Condensation at a basement window (C) Daniel Friedman Dehumidification of Indoor Air
Central A/C Not Dehumidifying - Diagnosis & Repair

  • DEHUMIDIFICATION PROBLEMS - CONTENTS: Air Conditioning System Dehumidification Problem Diagnosis & Cures. Inadequate indoor air dehumidification with central air conditioning or heat pump systems: causes, diagnosis, repair. How to diagnose & repair an air conditioner that is not dehumidifying the air - Cures for inadequate dehumidification by an air conditioner. Over-sized central air conditioners may make the air cool without adequately removing moisture. Why can't our air conditioner reach our target indoor humidity level?
  • POST a QUESTION or READ FAQs about how to diagnose and cure inadequate building dehumidification - excessive indoor humidity levels

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Diagnosis of inadequate dehumidification in buildings:

Tthis article discusses the causes of inadequate indoor air dehumidification when you are running a central air conditioner or heat pump system. If your central air conditioning system cools the building air but the indoor humidity remains too high, the system may be over-sized, as we explain here.

We also consider other sources of excessive water in central air systems, air handlers, and ductwork.

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Air Conditioning System Dehumidification Problem Diagnosis & Cures

Sketch explaining that oversized air conditioners are a mistake (C) Carson Dunlop Associates

Air Conditioning Dehumidification Problems: air conditioning system cools but does not dehumidify the room.

The most common cause of inadequate dehumidification by an air conditioning system is the installation of a cooling unit which has too much capacity, or is "over-sized" for the space it is being used to cool.

[Click to enlarge any image]

What happens is simple:

If an air conditioning compressor unit is oversized (too many BTUH of cooling capacity) what happens is it cools the room so quickly that the system does not move enough total volume of air across the cooling coil to remove much moisture before the room temperature has dropped to the A/C cut-off point.

In other words, an air conditioner needs to run longer, and move more total volume of air through itself to drop room humidity than it does to just cool the air. So "bigger" cooling capacity or higher BTU capacity for an air conditioning system is not necessarily better, and it can actually be a problem.

Key Air Conditioning Inadequate Dehumidification Symptom: If your cooling system is otherwise working normally, and it cools the room temperature quickly but the room humidity stays high, an over-sized unit is likely to be the problem.

Cures for inadequate dehumidification by an air conditioner

Air conditioner that was not dehumidifying (C) Daniel Friedman

  • First make sure that the equipment is operating properly: is it cooling - check that temperatures are dropping normally when the system is running.
  • Check that there is not an abnormal moisture source such as leaks into the building or its wall or ceiling cavities from any other source. Check to see if the condensate drain from the air conditioner is producing water.

    If you see lots of A/C condensate dripping out of the system condensate drain but room humidity is still high, we suspect that there may be an abnormal humidity source.

    At the end of this list we discuss adding additional dehumidification capacity.
  • Adjust the A/C unit controls to increase dehumidification: discuss with your HVAC service technician whether there are technical changes that might be made to fan speed or other controls that might improve A/C unit dehumidification. We're doubtful there is much mileage in this but it's worth asking.
  • Increase the cooling load on the installed air conditioner by making it cool a larger area - like maybe leaving the doors open to other rooms - which we doubt is suitable in most cases. Our photo (above left) shows a wall mounted air handler from a Sanyo™ split system air conditioner that was not adequately dehumidifying the bedroom where it was installed, even though the room was cold enough to hang meat.

