I have a mold problem in my apartment, in certain areas there is condensation.

I believe I have traced this to the air conditioning, in the apartment the humidity will vary from 60% to 85% RH at a temperature of 23.5 degrees Celsius.

Outside it is very hot, around 37 degrees Celsius with high humidity.

We have two fan coil units (one for the living room, the other shared between 2 bedrooms) both fed of a central chilled water system. We are on the 10th floor of a 15 story apartment block.

Both AC outlet vents put out humid air, one of them is worse than the other. The air is cold at the required temperature, but it feels wet - 90% RH at 23 degrees Celsius.

We have no leaky windows, no vents that are sucking in outside humid air - only outlet vents for the kitchen and bathroom extractor fans. We leave the AC running 24/7 at set temperature of 23.5 degrees.

Landlord said there is no problem and doesn't want to fix it - he sent his AC guys twice who just cleaned the filters nothing else.

It is very hard to find professional tradesmen here in this part of the world where there is an illogical infatuation with low cost labor - most "engineers" are self trained air conditioning experts.

Since I don't accept their answer that me mopping the floor is causing the humidity (i don't own a mop), how would i go about fixing this:

  1. Any idea what the cause is of cold but damp air (90% RH at 23 degrees Celsius)? Are there multiple possible causes?
  2. Can i service the AC drain, filter and anything else that may be causing a block myself?
  3. Is this likely a problem with the building or confined to my fan coil units? I am concerned about health issues so don't want to be troubleshooting this for long.


  • It seems unlikely that the air could be chilled that much yet still have such high humidity. Is it possible the meter is defective or needs calibration? Also, is it really sampling the output how you think. In the photo, it looks like room air possibly could be drawn upward behind the meter and thus provide a bogus reading. – wallyk Oct 3 '14 at 19:11
  • @wallyk I had the same initial reaction. But bear in mind, as air temp drops, RH goes up without the moisture content of the air changing one iota. Also, if the poster's information is correct, the unit is actually drying the air somewhat. 91% RH at 23.6C is about 42% RH at 37C. He said the unit is taking fresh air from the outside, and that the humidity is "high" outside. 42% is by no means high. So, I think the unit is pulling some water out, just not enough... – bobfandango Oct 3 '14 at 20:14
  • around the house there is humidity at 80% and condensation, this is well away from the air duct and was down by a table. The sensor also reads the same humidity as the dehumidifier. Holding the unit near the air flow of the FCU air the humidity increases by at least 10%. Outside it is typical for temperatures to be 40 degrees Celsius and 70% humidity, at night outside drops to around 30 degrees Celsius and humidity increases up to 90%+! The more i think this could be problem with FCU not pulling enough water combined with an issue with AHU unit on the roof supplying air that is too humid – morleyc Oct 4 '14 at 6:29

UPDATED (not enough reputation to comment, so editing this answer):

Another interesting thread here: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/77152/afv/topic/Default.aspx

Synopsis: if the chilled water is not cold enough, the fan coil unit will not remove enough moisture from the incoming air and RH could rise to mold inducing levels.

To avoid mold, the RH should be kept below 70%. 70% RH at 23.5C is a dewpoint of about 17.7C. I think the incoming water will need to be a fair bit below that to get your air dry enough... Indeed, the forum post linked to above suggests a temp of around 10C. Try to measure the temperature of the incoming water to see if it is too high.

More generally, you already have ample evidence (high RH of the cold air), IMHO, that the unit is not functioning correctly. The landlord is full of beans and too cheap to fix this. But more data/evidence (i.e. temp of incoming water) may be enough to convince them to actually fix the thing... Remind them too that mold is a health issue....

OLD ANSWER: I was intrigued because I thought AC dehumidifies, not the other way round... Per Google, so it is. But not always apparently.


I know nothing about AC systems, but the answers linked to above sound superficially authoritative. Perhaps it'll put you on the right track. Best of luck.

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  • 1
    If the link 404s there will be no actual info in this answer for future viewers, it is better to quote or paraphrase the relevant parts of the page you link to. – RedGrittyBrick Oct 3 '14 at 8:58
  • Fair point RedGrittyBrick. But the content of the page is too dense for me to parse. As I said, I know nothing about AC. – bobfandango Oct 3 '14 at 18:31

Please first check your measuring tool by putting it to your refrigerator.it will be around 8C.

Can you measure the outside temperature and humidity too if your measuring tool is working?

It seems there is a problem in your fan coil supply air temperature because of it high degree;21C. It's normal temperature is 15C in hot summer day.

The 21C may lower the room temperature but it will never remove the moisture but it will make indoor more humid due to only sensible heat transfer.

After assuring the measuring tool is working correctly,there will be following probabilities:

1-If other apartments in your floor haven't your problem you should check the circuit of your fan coils(pipes,valves and hoses).Sometimes one or both of valves are fully or partially closed and everybody thinks they are opened.And sometimes the internal diameter of hoses is small ,you should change them with next size.

2-If other apartments have your problem, there is a problem in your central chiller.

3-Or may be,both problems exist in your apartment.

May I ask where do you live?I think I know there!



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  • Thanks for the reply Mazdak, in the end i moved - as soon as I was in the new place the same humidity measuring device goes from 40% to 50% at 24 degrees. Much more comfortable to live and the floor isn't sticky when you walk bare feet. I guess this is proper designed humidity levels the old place must have an issue as you have mentioned with the central plant, or perhaps because they used to leave the basement door open to create a draft for the people living in the stairwell... I guess that was sucking in lots of moisture too as humidity here (the Middle East) hits 100% at times – morleyc Sep 26 '15 at 19:53

From the picture it looks like this might be a window A/C unit. Do you have the option to recirculate air, or is it always pulling fresh air from outside? If it can't recirculate, I doubt the AC unit will ever be able to dehumidify the air.

As a workaround, you might want to look into installing a dedicate dehumidifier. This will not pull in any outside air and instead will remove the humidity from the inside air.

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  • I wish it were Window AC would be much simpler! Its central AC with 2 fan coil units in the ceiling fed by chilled water, i believe fresh air is supplied to these FCU by an air handler unit on the roof. – morleyc Oct 3 '14 at 13:06

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