Humidifiers and Dehumidifiers - Confused about Dehumidifier and A/C in Basement

Not sure if this is the right forum (maybe I will also post it on the A/C or ventilation page?), but here goes:

I'm confused about controlling humidity in my cool, damp basement in the summer months. I have central air and operate a portable dehumidifier. There are three supply vents at the ceiling and one 6" return air duct almost to the floor for return air. Floor is concrete. Exterior walls are insulated, vapour barriered and drywalled.

Should I leave the supply ducts open or close them during the hot hot, muggy days? Would adding more return vents in basement (easily done, as basement is unfinished) help with humidity?

Also, I have a fresh air intake duct that is screwed to the side of my natural gas furnace. This obviously lets humid air get into the basement. I've thought about blocking it in the summer when furnace does not fire, but maybe this isn't a good idea?

Any and all help appreciated. Thanks.

I've always suggested to people to leave at least one vent open in the summer months. If your system runs pretty cool, more than one vent may make the basement too cool.

I've ways turn up the control on the dehumidifier a little bit when I ran the A/C.. It can take over of the humidty level gets too high.

FYI, if your basement is too cool, the coil may freeze on you on the dehumidifier

For the fresh air intake do you mean it's being pulled into the home on the return, or is it just going into the furnace room? If it's tied into the return, I would remove it, and just have it go into the room itself. That's what I did with mine.

Thanks and sorry if I wasn't clear about the fresh air intake. It is just a 6" duct that is screwed to the outside of my furnace, several inches off the ground. It is not connected to the return, but simply goes to the outside of the home. My understanding is that it delivers fresh outside combustion air to the furnace. Of course, it's also a direct path for warm humid air in the summer. I wondered if I could plug it up in the summer, but not sure if that is safe (I also have a Power Vented natural gas hot water tank close by)?

Any thoughts about what effect adding a couple of cold-air returns in the basement would do for humidity?

I wouldn't suggest closeing off the pipe unless your house is not tight.

if the basment a pretty good sized area or has a few rooms like bedroom.. wouldn't hurt to have more return there.. Itl'll help in the winter months as well.