You are asking a complex question which can't be answered without knowing how much energy the AC and dehumidifier use, and the temperature and humidity you find comfortable. The best way to figure this out is to experiment.

The statement that an AC doesn't run optimally when a dehumidifier is running makes no sense. The dehumidifier does put out some heat, but I think it is negligible compared to the total cooling load of the house.

You may save some money by lowering the humidity to the 40-45% range and then raising the temperature setting on the thermostat. I think are going to have to raise it 2-3 degrees in order to see any savings.

Consider buying a good dehumidifier like the Sante Fe instead of the cheap ones from the big box stores. They are a lot more expensive, but they use less energy will have a longer life.

First optimize your house envelope eliminating air leaks the best you can. Make sure that your ducts are not leaking if they are outside the house envelope. (If they are outside, any leaks turn your HVAC system into a power vent.) Leaks let in humid outside air. Duct leaks will suck in a lot of humid outside air. Variable speed cooling systems, (compressor and blower) offer better humidity reduction. After you do all this, minimal additional dehumidification will be needed so it won't make any difference how efficient your dehumidifier is.

Second - the A/C evaporator is a dehumidifier that cools as it dehumidifies. This can result in a cool and clammy environment under low heat load, humid conditions. This can be remedied by reheating the cooled air. This is exactly what a dedicated dehumidifier does.

@ionized: So how many people run both? 0.0001%? If you want to save $0.25 a month, follow Ionized advice. Pure fiction.

OP, if you want to run a dehumidifier to improve comfort and health, by all means, go ahead. There is nothing wrong with that. I expect that you want one during shoulder seasons when little heat or cooling is needed in the house. A high end dehumidifier will make less heat and use less electricity than a typical store-bought brand as Mike suggested. They make less noise too and can be made "central". The EPA rates them for efficiency.

On a purely theoretical basis, a system that has two condensing coils, one outside as normal, and a second in your air handler would be the the most energy efficient way to do this. When more cooling is needed, condensing of hot refrigerant gas would happen outside as normal. When only drying is needed, condensing would happen in your air handler in the coldest air flow after the evaporator coil just as it does in a stand-alone dehumidifier. You would not have the waste heat and annoying noise associated with running a portable dehumidifier. These would be custom systems rather than off the shelf, and you'd never come close to recovering the costs in a home. This is how it is done every day in commercial systems.

With a well-designed cooling system and a well-sealed home, you should not need a dehumidifier in hot-humid weather. If you have an oversized system or the blower capacity is mismatched to the evaporator, or you have a very leaky house, you might have excess humidity. Dry indoor air in the winter is also indicative of a leaky house. If you can tighten up your house envelope, you will be more comfortable. In my humid climate, there will always be a desire for a dehumidifier for some days out of the year no matter how tight my house envelope is.

you might find this case study enlightening:

Conditioning Air in the

Humid South—Creating

Comfort and Controlling

Cost

Research Report - 0214

November-2002

Building Science Corporation

The primary goal of a dehumidifer is to remove the wetness from the air, collect it, and turn it into water that can be discarded. The other thing they do is heat the house. The wasted electrical energy is turned into heat that is vented inside the residence.

In the winter season, taking excess wetness from the air and putting heat into the residence are both great.

In the hot weather, the heat created in your house is all lost. When the air conditioner has to work harder to kick the heat out of the house, the waste is doubled.

My basement dehu runs my bills up far more than my two half-ton window units.

The problem with higher end basement dehumidifiers is repairs. So you have your 150lb expensive energy efficient dehumidifier in your basement. When it breaks, the repair is a truck shipment back to the factory.

I was willing to buy a more expensive, energy efficient basement dehumidifier. But the repair scenario is unacceptable.

After a lot of research and thinking, I use the typical $250 frigidaire.

Whole house dehumidification doesn't make sense for 95% of the U.S. A properly sized two speed central AC compressors are the better choice for most people wanting good dehumidification without making the house too cool.

