Both AC units and air ducts make great places for mold to hang out.
With AC units, water leaks out of them and makes a nice place for mold. In these water pools mold grows, and then their spores are passed along through the ducts to every part of the house.
Your heating and cooling system also can suck air out of different parts of the house. This means that mold can be passed all over the place, every which way, by your ventilation system.
The HVAC unit that sits outside your home is perfect for mold, because there’s always water around it. Mold often grows in the unit’s water pools. The evaporator coil is also a major trouble spot.
How Can You Tell?
First, there are the regular mold symptoms which usually resemble sinus trouble; runny nose, stuffy nose, and sinus headache. If you can smell mold when you turn on or run the AC, you definitely have a mold problem there.
To make sure, you can always get a home testing kit from your hardware store. This might be a good idea anyway, and it can also tell you if the source of your mold troubles is your AC unit.
Take the testing gear and check each room. Check around all the vents. Also, check around the unit itself outside, especially where there is any standing water.
What Can You Do?
If you have mold in your air ducts, it doesn’t do much good to clean them. That’s not just my opinion; that’s what the EPA recommends. If you have mold in your ducts, it’s symptomatic of a mold problem elsewhere in the house, and until you address that, you won’t make any progress on the mold.
For the AC unit, you have to clean it on a regular basis to keep them mold free. Of course, you have to thoroughly clean up any mold that you find around the unit, or in the little pools of water around it.
In general, keep the fan mode switch on your AC on “auto” mode. If you put it on “on,” it will recirculate all that moisture that has collected on the evaporator coil through your house and air conditioning system. You don’t want to raise the humidity level of your house; that will cause mold problems all over.
If you are building a home or buying a new AC unit, look for one that has options to control the humidity. Some have variable speeds, or different modes like “moisture removal.” This will help greatly in controlling your house’s mold.
Another tip on picking an AC unit is that bigger machines usually increase humidity. For some reason, people generally choose oversized AC units for their homes. This is actually not a wise choice, because a bigger unit will create more moisture and circulate that moisture through your home.
If your heating and cooling system is infested with mold, you will probably have to call a professional and have the whole house tested and remediated.