How To Look At A House

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Why does my home still feel humid and muggy with the air conditioner on?

Thursday, June 2, 2022

An air conditioner that's cooling your home without adequately dehumidifying is only doing half its job, and here are our “Top 10" possible reasons why: 

1) System too large - Dehumidification occurs when the moisture in indoor air condenses on the evaporator coils of the air handler (indoor unit) as air flows over them. Then it runs down the surface of the coils into a trough that drains the water to the exterior. This is similar to how humidity condenses on the outside of an iced-drink glass and drips down the side.

    But when an a/c system is too large, it chills the air too quickly—without enough volume of air flowing over the coils to let more humidity drip out. Newer homes in Florida are required to have an energy code evaluation that includes calculating the correct BTU size for the home, and the HVAC specs from the code are often taped to the front of the unit. When a homeowner replaces an old, right-size a/c unit with a much larger one “to keep the house extra-comfortable,” it often backfires into cold, but muggy, interior air. This is especially true with mobile/manufactured homes.

2) System too small - This problem is a little more obvious. If the system is too small keep the home cool on hot summer days, then it is also not keeping up with dehumidification.

3) Duct leakage - Any duct leaks in the attic or crawlspace alternately pull humid outdoor air into the home and push cool, conditioned air to the outside. While this might not be the primary cause of humidity problems, it is often a contributing factor. Having an a/c contractor check and seal any duct leaks is a good solution.

4) Older system - The humidity-reduction ability of an a/c system deteriorates as it gets close to the end of its lifespan, especially as it ages past 15 years old. This too may be just one of several contributing factors. It could be time to get a new air conditioner.

5) Humidity created inside home - Every person inside a home exhales humid air continuously, so more people means more humidity. Indoor plants and aquariums also add moisture. Showering and cooking dump the most moisture all at once into the air, but that can be easily reduced by regular use of the bathroom and range hood fans to exhaust the moisture-laden air outside. 

6) Dirty evaporator coils - If the air handler runs without an air filter installed, or the filter buckles and allows unfiltered air to pass around it, then the fins of the evaporator coils act as a filter and bits of dust start to build up and coat the coils. This makes them much less efficient at removing humidity. 

7) Refrigerant leak - A refrigerant gas leak in the lines gradually squeezes the cooling and dehumidification ability out of a system until it’s barely cooling or dehumidifying at all. Eventually the suction line freezes over near the condenser.

8) Extreme outdoor humidity - An afternoon thunderstorm on a hot, humid day can saturate the air with humidity. This may briefly be more than your system can remove. One solution is to set the thermostat a little lower than usual, so that the system runs longer and has more time to remove the moisture from the air.

9) Condensate not draining properly - If the condensate drain line backs up, or the water drops off the evaporator coils instead of sliding down them, then it can soak down the return air plenum below the air handler, which then recycles the removed moisture back into the air. Mold may also grow in the wet material over time.

10) Moisture intrusion - Roof, wall, or plumbing leaks can sometimes go undetected, while continuously adding moisture into the indoor air. Air leakage due to poorly sealed doors and windows also allows unwanted moisture into the home. A home inspector’s infrared camera can often locate these types of problems, as shown at the inset in the photo above.

    You might need a visit from you local air conditioning contractor to figure out exactly what’s causing the problem.

• • • • • • • • • • • • • • • • • • • • • 
Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at for $19.95.
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  To learn more about heating and air conditioning systems, see these other blog posts:

Why is my air conditioner not cooling enough? 

How can I find out the SEER of my air conditioner? 

My air conditioner won't turn on. What's wrong? 

How can I find out the size of my air conditioner? 

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

Where is the air filter for my central air conditioner and furnace? I can’t find it? 

Does an old air conditioner use more electricity as it ages? 

How did homes stay cool in Florida before air conditioning? 

What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while? 

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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