My understanding of the dehumidify mode for air conditioning systems is that they slow the blower and maintain the coil at the lowest temperature possible without freezing it. This biases the system to dehumidify with minimal cooling, but cool significantly it will. My perception is that my mini splits go into that mode and I think it likely that many if not all all variable speed systems go into a mode like that as they approach set point. That means that they go in into dehumidify mode whether you set it or not, in other words.

I am very familiar with your problem. I think that if you want comfort in your basement, you will want both. Portable dehumidifiers are generally noisy and inefficient, but you probably won't want to invest in an efficient ducted model. They are very expensive. The mini split will keep the temperature in the comfort zone counteracting the waste heat from the dehumidifier and at the same time, take away some of the moisture removal load,

Is part of the basement unfinished like a walled-off utility area, equipment room or laundry room? If so, you can make your own plenum and duct system for a portable unit locating it away from your finished area making for quiet. See the link below and modify it for use without a central AHU adding a small blower. If you site the equipment cleverly, you might share the condensate removal means with the mini split.

Alternatively, you might be able to use a minimally ducted mini split and duct the dehumidifier into it. I don't know if you can find that kind of mini split AHU small enough though.

Please be careful about how you handle the insulation with respect to moisture migrating in from the soil. You really have to know what you are doing if you don't want a serious mold problem.

Thank you for your input. We have decided, after this insane heat wave to go at least with a mini-split AC unit that has a dry mode, and work from there. The Dry mode removes about 1.5 pints per hour. We'll see how it goes and work from there, augmenting with a floor dehumidifier that we empty. We only need it a few months of the year. A smaller floor unit will probably kick out less heat and the jumbo one we have now. A newer floor unit, now that we are wiser will be able to let us set the humidity level we are aiming for.

Our installer is very experienced and comes highly recommended. Our contractor had some great questions about the where the water from the split unit would go, and how to get it as far from the foundation as possible.

Thanks again.

Thanks for reporting back. Please let us know how the installation goes and how well your plan works. My parents used to have a dehumidifier that had a sort of temporary off button. It turned itself back on after an hour or so after they pushed the button to silence it when the noise was driving them nuts.

You could do the same thing with a plug in timer with removable pins. Just pull out the stop pin, set the dial to a time past "now" and turn off the power at the timer. You can order them from Intermatic (TN311) directly, but they are hard to find in stores these days in favor of switches with captive pins. You can order spare pins at the same time :-)

It looks like I forgot to post this link earlier. Look at the FILTER-VENT (FV) WITH DUCTED DEHUMIDIFIER and imagine it with a dehumidifier with its own dedicated duct to unfinished space in your basement and a blower. A simple cabinet will do and a small blower is needed. A filter would be good. It could work and make the noise go away. If your water removal system for the mini split is close by, you might be able to double up on it.

Conditioning Air in the Humid South—Creating Comfort and Controlling Cost Research Report - 0214 November-2002 Building Science Corporation

Heat generation will be proportional to the operating time and moisture removal of the dehumidifier, small or big, pint for pint the heat will be the same. It will be on less as long as the mini split is running and now you have a way to counter the heat. I think that you will be happy. That moisture removal figure is highly dependent on the starting humidity and the temperature. It is hard to calculate if you will use less electricity by running the dehumidifier more and the mini split in regular cooling mode, or run the mini split in dehumidify mode. I'll bet you end up the same either way because the mini split, I believe, ends up in dehumidify mode as it approaches the set point anyway. Just be sure it is sized right and not too big.

@Lina - sounds like a reasonable approach, but you may want to put off buying an additional dehumidifier. The mini-split should probably handle the moisture, 1.5 pints an hour is a fair bit. Since you already have a larger dehumidifier, just keep that in reserve for times when extra is needed. Running it once in a while for a few hours or a day (with timer if you want) shouldn't throw things off down there too much or be much trouble. At least wait until you see how the minisplit works.

Armoured, Running a mini split alone is a NE inuninsulated basement with 50 degree soil temperature will surely over cool a basement unless there is a significant heat source. That heat source will be a typical inefficient portable dehumidifier unless it is a crowd of people.

Ionized: perhaps. At any rate, all I'm suggesting is that she not replace the existing dehumidifier until she sees how it works in her specific application. In addition, if the mini-split has a heat function, it may be more efficient to provide the extra heat using that (rather than from the waste heat of the stand-alone dehumidifier) - assuming the mini-split has auto modes to cover this (I believe mine does, but not certain).

Repeatedly cycling the mini split between the cooling mode (that is biased to dehumidification) and heat is not going to be an efficient dehumidification process. It will tend to re-evaporate the water from the coils right back into the room. I can summarize the situation like this. The basement needs dehumidification. If that could be done without adding waste heat to the space, it would work. If that could be done without adding cool to the space, it would work. The mini split does one, the dedicated dehumidifier does the other so you put them both in the same room and let them fight it out. The most efficient scheme to do this would be a system that does variable refrigerant flow so the mini split could switch between "rejecting" heat to the outdoors and inside. Commercial$$$ units do $this$ with at least two indoor units.

A highly efficient dehumidifier will also cost less to run (Aprilaire etc.), and produce less heat, but the investment might not pay off. It depends on how much it runs and how much it costs to keep the temperature where you want it. You could also look into rotating desiccant disk dehumidifiers, but that will certainly be an even higher upfront cost.

I did not have any problem with the suggestion that the new dehumidifier (smaller) could wait. That is wise for the frugal. My problem is that, over and over, marketers of mini splits imply that they act as dehumidifiers. People not familiar believe it. It really sucks for them so I tend to react strongly, no no no.

Mini split air conditioners don't act like dehumidifiers . Dehumidifiers work by cooling air and reheating it with a net heat injection due to mechanical inefficiency and quite a bit of heat it is. Mini splits dehumidify air by cooling it. There is no reheat. All the heat goes outdoors, waste heat and the latent heat from the condensing water. The room that needs no cooling gets cooler. They don't make as cool as in full cold mode, but there is a lot of air cooling going on.

P.S., the auto mode is going to toggle back and forth between heating and cooling. There are two more problems with dehumidifying like that. They are set up with enough differential between heating and cooling call temperatures such that there would not be much of either going on. Second, the cooling mode is not as good at dehumidifying as dehumidify mode. As my variable speed cooling system approaches set point it goes into a mode that is very much like or identical to dehumidify mode, the coil gets very cold and the blower gets very slow. When it is far from the set point, fan is fast and the coil is probably as cold as the system can make it at that blower speed. They cool first and dehumidify later. Old school single speed systems that i've had the opportunity to observe with instruments tend to dry first and cool later.