Questions & answers about inadequate dehumidification in buildings:

These frequently-asked questions about building humidity, dehumidification, and air conditioning system's ability to dehumidify indoor air help diagnose problems with indoor moisture or moisture and condensation in air conditioners.

This article series 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.

Air Conditioning System Dehumidification Troubleshooting FAQs

These questions & answers about problems with dehumidification when running air conditioning were posted originally

at DEHUMIDIFICATION PROBLEMS - be sure to also read the diagnosis and repair advice we give in that article.

Article Index

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• [We're in process of editing the Q&A below - Ed.]

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Very high indoor humidity problems after installing a new blower fan motor

We had a new fan motor installed on our 20 year old unit, a week ago. I have noticed major moisture build up on the inside , and outside of the air handler unit in the garage.

The air handler is very soggy to the touch, and mold has increased on the outside of the unit. Could a new fan be causing the excess moisture? On 2018-08-06 by Jack

by (mod) - Could a new fan be causing the excess moisture?

Possibly indirectly, Jack as the new fan may be moving more air through the system than previously, causing more dehumidification of indoor air over the same or a shorter time period.

It sounds as if there is either a problem with

- a clogged condensate drain - as water should not be accumulating in the air handler

- too-rapid air movement across the cooling coil, blowing condensate past the coil into the air handler or ductwork rather than allowing it to fall into the condensate collection pan

On 2018-06-29 by Roger

Does a humidity level of 82% at the air duct register when the air conditioning is running indicate that the blower is blowing some condensate into the ducts rather than letting all drip to the pan and out the drain?

by (mod) - condensation in the ductwork

I can't say with confidence, Roger. I would look in the duct work just downstream of the cooling coil for actual water or condensate blow-off, and perhaps check the fan speed.

But high RH in the supply ducts could mean that either your environment is very humid to begin with or your system is not dehumidifying.

On 2017-10-10 by Sandra

I have a year and a half old 4 Ton American Standard System which includes a Heat pump Model #4A6H4048D1000AA and an Air Handler Model #TEM4A0C60S51SAA.

It worked fine for the first 16 months, but now is not removing the humidity. It was last serviced in April.

Could there be something plugged. It cools fine.

Thanks,

by (mod) - is the cooling coil blocked?

If the system is delivering cool air with no reduction in air flow then I doubt that the coil is blocked. But if the system has lost refrigerant, isn't delivering cool air, OR if air flow is reduced then yes those are things to have investigated.

On 2017-10-09 by (mod) -

Dehumidifier Reviews said:

Air conditioner dehumidification is also a good option for users. but, sometimes we need a dehumidifier which is able to remove all mositure from our room/home etc.

by (mod) - we never ever remove "all moisture" from a building,

Thanks for the comment, D.R.

Actually we never ever remove "all moisture" from a building, but we do attempt to keep moisture levels in a comfortable range and in a range that avoids mold problems.

See HUMIDITY LEVEL TARGET

On 2016-07-20 by Damp in Texas

Last week there was no condensation on my windows when I got up. Up until that day, there was condensation every morning and the air at night was crisp. My AC is slightly oversized because my 1874 home is drafty but I've had no problem in 10 years.

It does have a 2 stage fan system and I'm wondering if one "stage" is no longer working? The air is cool but not dehumidified.

by (mod) - Oversized A/C units are likely to fail to dehumidify adequately.

Oversized A/C units are likely to fail to dehumidify adequately. But if your fan can run at a slower speed that might help the situation.

Dehumidification in AC systems is a combination of coil temp vs air temp moving across the coil as Delta-T and time.

Not sure the age of this thread however 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 temp vs air temp moving across the coil as Delta-T and time.

What is not commonly told is that new systems mandated by 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 reduce moisture removal as air travels across the coil.

So in effect, what can happen is the aid 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. On 2015-08-09 by BAR

by (mod) - how to understand dehumidification in an A/C system

Thanks for the comments, Bar, we'll include them in the article DEHUMIDIFICATION PROBLEMS.

Why is our Air Conditioner Rusting on the Inside?

We have lived in our new home for 1 year and 7 months. Our air conditioner is rusting on the inside.

The blower fan and all the internal areas are rusting. We have blown insulation in the attic and exterior walls. Living area is 2400 sq ft and we have a 2 speed, 3 ton Lennox AC unit.

The Lennox inspector looked at the whole set up and said it was a moisture problem.

That's hard to believe that that is the problem. We had a manual J load done on our home to determine the size of the AC before it was installed.

We have followed everything that we were told to do. 2 different electrical components have failed due to rust.

The AC guys said they have 3 more units rusting the same exact way. 2 are in closets in the home and one is in an attic with regular fiberglass insulation. All 4 units are Lennox 2 speed units.

