Mold growth in homes is a serious problem costing homeowners millions of dollars in repairs as well as serious health issues.  There are numerous methods proposed to mitigate these problems, most of which involve expensive and toxic cleaning services.  The solution, however, is simple and cheap -- but largely unknown because it requires an understanding of psychrometrics and how air conditioning systems work.


As soon as indoor humidity gets a bit too high, mold will start to grow.  As a general rule, you don't want the relative humidity inside your house to exceed 60% -- at least, not for very long.

Prior to the energy crisis of the 1970's, the standard air conditioner would do a pretty good job of keeping indoor humidity within acceptable limits.  It helps to understand how an air conditioner accomplishes that feat, though.  Inside the "air handler" -- the box inside the house that has a blower that moves air into the living space -- there is a "coil" (engineer-speak for an item that looks kinda like a car radiator) that is cooled by the refrigerant flowing through the tubes inside it.  Under arid conditions, the air blowing over this coil is simply cooled, and you can measure how much it's cooled by the change in temperature between inlet and outlet.  Under humid conditions, though, this coil starts getting condensation on it the same way you get condensation on a glass of iced tea.  The cold temperature causes the moisture in the air to settle on the surface.  When enough of this moisture condenses to get the entire coil wet, it starts running off in streams.  The coil has a condensate pan or trough that catches all this water and drains it somewhere outside your living space.  Hence, the air conditioner is not only cooling your living space, it is drying it as well.

Things started to go wrong when the energy crisis hit in 1973 and President Carter and everyone else began to be concerned about how much energy we consume.  The state of Florida passed legislation requiring that air-conditioning units meet minimum standards for EER, or Energy Efficiency Ratio, the amount of cooling per the amount of electricity used.  The problem was that the EER was never intended to be used as a legal requirement, only as a guideline.  It's too easy to "cheat" to attain high EER numbers.  One very popular method of increasing EER numbers was to increase the airflow through the air handler.  By flowing more air, the system can provide the same total amount of cooling without getting the air as cold -- and getting it cold requires more energy than just getting it cool.  The problem, of course, is that when you don't get the air cold, you don't get as much moisture out of it via condensation -- and the interior spaces start to get humid.  And mold starts to thrive.  That's why mold problems began to appear after the energy crisis; it's about the air conditioning systems themselves.  It has nothing to do with the fact that houses are sealed better, which seems to be a popular myth; in fact, houses being sealed better actually helps prevent mold.

Here in northern Florida, the situation is especially bad in spring and fall.  During the winter, it's cool enough to have the heat on at least during nighttime hours, and heated air is dry.  During the summer, it's so hot that the A/C runs many hours of the day, and when running that hard it'll usually do a decent job keeping the humidity in check.  But the spring and fall are known for moderate temperatures combined with high humidity.  If the A/C runs at all, it only runs long enough to get the temperature down a couple of degrees and then shuts off.  It doesn't run long enough to get the humidity under control, and mold starts to take over -- and it may be a couple of months before anything puts a halt to its growth.

The moderate spring and fall temperatures also result in the A/C running only in short bursts.  This also inhibits the system's capability to remove moisture from the living space.  When the A/C starts up, it starts cooling the air immediately -- but it takes a while for enough condensation to accumulate to start running off the coil and out the drain.  If it shuts off quickly, very little water goes out the drain, and the condensation that remains on the coil just sits there until it gradually evaporates -- back into the living space!  The energy expended to condense that moisture is utterly wasted, and the mold is happy about it.


Virtually all air conditioners and air handlers have blower motors with multiple speeds, typically at least three and sometimes more.  And virtually all "central" air conditioning systems have those blower motors hard-wired to run on the highest speed whenever the unit is running.  This results in the maximum cooling capacity, which means it does the best job of getting the living space cool in a hurry when you come home on a hot summer day and turn it on.

That's not the problem in the spring and fall, though.  What you'd like to do is run that blower on a lower speed during moderate weather.  By slowing the blower down, you move less air over the coil -- which means the coil gets that air colder.  That, in turn, increases the amount of condensation generated and drained, leaving the living space drier.  And because the system won't cool the living space as quickly, it runs longer each cycle, which again helps it remove condensation more effectively.

As a fringe benefit, the blower running on a slower speed is quieter.

If the same blower is used for heating during the winter, by all means leave the blower in the slower speed through the winter!  This will result in less airflow for a less "drafty" feel, and it also results in the air coming through the vents being warmer -- the same amount of heat applied to a smaller amount of air.

