Condensation at a basement window (C) Daniel Friedman Dehumidification of Indoor Air- FAQs
Questions & answers about indoor air dehumidification

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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.

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Air Conditioning System Dehumidification Problem Diagnosis & Cures

Sketch explaining that oversized air conditioners are a mistake (C) Carson Dunlop Associates

Question: 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

Question: can water in the air conditioner/heat pump condensate drip pan or on the cooling coil cause high indoor humidity in the building?

The short answer is no. The cooling coil is removing humidity from air passing over it, not adding humidity to it. Edgardo, a reader, questioned whether or not running the blower fan in the air handler full time would increase building humidity by picking up residual moisture from an otherwise properly working cooling coil and condensate drip pan when the cooling system is off.

Our opinion is that the volume of water remaining on a wet cooling coil or condensate drip pan (one that is draining, not keeping a reservoir of water in the system) is negligible and very unlikely to explain high indoor humidity when the A/C is off. Details of this discussion are found at Questions & Answers about whether or not to run the heating or air conditioning blower fan continuously which is found within the article titled BLOWER FAN CONTINUOUS OPERATION

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

(July 20, 2011) ViennaGA said:
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.


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?


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.

Question: dehumidification problems and indoor mold contamination

(Sept 9, 2011) Kristin said:
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 13, 2011) Steffie said:
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.


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.

Question: Tommy opines that running the air handler blower increases indoor relative humidity

(Jan 15, 2012) Tommy said:
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.


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.

Question: AC has never dehumidified well

(Mar 23, 2012) kevin said:
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?


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.

Question: AC system icing problem

(July 13, 2012) Joyce Henry said:
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?


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

Question: moisture inside the AC unit

(Sept 30, 2012) Bruce said:
Had new AC/Heat Pump ionstalled June 2007,. Have had system checked twice annually with no problems reproted.Had this years fall check up in Sept. and the tech showed me there was moisture all over the inside of the unit, on the electrical connections and boards, in the fresh air returns, etc and a check with hygrometer shows inside humidity at 80%. Had duct work checked, no apparent leaks, present insulation at about R17.

Am having ducts resealed and adding insulation to R 38 with no guarantee this will solve the humidity problem. Would appreciate any and all tips and advice

Question: using a dehumidifier to dry out a flooded house - a risky idea

(Jan 5, 2015) Celia Angel said:
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?



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 - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR CONDITIONER BTU CHART - where we discuss the air conditioning system sizing problem
also see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.



Or see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs

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


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