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.

Ventilating a basement can be counterproductive in warm seasons in humid conditions. The basement can be at a temp below the dew point and you will be importing moisture. A dehumidifier is about the only thing you can do in that case. No windows that open? A dehumidifier will make waste heat so it might get hot down there in the summer. You might need cooling too so maybe a window unit can be fitted to a non-opening window so that it can be removed in the cold season if you have one. Where do you live? (What is the climate like?) The need for humidifiers during the winter in temperate climates is greatly minimized if homes are well-sealed. Leaks remove the moisture, that is naturally-generated from living activities, to the outside and let dry outside air in. If you are moving to a high efficiency furnace or boiler from a low efficiency unit, you will have less outside air infiltration. That might be good news for the dry air upstairs, but might be bad for the basement. On the other hand, some basements need dehumidification in the summer and never in the winter. Dehumidifiers are of two types with very little variation within the two types. There are inexpensive portables on the one hand, and more expensive ducted units. The latter are less costly to run and less costly in the long run. Look at the Energy Star web site for efficiency figures. You should only need one if you leave the door open between the two basement rooms and maybe even if you don't. Get a couple of inexpensive hygrometers and monitor the humidity. There will be some good water heater info for you to read at the Energy Star web site too. Gas is cheap right now so as mike_home wrote, there is not much incentive to change it for a marginal improvement. (More efficient gas heaters cost a lot more and don't save that much fuel.) I'd seriously consider hiring a local energy rater to look over your house. They will be familiar with local construction and climate. They will know local energy efficiency programs that might save you some money with rebates or pay for part of the improvements. They can identify the low-hanging fruit for saving money and help you with your moisture problem. www.resnet.us

I think installing a condenser in your attic would be a big mistake. It will perform poorly, noisy, and difficult to service. Do any of the other townhouses have an outdoor condenser? Where is it located? A condenser can be installed above ground attached to the side of the building with a bracket. This is very typical for townhouses that have no adjoining land. You would install it at a height that you can walk under it, but not too high so it would be difficult to service. I would think a 1 or 1.5 size unit for your house in the Bay area would be sufficient. You need a load calculation to determine the proper size. It is difficult to say how much it will cost given it is a new installation in a more difficult set up. Stay away from the big box stores. Get estimates from independent dealers. You can save a little money when the temperatures are mild and rebates are offered.

Our basement bathroom has slight signs of mould and we are planning to replace the shower for this reason. In the bigger picture, we have efficient air conditioning and heat in the basement but does it make sense to add a dehumidifier? If so, advice on what kind would be appreciated. Thank you!