In fact, you might find that you can leave the blower in this slower mode year-round.  The only question is whether or not the system will have enough capacity to keep the space cool during the hottest days of the year.  Those peak days are surprisingly rare and often only last a couple of hours per day.  Once you set the blower to a lower speed, you might never find the need to bump it back up.
  Even if it doesn't succeed in holding the desired temperature on those hottest days, it'll probably only miss by a couple of degrees, and the indoor air will be dry enough you'll feel comfortable anyway.  If your primary concern is mold rather than strictly holding room temperature on the hottest days, you definitely don't need to bump it back up.  If the air conditioner is a bit oversized for the job, you definitely won't want to bump it back up; just leave it on the lower speed permanently.

Regarding energy consumption:  There's a bit of a mixed blessing here.  First off, running the blower on a lower speed will save a lot of energy running that blower.  Cutting it in half is entirely plausible.  The system also runs more efficiently when it runs longer cycles than when cycling on and off all the time.  However, it'll also run longer, as will the compressor and outdoor fan, and these are the lion's share of the energy use of an air conditioning system.  The fact is you'll be removing more moisture from the air, and it takes energy to remove moisture from air, so you're going to be using more energy there.  Between the increased dehumidification and the reduced blower power consumption, the difference in your electric bill may not be noticeable one way or the other.  The next time you have to throw away a belt or a pair of shoes because they got moldy, you'll decide it'd be worth it.  The next time you have to call a contractor to come tear out some walls due to mold damage, you'll really decide it's worth it!  Don't even get me started on your children coughing and wheezing with all that mold in the house.

There's also a comfort factor.  Getting the humidity out of the air makes it more comfortable inside the house.  It doesn't feel so "clammy".  As a result, you may find yourself setting the thermostat a degree or two higher than you did before, saving some energy there.  You also may have less need to run ceiling fans or other fans to stay comfortable.  And if your clothes dryer is inside the house, it will dry your clothes in less time, saving some energy there.


Some modern air conditioning systems have elaborate blower controls that gradually speed up the blower upon startup and gradually wind it back down when it shuts off.  I am not familiar enough with how these systems work to speak on how to adjust the running speed -- but I'll bet it's possible.  It might be worth a look through the specifications and schematics.

With the older style units that simply cycle on and off in response to a thermostat on the wall, the implementation is relatively simple -- but if you're not savvy enough to do such things yourself, you'll probably need to find someone who is.  The open-your-checkbook method:  Call up an air conditioning repair service and tell them "Please adjust the blower in my air conditioning system to operate at the next slower speed."  It won't take them long, but they'll have to charge you for making the trip out.

To do this yourself, the first thing you'll need is the schematics of your air handler.  That's not as challenging as you might think; a set of schematics is usually hidden inside the unit itself.  You'll have to remove some screws and take a panel off, and it needs to be the panel that leads to the electrical connections.  Obviously, turn off the power at the breaker before opening any panels.

Study the schematics and locate the blower motor on them.  You'll see the wires from the motor, typically clearly labelled H, M, and L or something similar.  You also need to find the actual wires themselves, which should be right where you're looking behind that panel.  You'll find the M and L wires taped off and not connected to anything, while the H wire is plugged into something.  So, for the simplest fix, just unplug the H wire, wrap it up with tape, and unwrap the M wire and plug that one in instead.  Put the cover back on the unit and go, you're done.

I don't recommend going all the way to L speed.  For one thing, it's usually not necessary.  For another, you can actually cause problems with an air conditioner using too slow a blower speed.  If you happen to have the airflow specifications on your system, here are some guidelines:  The typical air handler is designed to move 400 CFM (cubic feet per minute) of air for every ton (12,000 BtuH) of cooling capacity.  Some of the energy-saver models flow more air than that.  To inhibit mold growth, you want to flow less air than that.  Under no circumstances should you ever set the system to flow less than 250 CFM per ton; this is the minimum airflow typically recommended by the manufacturer.  But 250 CFM is a lot lower than 400 CFM.  By dropping to the next lower speed than H, you'll never risk a problem.