What could possibly be our problem? Does Lennox have a problem that their not sharing with us? Any help would be appreciated! We don't know what to do next. - R. & C. R. - South Louisiana

Reply: Check your A/C BTU Rating and Sizing Specs

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, and we are just guessing, as we have so little information, but

• Oversized A/C? Your air conditioning system could be oversized for the size of your home - we discuss this problem
  
  at DEHUMIDIFICATION PROBLEMS and
  
  at MOISTURE PROBLEMS: CAUSE & CURE
• Condensate Problems? Also, your air conditioning system condensate may not be being handled properly.
  
  See both CONDENSATE HANDLING, HVAC
  
  and CONDENSATION or SWEATING PIPES, TANK

 

 

AC condensate drain clogged and air conditioner recirculates moisture rather than removing it

Our 5 ton systems drain line clogged, but was unclogged several weeks ago.

Building up to the clog - and ever since - the humidity in the downstairs has increased to 70% and will not come off. The drain line does not appear to be producing adequate runoff yet the drain pan also is basically dry.

The upstairs humidity has not jumped up as a second 3.5 ton unit services the upstairs. Can you add more detail about troubleshooting when the A/C condensate appears to be limited?

We cannot find any abnormal source of moisture and when comparing condensate from either system the 5 ton just seems to be having a problem. Technician confirmed good pressures in the system.

I wanted to add that the system is cooling the space, it just really appears to be circulating the moisture rather than removing it. Lowering the set-temp overnight also had no affect on the level of humidity. (July 20, 2011) ViennaGA

Reply:

ViennaGA
An air conditioner that cools but does not dehumidify adequately can be a signal that it is over-sized for the space to be conditioned. The other place to look is for source(s) of excessive indoor water or moisture.

What happens if your system is too large or "oversized" is that the A/C unit can drop the temperature of the room or space just fine, but it does it so rapidly that there is not enough on time or run time to also remove enough moisture from the air.

Put another way, a big or oversized cooling system can drop the room temperature much faster than it can remove moisture from the room air.

(July 20, 2011) ViennaGA said:

I know but the unit has been in its configuration since we bought the house some odd 6 years ago. It just all the sudden has gotten here.

Or, if I hear you correctly, that the humidity got up it still cannot be removed until we get the unit to not short cycle - which could mean fully opening doors to warm areas or closing registers near the cool air return? What kinds of parameters would tell us the expected cycle time?

Anything related to the TEV or control boards that you have seen happen that might introduce this so suddenly?

Reply:

Thanks ViennaGA - that observation is that your system is probably not oversized. And I agree that if your A/C system is short cycling, that needs to be cured. If the A/C system comes on for just five or ten minutes and then is off for that long or longer I suspect it's short cycling - that's a somewhat arbitrary OPINION and your HVAC tech may have a different guideline.

Here are some other things we do to improve dehumidification:

1. As you suggest, open doors or registers to give more building air access to the HVAC return register - (by the way check your airflow - if it's blocked that's the problem ?)

2. I will sometimes place one or more house fans to improve air circulation and improve air movement to the return register from various rooms.

3. If short cycling is even less than I cited there could be a control problem - an electronic control problem; if the A/C cycling is much longer and the system is staying off for too long, I suspect a freeze-up at a coil or even a refrigerant metering control problem.

(July 25, 2011) ViennaGA said:

I was reading 56-59 deg at the register with a 76 set point at the thermostat right over the cold air return. System cycling on for 11-15 minutes, off for 5-7 minutes. One night at the end of the week I dropped the set point down to 72 hoping the unit would run and push the humidity out.

That did not work as I still had 68% in the next morning. However, when I moved the set point back up, I hit the fan control which sent the fan out of Auto and into On mode.

The fan ran all day. By 5:30 the humidity had dropped to 58% and I observed a good stream out of the drainage pipe – finally! Ran the fan a few more hours and turned it back to Auto at bed time. By morning the humidity reached 48%. Been great ever since.

Dehumidification problems and indoor mold contamination

My problem: recently high humiditiy from the room. small room and cools well if not too well, so maybe oversized. but why only change recently? not due to warmer weather lately because was hotter than this a month ago. seems to bother my breathing. if something broke, is it possible that something beyond water vapor is entering the room (like the gas coolant?) (Sept 9, 2011) Kristin

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How do I get my humidity upstairs down? I live in a multilevel house and just can not get the humidity under 60% on the top (bedroom) floor.

Have central air and a dehumidifier in the hall near the bathroom.

The floor below runs around 54% (kitchen/dining/living room) and the 1st floor ground level small area on a slab with front door ground level with driveway and street runs around 52% with dehumidifier and air purifier and the basement runs 55% with dehumidifier and air purifier. I do have a sump pump and mason could not fix where water comes in through chimney door from underground.

I spent $30,000 on mold remediation 7 years ago and seemed OK until this year - I have been sick all summer and still now and allergist said I am allergic to mold so do not know if it is from past exposure or there are still mold spores in the house. Being sick I have been unable to keep up with dusting, etc. like I should. I am 74.

Please if you could give me any suggestions as I was thinking it was coming from the attic but then the kitchen floor would be high also as that is under the attic as well. If I do get the humidity down into the 50's it does not last long until the heat comes on and that is a long way off yet. It runs around 40% in the winter. Thank you.