    The occupants experimented with leaving the bedroom door opened, as the rest of their home did not have air conditioning. That proved just enough to cause the system to dehumidify beautifully.
  • Adjust the building HVAC system ductwork: if a non-dehumidfying central air conditioning system (as opposed to a window unit or a wall-mounted split system unit) is otherwise working well, it may be possible to adjust its load and thus improve its dehumidification by making changes in the duct system, such as increasing the return air to the air handler from additional building areas.
  • Replace the oversized air conditioner: Unfortunately the most likely fix for an A/C system that is not dehumidifying may be an ugly one - either replace the unit with one that is properly matched to the size of the area to be cooled
  • Select an A/C unit of the proper size.
    See AIR CONDITIONER BTU CHART for the data useful to do this. Whoever selected and installed the air conditioning unit needed to relate the room size to the AC capacity. Also
    see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs
    see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.
  • Check that high indoor moisture levels are not due to a building water entry or leak problem. There could be a source of indoor moisture that just exceeds the ability of the air conditioning system to handle.
  • Check that the air conditioning system condensate capture and drainage systems are working properly. Condensate in the air handler that is not being adequately captured and drained out of the blower compartment can wet nearby ductwork causing rust and sometimes air conditioning system duct mold.
  • Add supplemental dehumidification. For wet conditions that exceed the capacity of the air conditioner itself, or when you require both unusually low temperatures and further dehumidification (such as a seed storage facility described by one of our readers), it may be necessary to add a separate free-standing or portable dehumidifier to the system to reach the temperature and humidity targets you need.

    In an area of our forensic laboratory we use a Sears® dehumidifier, model 580.54501 - 50 pint (also available as 580.54701 - 70 Pint Low Temp) (cost about $200.). The Sears portable dehumidifier  instruction manual (Part# 3828A20803B) confirms on p.7 that the humidity control can be set anywhere between 35% and 70%.  You can also adjust the fan speed (low / high) and also set an on-off cycle period of 3 hrs or 6 hrs.

    Operating a dehumidifier (which uses the same technology as an air conditioner) will have a side effect of pumping a little heat into the dehumidified area - the output side of the dehumidifier produces air that is warmer than its input side. Depending on the size of the area being conditioned, that added heat might help compensate for an air conditioner that was a little oversized for the space.

See LOST COOLING CAPACITY if not enough cool air is provided by your air conditioner, or if the air temperature is not cool enough, or if you just can't get your A/C unit running, this article helps diagnose and correct the problem with step by step things to check and links to more detailed explanation when you need it. We provide links to other air conditioning system diagnostic articles too.

Troubleshooting High Indoor Humidity Levels

Reader Question: 11/30/2014 Glen de Valk said:

Last May we replaced our central air unit in our 2400 sq.ft. home in Sarasota, FL. It was replaced with the same size unit that had been in place for over 20 years (3.5 tons) and cooled and dehumidified very well. However, the new unit does not dehumidify the air regardless where temp. is set and keeps the humidity level no lower than 60% and as high as 72% when we raise the thermostat to our comfort level around 75 degrees.

We had an independent inspection of the system done and detected that the air humidity at the registers was 83-85% when the unit was running; when the compressor shuts off and the fan runs for a pre-set 90 seconds more, the humidity level goes to 100%.

The installer cancelled the 90 second period of running but the humidity level still stays at 82-85% coming out of the registers and around 60 to 65 % at the returns. The house was also thoroughly tested for plumbing leaks, etc. and there are none.

The A/C contractor thought the 3.5 ton unit was too big and replaced it with a 3 ton unit so it would run longer cycles. That made absolutely no difference in dehumidification. The contractor seems to be at a loss to cure it. Please advise. Thanks.


Glen, usually when an air conditioner is not adequately dehumidifying the indoor air it is because its cooling capacity is too great - the air cools and shuts off the cooling system before the humidity has dropped.

  • Continuing to run the fan after the cooling cycle stops will not reduce humidity at all - that only occurs when the cooling coil is in operation.
  • Though you indicate that the capacity of the new unit was the same as the one it replaced, perhaps it was more efficient, cools faster, or was using a different air velocity, or a different sized cooling/evaporator coil. Moving to a smaller capacity unit sounds sensible to me.
  • But as that did not work I'd want to look more closely at the air handling system: duct sizes, air flow rates, cooling coil size, return to supply air balance, etc.
  • Also check for cooling coil icing - if the coil is partly blocked that might be a factor.
  • Also ask yourself: what else was changed in the home: occupancy patterns, cooking patterns, doors kept open or shut, windows open or shut, house air leakiness, sun exposure, etc.
  • Take a look at Simonson (2002) and also at Zhang (2003), cited below, for some techincal research on problems with decoupled cooling and humidity in humid climates (such as yours in Florida) that may be helpful
  • Finally it would be smart to also survey the home carefully for unusual moisture sources that might not have been noticed.