Now, swapping those wires around was easy, but you probably don't really want to have to do that twice a year -- down to M in fall, back up to H the following summer.  So, with a bit more effort, you can install a SPDT switch somewhere -- perhaps right on the side of the air handler, or anyplace inside the closet with the air handler, or perhaps on the wall next to the thermostat, wherever is convenient for you.  Just run leads from the H and M wires from the blower to the contacts on the switch and a common wire back to the air handler.  You can then go from H speed to M speed and back with the flip of a switch.  Label the switch something like "Summer Only" and "Fall, Winter and Spring".  Now you have a switch that you flip twice a year.  Or, you can label it as "Hot Weather Only" and switch it to H only on scorching days.  And yes, it should be OK to flip the switch while the system is running.


There is such a thing as a "2-Stage Cooling Thermostat".  It's a single thermostat that is designed to control two cooling systems; it brings on one first, and if that doesn't do the trick and the temperature rises a couple more degrees, it brings on the second.  Honestly, there are not many applications that use such thermostats, so finding one won't be easy; you'd probably have to contact Honeywell or Johnson or some such, and perhaps order one online somewhere and pay entirely too much money for it.  But once you have one, you can wire it up to control those fan speeds automatically.  When the system first calls for cooling, it brings on the blower on M speed.  If the temperature continues to rise a couple of degrees, the thermostat sends another signal that energizes a SPDT relay (you'll have to buy that, too) that kicks the blower up to H speed.  Now you've got a system you can forget about, no need to manually flip any switches or fiddle with wires, ever.  It'll run on M speed virtually all the time, but it'll kick itself up to H speed as needed.


Running the air handler on a lower speed is so effective that it would end all concerns about household mold.  Period.  So why isn't it commonly done?  Frankly, I have no idea.  One wouldn't expect legislators to understand such technical issues, so they couldn't be expected to pass legislation to require it even though they arguably should require it in humid climates to prevent mold issues becoming a health concern.  HVAC engineers (such as myself) and air conditioner manufacturers understand psychrometrics well enough to understand this proposal, but evidently it hasn't even occurred to many of them.  In a better world, all thermostats would automatically run air handler blowers at reduced speeds except during peak loads.


The technical term for the temperature an air-conditioning system's coil operates at, and hence how much moisture it removes from the air, is the "Apparatus Dew Point", or ADP.  This term is not commonly used even by HVAC engineers, even though they readily understand the concept.
  Furthermore, the ADP needed to avoid mold problems is 53°F.  Exactly.  You simply wouldn't believe how precise this requirement is.  54°F is flirting with trouble.  By the time the ADP reaches 56°F, you've got a serious mold problem on your hands.

It should be noted that ADP is not the same thing as the temperature of the air coming out of the vents, although it is closely related.  When the air flows through the coil, some percentage -- typically about 5% -- manages to slip through without touching the coil and hence doesn't get cooled or lose its moisture.  So, you mathematically combine 95% of the air being cooled to the ADP with 5% of the air still as warm and moist as the incoming air and you'll get approximately the temperature of the air coming through the vents.   For a system operating at an ADP of 53°F, the temperature of the air coming through the vents will be about 55°F, maybe 56°F.  This provides a handy way to check to see if your system is sufficiently dehumidifying your space:  With the system running normally, measure the temperature of the air coming out of the vents.  If it's any higher than 56°F, you need to slow down the blower speed.  If it's colder than 54°F or so, you could actually get away with increasing the blower speed -- unless you like it really dry indoors for some reason.

The ADP term could be used to apply legislation to the mold issue.  For example, the Florida law mandating energy efficiency could easily be modified to require a minimum EER at an ADP of 53°F.  This would eliminate the option of simply moving more air to attain that desired EER.  As things stand, periodically the legislature simply increases the minimum EER requirements in the name of energy conservation, but they'd be better advised to leave those EER requirements unchanged while adding the ADP requirement of 53°F -- which would be a more serious challenge to the equipment manufacturers, requiring real efficiency improvements rather than just fiddling with the airflow.


It should go without saying that all of the above presumes the air conditioning system is functioning normally.  Most notably, the condensate pan must be collecting the condensate and draining it outdoors.  If the drain is plugged up, the condensate just collects in the pan, and either evaporates back into the indoor space or -- much worse -- overflows onto the floor and soaks into the carpet and trim, causing immediate mold problems.  For most residential applications, the condensate drain is simply routed to a little pipe coming out of a wall outside the house somewhere.  When running, there should be cold water dripping from this pipe.  If there is not -- and you don't live in Arizona -- you need to be figuring out what's up.