I think i'm being sold bunk by a mold company, but not sure.

We live in the tropics, we recently had our bathroom ceiling flood due to a burst pipe, and went away immediately after. (the landlord had the bathroom ceiling removed a bit under a week into our holiday)

Before we left, we called a mold control company and said we were going to leave the A/C on to keep the humidity down to prevent mold growth (we have a new A/C, very efficient, was going to keep it on the lowest setting just to dry the air a bit, so the energy usage would not have been a concern). They said not to do that. They're the "experts" so we followed their advice.

When we returned, the apartment had bloomed (literally bloomed) with mold.

Our landlord now is saying that this is because we use the A/C a lot (we don't), and need to open the unit windows more, and buy a dehumidifier. If we actually did use the A/C a lot, wouldn't the cooling and drying of the air have prevented the mold growth more than opening the unit?

We live next to a rainforest, so I would've thought that opening the unit would invite spores and damp air in rather than prevent mold?

Thanks in advance for any help

  • 3
    Odd that both parties would give you that flawed advice. I'm guessing that you don't pay utilities separately from your rent, giving the owner motivation to discourage AC use.
    – isherwood
    Commented Jan 7, 2019 at 14:53
  • 4
    There's one circumstance where I could see it being an issue -- if you run the AC aggressively during the day and cool down the house to, say 65 degrees, and then at night you open the windows and let warm moist 85 degree air in, it could cause condensation on walls and other surfaces.
    – Johnny
    Commented Jan 7, 2019 at 19:58
  • 6
    "and buy a dehumidifier" A/C is a dehumidifier. Commented Jan 7, 2019 at 23:13
  • 2
    The lease contract places onus for repairs above $150 on the landlord - the mold outbreak occurred after slow repair of the waterlogged roof, so the landlord is incentivised to blame us for the mold. The A/Cs are about a month old, so unlikely they are the source of the mould
    – ChrisJ
    Commented Jan 8, 2019 at 11:57
  • 2
    @JimmyJames Well, those in e.g. office buildings and hotels almost always have but a lot of newer systems for home use have a fresh air intake as well. It allows you to filter out pollen and pollution from the outside air for example. You can also pre-cool the intake air by routing it through the ground to increase efficiency.
    – Christian
    Commented Jan 8, 2019 at 17:40

6 Answers 6


Yeah, that advice was super wrong. Moisture encourages mold growth.

After a water leak you need to go to extremes to dry the air to get wet things to evaporate into the air, which you then continue to dry. I would run dehumidifiers, or shoot, if your house has A/C, just run the A/C since that is a dehumidifier.

The only risk is if you excessively chill the air, because cooler air has less capacity to hold moisture. This drives the British crazy, their masonry buildings often have utility spaces with "the damp" which are 40-50F (5-10C) and freon dehumidifiers just don't work well at those temperatures. They are forced to heat the space just so the dehumidifier will work effectively. I've witnessed it myself, a dehumidifer that fills in 8 hours at 75F takes a week to fill at 45F. The goal is measured in gallons, and time is a factor, so that is a total lose. And mind you, dehumidifiers do heat the space since all the energy they use winds up in the room as heat. Air conditioners remove the heat.

So you need to also heat the house, which fortunately is something solar load (i.e. The sun hitting your roof) does automatically. All you need to do is have a system whereby the air conditioner doesn't cool the house more than it's being heated by the sun... Some sort of, gosh, I don't know, thermostat :)

Oh yeah, you have one of those. Set it aggressively enough to run a lot, but not so aggressively that you create the British problem. Something like 60-ish (16C), let it go nuts.

  • Yeah, i was sure that ACs dehumidify and opening up the windows in the tropics would bring in moisture, was starting to question myself after hearing it from two different sources
    – ChrisJ
    Commented Jan 8, 2019 at 11:58
  • I don't think this is right. If you want to dry out your house, you need to have the AC running a lot. The reduced capacity of the air is inconsequential as the condenser will continually remove moisture and keep the ait below saturation.
    – JimmyJames
    Commented Jan 8, 2019 at 14:53
  • @JimmyJames You'll continually reduce the temperature, reducing evaporation. Cycling on and off will give the temperature to stabilise (obviously, to stay at the thermostat temp) and moisture to evaporate, before AC kicks in again to circulate and dehumidify what's airborne.
    – Dan
    Commented Jan 8, 2019 at 15:29
  • @Dan What you are missing is that the condenser is always colder than the air. It's like soaking up water with a damp sponge. A damp sponge can't hold as much water as a dry sponge but you can continually saturate it and wring it out in order to clean up more water than it can hold in total.
    – JimmyJames
    Commented Jan 8, 2019 at 15:35
  • @JimmyJames Yes, but that assumes it always has humid air passing over it. Constantly decreasing temperature will produce diminishing returns. It'll work, yes, but you're probably wasting energy for the same outcome.
    – Dan
    Commented Jan 8, 2019 at 15:41

You are being sold.