Whole house dehumidifiers in the duct system are more for Seattle, not Virginia or Illinois. Most people in zone 5 midwest don't even pay for two speed central AC. Just a one speed AC, which a dehumidifier in the basement when needed.

Even with a 3-5 year life, the inexpensive basement dehumidifier is probably the way to go for most people.

Don, there is a lot of sense to what you say. I'll add that even with a cooling system that is great at dehumidification, Mitsubishi mini splits, I still find a lot of seasonal use for a portable dehumidifier. You might find this study interesting. You might think that that the 2-stage compressor and ECM blower could provide adequate humidity control, but it really did not.

Conditioning Air in the

Humid South—Creating

Comfort and Controlling

Cost

Research Report - 0214

November-2002

Building Science Corporation

If you have central air conditioning, I'm not sure why you'd need a dehumidifier. I live in the Southeast (and in a 22 year-old home that leaks air like a sieve!), the most oppressively humid corner of America, and the addition of dehumidifiers to my HVAC systems has never been suggested by either the company that installed them or the folks who service them—and they're the ones with the most to gain by suggesting them!

Many people with AC need a basement dehumidifier in the summer, especially in older houses as well as areas with a high water table. But above ground use of a dehumidifier in hot humid weather only makes sense when the main HVAC is poorly designed.

If additional dehumidification is needed, the right tool is a smaller window AC. This is simply a dehumidifier that puts the hot air produced where it belongs - outside. When my basement dehumidifier runs I can watch the air temp rise on my home automation system. Increased temps is good in my cool too humid basement in summer. Raising air temps is obviously not desirable upstairs in summer.

Inability to dehumidify with AC is the result of too much AC cooling too fast. This is why two speed compressors central AC compressors are used in hot and very humid areas. It has been understood for years that oversized ACs are poor at dehumidification.

Ducted dehumidifiers installed as part of an expensive HVAC design are used in damp winter weather. AFAIK, summer dehumidification is always done with only two speed AC compressors, variable blower speeds, and controls that understand inside and outside conditions.

Anyone in Atlanta with high end HVAC have a ducted dehumidifier installed for summer use?

Some window AC units are better than others for dehumidification. They need to have a setting that turns the blower off with the compressor to be most effective.

My dehumidifier runs in times when little or no heating or cooling is needed, but the outdoor humidity is high. The Building Science paper provides a good description of the problem, tests some solutions, and gives results and costs.

Preventing fan run with a wet coil is also an advantage of controlling a portable dehumidifier externally: The fan is cut off with the compressor. Normally the fan run across a wet coil with the compressor off re-evaporates the water on the coil.

Reportably this is also a problem with short whole house AC cycles and dehumidification. Several times an hour the fan dries the wet coil with house air.

AC's with variable speed compressors, like newer mini splits, can to be set up and operated with long run times. If the compressor seldom shuts off there little problem with a wet coil.

I'm sure a ducted dehumidifier does run when there is little AC load. The question is, does it matter? Say the house is closed up overnight and the outside temp cools off. The humidity in the house rises, so the dehumidifier runs as the house air temp is still good. In the morning, the system is shut off and the windows opened.

Does it matter that humidity increased overnight? In a museum, yes. But in most houses in most places the is just spending electricity to hit a lower humidity number for a few hours before dawn. If I had a mold problem above ground in a properly ventilated house I might want that late night dehumidification. But most people adding central dehumidification for summer use are simply making up for insufficient HVAC design. A few hours per day above 50% humidity probably doesn't mean anything.

Most people don't want their house to have windows closed and HVAC on when there is little heating or cooling load. The "perfect environment" pitch from the HVAC industry is simply to sell more stuff. The more time people spend in a tightly controlled environment, the less tolerant they become of the natural environment.

Where I live, the RH is upwards of 80%, probably, on most nights. It is relatively inexpensive to reject heat at night time temperatures. Opening windows would put a huge load on the cooling system first thing in the AM to remove all the "latent heat". Running the dehumidifier keeps it comfortable and probably is not all that costly. The Building Science case study does a good job of defining the problem and compares some solutions. Their real-world data says that the variable speed systems don't cut it.