to restore airflow and allow moisture to condense properly.
Ensure supply and return vents are open to maintain balanced airflow.
Inspect the condensate drain line and pump for clogs or failures; clear standing water.
Check the refrigerant lines for ice. If present, turn system off and run the fan to thaw.
Call Efficiency Heating & Cooling for professional humidity diagnostics, airflow balancing, or dehumidification upgrades.
Common High-Humidity Culprits in Portland
Moisture loves Portland. From the Willamette’s damp breezes to spring rainstorms, local homes are constantly challenged by humidity. Your AC is a powerful dehumidifier when tuned correctly. The ten causes below explain why humidity spikes despite cooling.
Oversized Air Conditioner and Short Cycling
Oversized systems cool the air temperature quickly but shut off before removing enough moisture. Portland’s mild summer temperatures make oversized equipment especially problematic because the system may run only a few minutes per cycle.
- •Symptoms: House feels clammy, thermostat setpoint is reached quickly, indoor humidity stays above 60%.
- •Solution: Adjust blower speed, increase run time with thermostats that extend cycles, or replace with right-sized variable-speed equipment.
- •Manual J load calculations ensure proper sizing for Portland’s maritime climate.
Low Refrigerant Charge
Insufficient refrigerant reduces the coil’s ability to absorb heat and condense moisture. Humidity rises even though the system runs continuously.
- •Look for ice on the evaporator coil or refrigerant lines, longer-than-normal run times, and warm supply air.
- •Professional leak detection and recharge restore dehumidification performance.
Dirty Evaporator Coil
Dust and microbial buildup on the indoor coil act as a barrier, preventing air from contacting the cold coil surface. Moisture remains in the air stream.
- •Cleaning involves removing coil access panels, applying coil foam cleaner, and rinsing with low-pressure water.
- •Combine with UV lights or electronic air cleaners to keep coils clean longer.
Clogged or Disconnected Condensate Drain
When the drain pan overflows or the condensate pump fails, moisture stays in the air handler or spills back into the ductwork, raising indoor humidity.
- •Inspect the drain line for slime or algae and clear with a wet/dry vacuum or compressed air.
- •Install safety float switches to shut down the system before water damage occurs.
Improper Fan Speed or Continuous Fan Operation
Running the fan continuously re-evaporates moisture from the coil back into the air. Incorrect blower speeds limit moisture removal.
- •Set fan to AUTO so it runs only during cooling cycles.
- •Technicians can program blower speeds to optimize dehumidification.
Leaky Ductwork Pulling in Humid Air
Return ducts in crawlspaces or attics depressurize the home and draw in humid outdoor air. Supply leaks dump conditioned air into unconditioned spaces, reducing dehumidification indoors.
- •Seal ducts with mastic or Aeroseal, insulate ducts, and install vapor barriers in crawlspaces.
- •Balancing the system ensures each room receives proper airflow for moisture removal.
High Outdoor Humidity Events (Wildfire Smoke Season)
During wildfire smoke or wet heat waves, outdoor humidity rises. Systems without staged capacity struggle to keep up, especially if filters clog from smoke particles.
- •Use MERV-13 filters and replace more frequently during smoke events.
- •Consider adding a whole-home dehumidifier for dedicated moisture removal.
Inadequate Return Air or Closed Vent Registers
Closing vents to “save energy” actually reduces airflow across the coil. Less air contacts the cold coil, so less moisture condenses.
- •Keep registers open and unobstructed. Use dampers for zoning instead of manually closing vents.
- •Add return air grilles in bedrooms or finished basements to balance airflow.
Aging Equipment with Weak Compressor
Older compressors lose capacity and cannot maintain the temperature differential needed to condense moisture.
- •Systems older than 15 years may need replacement to restore humidity control.
- •New variable-speed technology maintains long, low-speed cycles that pull more moisture from the air.
Home Envelope Issues
Unsealed crawlspaces, foundation cracks, and unvented bathrooms introduce constant moisture loads that exceed the AC’s dehumidifying capacity.
- •Seal crawlspaces, add vapor barriers, and ensure exhaust fans vent outdoors.
- •We partner with building performance experts to address whole-home moisture problems.
Long-Term Humidity Control Strategies
Call Us When You Notice:
Indoor humidity stubbornly above 60% despite long cooling cycles.
Musty smells, condensation on windows, or visible mold growth.
Condensate drain backups or water around the air handler.
Frequent family allergy flare-ups or respiratory irritation from damp air.
Humidity FAQs for Portland Homeowners
Enjoy Crisp, Comfortable Air All Year
Efficiency Heating & Cooling delivers precision humidity control solutions tailored to Portland’s climate — from duct sealing and system tuning to whole-home dehumidifiers. Schedule your evaluation today and breathe easier tomorrow.