The evaporator coil in the AC condenses water vapor in the air. It does this by blowing the warm air from your home over the cool evaporator coil. The cool coil pulls the moisture from the indoor air, removing it and draining it from your home via the condensate array. This happens with every air conditioning cycle. If the AC unit can't keep up with the humidity, the air will feel clammy and cool when it should feel dry and cool.

Get a humidity measuring tool so you always know what it is at (generally around $10 US). I don't know how bad the mold is but if it only on the surface, run the AC and in the meantime put on a respirator and use a bleach mix to remove/kill it.

  • 6
    I use 3% hydrogen peroxide and water it kills the mold and you don't have the bleach smell. I do agree running the AC will reduce the humidity all the condensate coming from the evaporator coil is proof of this the only difference in a dehumidifier and AC is the AC dumps the heat outside where a dehumidifier dumps the heat in the same room so there is no cooling but they work the same.
    – Ed Beal
    Commented Jan 7, 2019 at 15:44

It is possible for A/C to cause mold issues but it depends on a few factors. The problem is this: when choosing a A/C unit for a home, often people (even 'professionals') will assume that bigger is better. The installer plugs in the numbers to the manufacturers model and comes up with a recommended unit size, then bumps it up to a larger one.

The problem with this is that the larger unit will cool the home too quickly. It comes on, cools the air down and then shuts off. But because it only ran for a short time, the condenser did not have a chance to remove much moisture. Now you have cool, damp air in the home which results in moisture condensing in the walls. It may be the case that the mold control company has been called into to deal with such issues and that led to them giving that advice.

In order to dry the house out you need to have the A/C running continuously. Setting it on low (higher temperature) may lead to the A/C cutting on and off and if you have an over-sized unit and/or it isn't especially warm outside, it could make things worse. Setting the A/C to the lowest (warmest) setting that still keeps it on continuously is what you should do if you want to get your house bone dry.

Here are some references that discuss and help explain the issues caused by oversized units:

  • The US codes limit 'bumping' condensors to one size larger, for several reasons including this one. Others are energy efficiency and power plant load smoothing (toggling the system on and off too frequently) and carbon emissions (less volatile condensate out in the world).
    – brichins
    Commented Jan 8, 2019 at 22:11
  • This. If your AC unit is larger capacity than roughly 1-1.5 ton per thousand square feet, your home will stay humid. If that's the case you should try to make the fools who pressured you to get an inappropriately-sized system (to make more money off it) replace it with a suitably-sized one. Commented Jan 9, 2019 at 3:26
  • Most A/C companies in tropical climates worth their salt should already know how to size a unit properly. I live in Florida, and the A/C in my current/previous homes (whether existing or new installs) have always been sized so that my unit is running fairly often during the summer. Commented Jan 9, 2019 at 14:45
  • @OgrePsalm33 The last link in the answer discusses a situation in Florida with an oversized unit. People don't always do what they should.
    – JimmyJames
    Commented Jan 9, 2019 at 14:48
  • @JimmyJames Yeah...I suppose I should have put emphasis on should Commented Jan 14, 2019 at 22:38

It can, but not for the reasons you think

Mold needs two key components to go wild like that

  1. Moisture
  2. A cool atmosphere

Your A/C can encourage mold growth by not running. If you leave your HVAC unit on cool and the surrounding air gets cool (i.e. a winter cold front blows through), the moisture in your house will condense and feed the mold. Then, when it warms up, the A/C starts up again, but now it's blowing the spores around. The humidity drop will help deter more growth, but what's there has to be dealt with to make the space habitable again.

We live next to a rainforest, so I would've thought that opening the unit would invite spores and damp air in rather than prevent mold?

The mold spores are already inside, A/C or not. Don't think of it as "letting more in". The issue with an open window next to a rain forest is humidity.

