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

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

Higher cooling coil temperatures may also reduce the dehumidification rate: a second explanation of why an air conditioner may not dehumidify properly was offered by reader Bar who noted that newer HVAC systems, in order to meet energy efficiency requirements, may operate at a lower cooling coil temperature. Here is what he noted [with minor editing]:

... I'll interject some of my expertise from my HVAC career. Dehumidification while often is an over sizing issue, it is not always the case especially in the replacement of systems. Dehumidification in AC systems is a combination of coil temperature vs the temperature [and relative humidity or moisture content] of air moving across the coil as Delta-T and time.

What is not commonly told is that new systems mandated by [U.S.] federal, state and local energy codes have a higher efficiency designs which tend to raise the indoor coil temps and reduce the Delta-T.

This causes a reduction in moisture removal as air travels across the cooling coil. So in effect, what can happen is the air is cooled somewhat more slowly on right-sized systems yet moisture removal is not as good as an older less efficient AC system.

This is where reducing the size of the system does not change how it dehumidifies and infac could make the situation worse because the coil temp is actually warmer.

Sometimes a simple solution (if the system is not over sized) is to reduce the fan speed if the system allows the wiring to be reconfigured. On some newer and more elite systems technology is applied which adjusts compressor and fan speeds to compensate for cooling load on warmer or cooler days, and humidity levels. All of this is more complicated than this brief description but hopefully this adds some understanding to the thread. - Bar 9 August 2015

Cures for inadequate dehumidification by an air conditioner

Air conditioner that was not dehumidifying (C) Daniel Friedman

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.

Research on Reducing Indoor Humidity Levels

Reader Question: Is there a Way to Scale Down Oversized Air Conditioner Units?

Some years ago now, I had 2 ACs installed in my home. It cost me about $10k and now I am told they “overinstalled” with 7 tons of cooling when I needed 4.5 tons. Problems keep coming, of course and the only fix I hear is to install new ones with less tonnage. Seems to me, there ought to be some way to scale down the ones I have to fit my home. Utility cost is out of sight, of course. I am told the savings to reduce the AC capacity would be significant. Is there some way I can scale down the volume of the units I have? Seems like a better fix for me. What say you? - W.S.

Reply: We are doubtful that you can squeeze down an oversized A/C system

A competent onsite inspection by an expert, especially if you employ an experienced HVAC designer or engineer usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider:

How to Diagnose an air conditioning system that is not working

If your air conditioning system won't work, follow our diagnostic guides

Schematic of an air conditioner compressor unit showing inspection points (C) Carson Dunlop Associates

See our complete list of air conditioning system diagnostic and repair guide articles just below.

Sketch courtesy of Carson Dunlop Associates..

Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.

If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.


Continue reading at AIR CONDITIONER BTU CHART - where we discuss the air conditioning system sizing problem or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see DEHUMIDIFICATION FAQs for diagnostic questions & answers about building air dehumidification.



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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