Do you live in an area with lots of humidity ? If so a central dehumidifier would be ideal especially with walk out basements.

The radiant slab won't really come into play during peak humidity unless you live in an area like the Pacific Northwest, where humidity as rain and cooler temps require heating. In the Mid-west and east, humidity is more a summertime issue. A whole house dehumidifier would require whole house ducting...a second system at odds with the radiant floor. It would give the opportunity to install air conditioning and that system might make the dehumidifier unnecessary. Hope this helps!

The house in SE Mass within earshot of the ocean. I am planning on using mini-splits for my A/C which I think will be seldom used. Except to dehumidify? Maybe I should expand the mini-split system to include the lower level.

Yes, do and then the whole house dehumidifier will work int he basement. Basements are generally cooler and moister than the upstairs anyway as cool air is heavier and wetter than hot air and collects naturally in the lower sections of a home.

The lower level is what most people would call a walk out basement. Outfitted with RFH and insulated exterior walls. The 'walk out' side faces East. Where would the cool, damp air be
coming from?

It falls down from above. One of the issues with air conditioning is that it uses the same system of delivery as the heating system...but the laws of thermodynamics apply and it takes more energy to cool a house than warm it because the system won't work ideally for both. Cold moist air falls and hot dry air rises...your stairs are the exchange area. Additionally, insulation only works if there is heat pushing the cold of the surrounding earth back into the middle of the insulating barrier. In summer the heat is off and the insulation is no longer a barrier...once it reaches the temperature of the dirt behind the foundation wall it is no longer an insulator..it is just as cool as the dirt and so is the room. When I was a kid and we didn't have central air, we used to grab sleeping bags and head for the basement on a hot summer night...

haha

I'm hoping you can help me. We purchased a whole house dehumidifier after moving in to our home 8 years ago. A few years ago, I started noticing that it felt clammy in our house so I called the company and asked if they could send someone out to look at it. They told me they don't service them because there is no need to. I asked, "How do I know if it is working." He told me, "You will have mold." My husband and I were baffled that you never have to take a look at it even though we knew something just was not right. A few weeks ago, with all the rain we have experienced, I noticed mold on my window. I called them and said, "Well, I think we have mold! Will you please come look at it." They did send someone and he opened the door, looked around and touched the compressor. He looked at me and said, "You have three leaks and the compressor is hot." He went on to say, "I will try to find you a refurbished on." I didn't say anything. I kept quiet because I knew he was up to something. Mind you, this is a company that does in home mold testing and restoration as well. He calls me the next day to tell me he can't find a refurbished on but he will build me two for $7,000! I already paid almost that amount for the one we have! He said that my basement must have been damper than usual and caused the dehumidifer to stop working!! Tonight, I went to the basement and there is water everywhere. I called him mad as all get out and he tried to blame it on other things. My husband is down there wiping it all up right now. Tomorrow I have to spend a ton of money for someone to come test everything. If it's major, I just don't know what we are going to do because there are other things pending we have to fix and this will send us right into bankruptcy. Now, if it's not that bad and we call restoration and pay them to clean it all up, I'm still going to need a whole house dehumidifier but actually, I never bought the idea it was for the whole house. We have three flours and I never felt like it worked upstairs. I have been scammed so many times, I do not know who to trust. Do you know a company that is dependable and trustworthy for a whole house dehumidifier? I've been looking at hospitals and they use company's but of course they don't do residences. I need somebody really reliable because this horrible company may just be putting us in the worst situation of our lives. Thanks..