Forgot to mention I have hot water heat not hot air so a/c unit is separate in attic. (Sept 13, 2011) Steffie

Reply:

Steffie, if the A/C system is oversized for the home it won't dehumidify adequately - things cool down too fast for the system to move enough air to dehumidify it. You may need a supplemental dehumidifier.

Regarding "could not fix where water comes in through chimney door from underground." if you don't fix sources of outside water you're fighting a losing battle trying to dehumidify in the home. I'd find someone else who can and will fix the water entry problems.

 

Reader opines that running the air handler blower increases indoor relative humidity

Running the blower continually will increase the RH in your home. The problem is not water in the drain pan but water that is suspened in the evap coil that is evaporated back into the air. If you watch the condensate outlet you will notice that when the condensing unit and the blower stop, there will be a noticable increase in water flow from the outlet.

Also if you have ever turn a unit off that has a clogged drain, you will immediately have greatly increased water dripping or sometimes flowing from indoor unit. I have seen many residences with 10 percent and more reduction in RH by cycling blower.

Commercial building usually benefit from continual blower operation because of more varied heat sources, air movement feeling cooler and more humidity sources (open doors, etc.). I do live in Florida where we use air conditioners year round.

An AC system that is not properly charged will also cause high humidity. An overcharged system can be more of a problem than an undercharged system because the slightly overcharged system will still bring down the temperature but will not dehumidify very well.

Usually when a system is undercharged you will notice a decrease in capacity. If you continually have a tech adding refrigerant to your system, it may be overcharged. I find more system overcharged than I find undercharged. (Sept 13, 2011) Steffie

Reply: how does overcharging of refrigerant increase cooling?

Tommy how does overcharging increase cooling?

Coolant flow rate into the cooling coil is set by the TEV or a cap tube, and is independent of refrigerant level as long as charge is sufficient. When the charge is too low, on the other hand, the result is coil frosting - until the charge gets so low that there is no cooling at all.

I appreciate the comment but need to understand better the explanation or mechanics behind it. Thanks so much.

There are a couple of dangerous speculations in Tommy's comments above:

1. While circulating air may increase the ability of building air to pick up moisture, only if there is a persistent moisture source might circulating air increase the RH measured in that air, and then only if the air conditioning is turned off.

2. Overcharging any heat pump or air conditioning system - putting in too much refrigerant - is never a good idea and is pretty much always dangerous as it risks liquid-slugging the compressor which then destroys that very expensive component.

 

Our AC has never dehumidified well - should we close off one of our air returns?

(Mar 23, 2012) kevin

I installed a 2 ton ac unit with a variable speed air handler in my upstairs. It has never dehumidified very well. When running returns, I ran out of space in the attic and made a space in the return (@ the air handler).

Would it be of any benefit to plug that particular return, forcing more of the affected air into the air handler?

Reply:

Kevin, thanks that's an interesting question. If the unit is oversized, it cools the air so quickly that the thermostat is satisfied based on temperature before the air humidity has been reduced.

Reducing the air flow through the system will probably make that condition worse ... or now that I think of it, maybe better. If you do anything that makes the unit run longer before the thermostat will be satisfied that may help the dehumidification problem - ask your service tech if you can restrict the fan to run only on low speed.

Only on newer air conditioners whose cooling coil operates at a higher temperture might we try slowing the fan speed (if the fan design and wiring permit) to improve dehumidification.

 

AC system icing problem

Our tech installed 1/2 HP FAN MOTOR INSTEAD OF THE 1/3 HP ONE WE HAD ON OUR AIR CONDITIONER The motor is fan driven. It now freezes the coild and we have to stop the air conditioner until it unthaws. What hapened? (July 13, 2012) Joyce Henry

Reply:

The system is not properly charged or its thermostatic expansion valve is not metering refrigerant at the proper rate

 

 

Using a dehumidifier to dry out a flooded house - a risky idea

Does anyone know if using a dehumidifier 24 hours a day for months [to dry out a flooded house] can lead to any health problems.

My family and I are suffering from excessively dry and itchy skin and very itchy scalps since the dehumidifiers have been installed. Is it likely to be connected to the dehumidifiers? (Jan 5, 2015) Celia Angel

Reply: Watch out: this is probably an unsafe (unhealthy) idea

Celia

With apologies that this sounds a bit glib, of course using a dehumidifier to try to dry out a flooded house can lead to health problems: a dehumidifier alone cannot possibly properly dry out a flooded home.

The result of taking too long (more than 24-48 hours) to dry out a flooded home is likely to mean that there is mold contamination: if not visible mold growth on wood and drywall in the home as well as on contents, there may still be hidden mold growth in floor, wall, or ceiling cavities both directly due to the flooding and even secondarily due to prolonged high indoor humidity levels.

See FLOOD DAMAGE REPAIR PRIORITIES for a list of the key steps and priorities of action to minimize flood damage to a building.

...

Continue reading at DEHUMIDIFICATION PROBLEMS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

A/C & De-Humidification 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.

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