Diagnose & fix indoor humidity or dehumidification problems with air conditioners:

Here we explain the cause & cure of an air conditioning system that is not adequately dehumidifying or removing moisture from building air.

This article forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperaturer, or an air conditioner that won't start.

Air Conditioning System Dehumidification Problem Diagnosis & Cures

A summary of indoor A/C or heat pump dehumidification problerm diagnosis is just below.

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.

The result of over-sizing an air conditioner or heat pump is often called "short cycling" that we will explain in more detail below.

First to summarize, 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.

"Bigger" cooling capacity or higher BTU capacity for an air conditioning system is not necessarily better, and it can actually prevent the air conditioner from adequately dehumidifying indoor air.

Sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Question: My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems

Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!

Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old

Five causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair

DanJoeFriedman (mod) said to Joe and Matthew (re questions just above):

1. Loss of air conditioner refrigerant 
   
   - a refrigerant leak in the system. You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak. Otherwise you will have to keep repeating the costly service call to just add refrigerant.
   
   See REFRIGERANT LEAK DETECTION.
2. A/C Coil Icing 
   
   - the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.
   
   See FROST BUILD-UP on AIR CONDITIONER COILS
3. Oversized air conditioner 
   
   - if the short cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled.
   
   A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled.
   
   Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat.
   
   See AIR CONDITIONER BTU CHART
   
   and OPERATING TEMPERATURES HVAC
4. A/C control problem 
   
   - it's less likely, but a damaged control board or switch could also be causing rapid equipment on-[off cycling.
5. Compressor damage or compressor start-troubles
   
   - I've seen these other causes of air conditioner short cycling:
   
   if someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor.
   
   Normally that head pressure bleeds off over time, making it easier for the compressor to re-start (against low head pressure) the next time it turns on.
   
   See HARD STARTING COMPRESSOR MOTORS

But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.

We have also see or a damaged compressor internal refrigerant valve causing high head pressures;

In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.

If your air conditioner or heat pump has the opposite problem, staying on too long,

see LONG-ON CYCLING AC COMPRESSOR.

...

Continue reading at DEHUMIDIFICATION PROBLEMS - details about fixing dehumidification problems, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see DEHUMIDIFICATION PROBLEM FAQs - questions & answers posted originally at the end of this page.

Or see these

Recommended Articles

• AIR CONDITIONER BTU CHART - where we discuss the air conditioning system sizing problem
• COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs
• DEHUMIDIFICATION BASICS
• DEHUMIDIFICATION PROBLEMS
• HUMIDITY CONTROL INDOORS
• HUMIDITY LEVEL TARGET
• MOISTURE PROBLEMS: CAUSE & CURE
• RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.

Suggested citation for this web page

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

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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