Research on Reducing Indoor Humidity Levels

  • Also see the REFERENCES of this article
  • Arens, Edward A., and A. Baughman. "Indoor humidity and human health: part II--buildings and their systems." Center for the Built Environment (1996).
  • Baughman, A., and Edward A. Arens. "Indoor Humidity and Human Health--Part I: Literature Review of Health Effects of Humidity-Influenced Indoor Pollutants." Center for the Built Environment (1996).
  • Dharmage, S., M. Bailey, J. Raven, T. Mitakakis, F. Thien, A. Forbes, D. Guest, M. Abramson, and E. H. Walters. "Prevalence and residential determinants of fungi within homes in Melbourne, Australia." Clinical and Experimental Allergy 29 (1999): 1481-1489.
  • Fletcher, A. M., C. A. C. Pickering, A. Custovic, J. Simpson, J. Kennaugh, and A. Woodcock. "Reduction in humidity as a method of controlling mites and mite allergens: the use of mechanical ventilation in British domestic dwellings." Clinical & Experimental Allergy 26, no. 9 (1996): 1051-1056.
  • Garrett, M. H., P. R. Rayment, M. A. Hooper, M. J. Abramson, and B. M. Hooper. "Indoor airborne fungal spores, house dampness and associations with environmental factors and respiratory health in children." Clinical and Experimental Allergy 28, no. 4 (1998): 459-467.
  • Padfield, Tim. "The role of absorbent building materials in moderating changes of relative humidity." Department of Structural Engineering and Materials, Lyngby, Technical University of Denmark 150 (1998). [Ph.D. Thesis] Excerpt:
    This thesis is about controlling the relative humidity inside buildings by using the water absorption properties of porous materials.
  • Peat, Jennifer K., J. Dickerson, and J. Li. "Effects of damp and mould in the home on respiratory health: a review of the literature." Allergy 53, no. 2 (1998): 120-128.
  • Simonson, C. J., M. Salonvaara, and T. Ojanen. "The effect of structures on indoor humidity–possibility to improve comfort and perceived air quality." Indoor Air 12, no. 4 (2002): 243-251. Excerpt from the article abstract:

    ... moisture transfer between indoor air and hygroscopic building structures can generally improe indoor humidity conditions. ...
  • Zhang, L. Z., and J. L. Niu. "Indoor humidity behaviors associated with decoupled cooling in hot and humid climates." Building and Environment 38, no. 1 (2003): 99-107.

    Radiant cooling with independent air dehumidification/ventilation is a complementary cooling and ventilation technology that has the potential to provide better thermal comfort, air quality and energy consumption than conventional all-air systems. However, in hot and humid regions, fears for the risk of condensation on ceiling panels limit its market penetration.

    To address this problem, in this paper, indoor humidity behaviors associated with decoupled cooling in hot and humid climates are investigated. Room mean temperature, mean humidity, maximum RH on the ceiling panel surfaces, annual condensation hours and annual primary energy consumptions are predicted with systems combining chilled ceiling with various air dehumidification and ventilation strategies, using a building energy simulation code ACCURACY.

    The effects of night air infiltration rates and the ratios of air flow rates of return air to those of fresh air on the indoor humidity performance and annual condensation hours are discussed.

    The results indicate that dehumidification and ventilation prior to cooling panels operation is required to reduce condensation risks in hot and humid climates. It is also revealed that a in advance dehumidification/ventilation in summer could completely eliminate the condensation problems.


Continue reading at AIR CONDITIONER BTU CHART - where we discuss the air conditioning system sizing problem or select a topic from the More Reading links shown below.



COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs

RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.

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DEHUMIDIFICATION PROBLEMS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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