  • Why would the air being cold outside cause moisture to condense inside?
    – JimmyJames
    Commented Jan 8, 2019 at 15:22
  • @JimmyJames This happened to my mother's house a few years ago. We moved her out but she still had some things inside. We left the A/C on. We then had a serious cold snap but no heater to keep the temperature level. We then discovered the mold had taken over when all that moisture condensed on the furniture.
    – Machavity
    Commented Jan 8, 2019 at 15:26
  • Sside from having an over-sized unit (see my answer) and assuming the water from the condenser is removed from the house (e.g. sump pump) and doesn't just dump in on the floor or something running the A/C is going to reduce the moisture in the home, not increase it.
    – JimmyJames
    Commented Jan 8, 2019 at 15:31
  • That's my point, tho. The HVAC will NOT run if it's set to cool and the house is already cool
    – Machavity
    Commented Jan 8, 2019 at 15:33
  • 4
    Running -> less moisture , Not-running -> no effect. Not seeing where the A/C increases moisture.
    – JimmyJames
    Commented Jan 8, 2019 at 15:43

My air conditioner specifically has a 'dry' setting, which removes moisture from the air better than the regular setting (although air conditioners in general do dehumidify to some extent; you'll notice they drop water outside). Reducing humidity can reduce mold, as others have said—however, humidity is probably not the most important thing you should worry about. I would be more concerned about a lack of air circulation.

Old or dirty air conditioners can introduce dust and things that might be unpleasant for some people to breathe. The particles could potentially include mold spores. However, the dehumidification and the air-flow will probably prevent those microbes from growing while the air conditioner is going (although after it stops, it could potentially grow faster than before the air conditioner was used—but I wouldn't count on it).

Although mold does appreciate warm temperatures for growth, it is my experience that heat combined with air circulation does cause a drier environment than cold combined with air circulation (notwithstanding air conditioners do dehumidify). I use a warm room and a box fan to dry seeds quickly, without mold (for gardening), and I turn the air conditioner on dry for a bit when it gets too humid (although with the fan going, I haven't found extra mold during humid times, but I do feel better having drier air, and not just dry surfaces). You'll notice that food driers blow warm air (the same principles work for drying other stuff). In my experience, mold doesn't grow easily if there is air circulation in the room (without regard to temperature), however, but things dry faster if it's hot.

I would personally recommend having fans going (like box fans; I use a box fan to keep black mold from growing on the walls in my room; I have one going almost all of the time, and I haven't had black mold there since; it's been years). Air circulation helps to keep things dry. I also use an ionizer to help prevent mold (supposedly, the negative ions can kill microbes in the air), and I personally think it works based on my use of it (but I haven't done a lot of experiments there, and I don't mean to be making any claims there; I had already had success with the fans before getting the ionizer).

Disclaimer: I live in a dry climate—so my humid times may not be nearly as humid as yours.

I should probably point out that for some reason I don't understand, central air doesn't seem to prevent mold as well as a box fan (in fact, I can see how someone might think that would promote mold, although I don't have any data to support that notion beyond how food got moldy pretty easily in the house with central air). Perhaps central air failed me with regard to mold because it spread microbes from every room in the house to every other room (just a guess), and the vents were old. I don't know that central air helps to dehumidify much, but regular air conditioners you put in the window can do so quite a bit.

While box fans may not exactly dehumidify, they do help to keep the surfaces in the room dry.


Lol, I didn't read the other answers because I'm sure they say run the A/C as much as possible with the windows tightly closed. I don't know who in their right mind would suggest turning of the air conditioner and using a dehumidifier instead. I would say max the air conditioner, also run a dehumidifier, large propeller fans and portable heaters. Heat lowers relative humidity given a volume of air, so the air becomes a better moisture sink. Propeller fans move alot of the air, moving the moist air away from the wet surfaces allowing dryer air to contact the wet surfaces. A dehumidifier is going to dehumidify and add heat to aid in moisture absorption. And an air conditioner is (especially in a tropical climate) a gigantic moisture sucking device that will literally pull pounds if water from the air every hour. It's a shame that the landlord and the "mold expert" are very uninformed about enthalpy. After all of this you should check the evaporator coil for any live mold and consider a coil mounted UV light. Being in a tropical climate your evaporator coil likely never has a chance to dry. And since mold spores are probably very present due to the rain forest, the evap is a breeding ground for mold and bacteria. Here in Canada we have short, hot, damp summers, with cold and dry winters. UV lights are not common but I can now understand why they exist.

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