You have a lot of moisture in the basement! Do have any water seepage through the basement walls?

What is the moisture level on the first floor? Does your AC short cycle?

My recommendation would be to get a dehumidifier. The 70 pint model from Sam's club is not going to be able to lower the humidity from 79% to 50-55%% for a basement of your size. You would need at least two of these, or buy a Santa Fe dehumdifier for a lot more money.

Installing a portable AC will help, but not as well as a dehumidifier. You will also be lowering the temperature into the 60s. It will be both cold, clammy, and not comfortable. If the cold temperature doesn't bother you, then have you tried installing a vent and return in the basement? One small vent and return should not affect the performance of the HVAC system. It will also be a benefit during the winter months.

Absolutely no seepage, basement spotless, almost no spiders. Horrendous rains here, 15 inches in 2 wks, with house-eating mudslides and roads washed out, but not a drop in our walk-out basement with no sump pump.
Upstairs is 70%, going down to 60% if it gets up to 90 out and the a/c runs a lot. Central a/c doesn't run much when temp outside only gets to low 80s: dew point is 65, so pretty humid.
I'm concerned that including the 69 degree basement in conditioned space would mean even shorter a/c cycles, with higher humidity throughout the house. A/C cycle while writing this lasted all of 6.5 minutes.
I hate heat. I could easily get to love a 60 degree basement in the summer!
DeLonghi a/c would function same as dehumidifier if allowed to exhaust into the basement. (This isn't the model that sprays condensate water on the condenser!) And the heat from this or any dehumidifier would also lower the RH.

This post was edited by jrh0 on Wed, Jul 17, 13 at 13:24

Why would adding the basement shorten the AC cycle? If anything it would be longer. You would be stealing some of the cold air from the first floor which is where I presume is the thermostat. You must have plenty of extra cooling capacity since the AC only runs for 6.5 minutes.

I don't think adding the DeLonghi is going to have much affect on the humidity in the basement. Perhaps others who have tried this can comment.

Adding a few hundreds watts of heat to a basement as big as yours will have little affect on the temperature.

Once you drop the humidity to 50% you will appreciate the 69 degree temperature. At 60 degrees and 50% humidity it will feel like a meat locker.

I guess adding the basement wouldn't shorten the cycle, but it would lengthen the time between cycles. A/C is set at 75, and basement is 69. Blending bsmt air with living space upstairs would cool upstairs even without any a/c on, if I didn't mind the extra humidity and radon.

The only difference, as I understand it, between an air conditioner and a dehumidifier is that the heat from the a/c condenser can be separately vented if desired. An a/c's dehumidification should equal a dehumidifier's, just as an a/c cools as well as a heat pump - all the same machine, except how the plumbing and venting is arranged. The only other considerations are that if it is vented, the a/c will cool the bsmt, as well as drawing in moist outside air, both increasing RH. In other words, the dehumidifier (or a/c in unvented dehumidifier mode) achieves part of its dehumidification by heating the space, or at least by not cooling it.

I think if the a/c unit can operate at temps in the 60s, it is preferred, as it has the flexibility of running in cooling mode (vented) or dehumidifier mode (unvented). I'm also thinking it might be more efficient, as EER is reported (even tho not great), while there's not much reason for dehumidifier mfr to make unit efficient.

The efficiency rating for dehumdifiers is pints/KWH. You are correct in that dehumdifier manufacturers are forced to post their efficiency ratings.

Below is a link to the Sante Fe dehumidifiers. These are more efficient than anything you can buy in a big box store, but they are significantly more expensive. These units can operate down to 60 degrees.

Keep us posted on what you decide to do and how well it works.

Here is a link that might be useful: Sante Fe products

although it doesn't have an outdoor makeup air ability.

Those single-hose models are horribly inefficient. Venting a cool, damp basement to the outside when the outside humidity is high is a bad idea as well. The outside air may be below the dew point in the basement increasing humidity further.

Dehumidifiers aren't rated for efficiency....

The DOE rates dehumidifiers, but they fall into two groups, normal and high efficiency. There is a very discontinuous distribution.

You should consider dual hose as it is more powerful than the single hose portable air conditioner.

Here is a link that might be useful: Check here

When my basement heat pump and oil furnace were installed, I added a return in my basement. This served to lower the humidity somewhat in my basement.

This may be relatively easy to have done added. YMMV

This post was edited by saltidawg on Wed, Sep 25, 13 at 11:40

Marc Well. Who would have thought that you simply plagiarized someone else's post?

Plagiarized Stuff by Marc Well

So in one sentence, the distinction between dehumidifier and cooling system is that the dehumidifier decrease the wetness level by drying the air creating it hotter and cooling system equally removes wetness by cooling the air. In energy use dehumidifier vs cooling system, the talk isn't thus wide as a result of there square measure many varieties of air conditioners and dehumidifiers. shift to electrical converter air-con units would be an honest various as a result of it'll abate your bill to up to seventieth. Dehumidifier vs cooling system Power Consumption. The dehumidifier power consumption vs cooling system is lower on the dehumidifier aspect. Dehumidifier air-con permits the wet air to tolerate cool coils and evaporate and returns it back to the area and also the result's cooler air. wet significant air causes you to feel the warmth additional on sweltry days and in these cases will your ac act sort of a dehumidifier and obtain obviate all the wet air? the sort of air-con you have got will get you the comfort and clean air you want. Dehumidifier and cooling system jazz band are you able to get cleaner air by reducing mould and mildew within the air and cooling it to your comfort.

Portable air conditioner-top