Why is my house so muggy when A/C is on?

AirConditionerCondenser
Summer has finally arrived here in Western New York, after what felt like one of the longest, wettest spring times ever. Air conditioner season is finally upon us! As a home inspector, I’ve been asked by a few homeowners in the Buffalo area “why does my A/C make my house so humid?”. As it turns out, there are at least 7 reasons for this common problem. This question at first seems strange, because air conditioning by design is supposed to remove humidity from the house.

This question at first seems strange, because air conditioning by design is supposed to remove humidity from the house. 

 An air conditioner should NOT make your house humid.  If it does, then something is wrong. There are a few reasons why this might be happening.

 

 

#1 The air conditioner is oversized

The primary job of an air conditioner is, of course, to remove heat from a home.  Removing humidity is actually a side effect of the cooling process that relies on air moving across cold evaporator coils, which in most systems are above the furnace or blower unit in the basement.  

Warm air moving over cold coils causes condensation, which collects, beads up and drips away into a drain.  The longer that the AC runs, the more condensation forms and so more humidity is removed from the air.  

Put simply, if the air conditioner runs for only 5 minutes every hour then there is very little humidity that can be removed from the air.

Why would a an air conditioner be oversized?  The quick answer is that it is not easy or fast to calculate the heat load of a house, which is a required step to determine how large an air conditioner should be.

Proper heat load calculations take into account our climate, the size of the home, what direction it is facing, the size and efficiency of the windows, size wall framing, insulation R-value, and if the house is clad with brick or stone siding, for example.  

The Penn State College of Earth and Sciences maintains a course syllabus which outlines the concerns with sizing air conditioners.  It states that:

“A system that is too large will cool the room or home quickly but will not provide the comfort that is needed, because the cool air reaches the thermostat quickly and the thermostat sends a signal to shut the system before the relative humidity is reduced to a comfortable level. As the cold air is distributed in the room, the thermostat realizes that the temperature is not at the set point and then turns on the air conditioner. This quick cycling of the unit (start and stop) reduces the lifespan of the equipment and increases the energy consumption. A larger air conditioner also consumes more energy.

A system that is small will have to work all the time and is not energy efficient. So the right size is very important for energy efficiency.”

Almost zero contractors perform these calculations, and use their best judgement.  Unfortunately sometimes they pick a bigger unit that they should have.  

At my own home, I had a new AC system installed last year and received three quotes.  Two were for 3-ton systems, and the other was for 5-ton. I asked them about their sizing, and all three stated that they thought the size was right for the house.  No basis was given for their recommendation.

Unfortunately oversized air conditioners not only can lead mugginess in the home, but short cycling (constantly turning on and off) can lead to a shorter lifespan of the unit and higher electricity bills.

What can you do if your AC system is oversized?  

My recommendation is to contact a reputable HVAC specialist, who can properly evaluate the system.  If the unit is oversized, you most likely will need to replace the compressor and evaporator coil with smaller units in order to regain efficiency and reduce humidity.

 

#2 The air conditioner is old or failing

The air conditioner’s condenser unit (the outside part) has several parts that just wear out over time, making the system as a whole less efficient at both cooling and removing humidity. 

Wikipedia’s article on air conditioning states that:
Air conditioning equipment will reduce the absolute humidity of the air processed by the system if the surface of the evaporator coil is significantly cooler than the dewpoint of the surrounding air.”

There are a number of reasons why an old air conditioner might be less effective than a new one, but what it comes down to is that if the indoor evaporator coil is not chilly while the system is on, then water will not condense on it and humidity will not be reduced.  

Reducing humidity may be as simple as replacing the refrigerant, which would also prevent the unit from properly cooling.  

I recommend that you call for service before giving up on the system or writing it off.

 

#3 The fan is on all the time

This one is less obvious, but is the easiest to fix.

Most thermostats have 3 settings for the fan:

Off
Auto
On

If the fan is set to “on” while in cooling mode, the fan will always run, yet the air conditioner will only run when the temperature rises above the set point. 

People sometimes use this setting for comfort reasons, and they feel as though it’s nice to have air moving all the time.

Other times people with allergies have invested in expensive inline air filters, and want to constantly filter dust and pollen particles out the air.

Why does running the fan constantly keep the humidity high?

Think of the cold evaporator coil as a glass of ice water.  If air is not moving over the coil, then water has a chance to condense on the surface of the coil.

If air is constantly moving, then the moist air is just blowing right over the coil back into the rooms of your home.  

In “auto” mode, air is moving for several minutes, then it stops and allows moisture to condense on the coils and drain, then moves again.  This cycling is healthy and is what allows the dehumidification to occur.

 

#4 dirty and dusty evaporator coils

Dirt and dust on an AC evaporator coil creates a layer of insulation between the warm air in your home and the below freezing refrigerant that runs through the coil. 

This insulating layer will greatly reduce the ability of the coil to absorb heat from the air, also reducing the ability of water to condense on the coil.


Severe dirt and dust build-up can cause ice to form on the coil.

How to clean evaporator coils?
I always recommend to my clients that they have their furnace and AC regularly maintained, if not yearly at least every other year.  Cleaning evaporator coils requires special techniques, tools and substances. You don’t want to damage your coil, so call a professional.

#5 Leaky Return Ductwork

Big Opening in Return Ductwork
A large cutout in return ductwork

Return ductwork is designed to pull air from the living spaces of a home, filter it, then heat or cool it and circulate it back into the rooms of the house.  

 

What if there is a big hole in the return ductwork in a humid, mildewy basement?  Or a poorly installed return ductwork system running through the attic?

 

It’s not hard to imagine the effects of this on the humidity levels in a home.  

 

An air conditoner will not be able to keep up with this type of problem.  AC systems are intended for ductwork to be reasonably tight. Not every seam needs to be taped up, but large gaping holes must be sealed up in order for humidity to be reduced.

 

What if there is a big hole in the return ductwork in a humid, mildewy basement?  Or a poorly installed return ductwork system running through the attic?

 

It’s not hard to imagine the effects of this on the humidity levels in a home.  

 

An air conditoner will not be able to keep up with this type of problem.  AC systems are intended for ductwork to be reasonably tight. Not every seam needs to be taped up, but large gaping holes must be sealed up in order for humidity to be reduced.

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How to Reach Us

Bradley Beck
Alto Home Inspection, LLC
Colden, NY 14033

716-222-2586
[email protected]