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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.

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:

• https://www.cooltoday.com/blog/3-problems-caused-by-an-oversized-air-conditioner


• https://aristair.com/blog/hvac-humidity-control-5-reasons-your-ac-is-ineffective/


• https://airexpertsnj.com/knowledge/entry/air-conditioner-not-dehumidifying-well-a-new-jersey-tech-explains-why


• https://energyair.com/signs-ac-system-large/


• https://trustcompleteair.com/signs-your-air-conditioner-is-too-big-oversized/


• https://www.greenbuildingadvisor.com/article/coping-with-a-wrong-sized-ac-system

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.

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.