Running cold water through a radiant system can cause condensation and dry rot in your floor joists. I have designed a few systems for slabs in very dry climates that work well, but you need to waste water or run a sprinkler to make them work, just doing it just for the domestic water will not create enough cooling.
  Solar floor heating is another system that some are promoting, I designed and installed a few of these several years ago and found them unfeasible. The problem is the amount of storage that it requires to store enough heat to heat through the night and time when solar is not available. So the back up ends up doing almost all the heating making the cost of the system to large for a return. I do recommend solar for domestic water heating, it is feasible.
  The bottom line is that there is no better investment in your future energy cost's than the thermal design and insulation values of your home. Spending the money that would have went into more expensive heating and cooling systems on more insulation, less glass or a advanced construction method such as SIP's will reduce the energy cost to such a low point that heating and cooling is never a problem.

Ducted cooling systems lose 30% to 45% in duct losses. Cooling ducts should never run in the attic.
The high velocity ducted systems are the least efficient.

   Ductless mini split air conditioners are relatively new to North America . Nevertheless, they have been in use throughout Europe , Africa , Australia , Middle and Far East for many years. They have a multitude of residential and commercial applications. The most common residential applications are use in multifamily housing or as retrofitted add-ons with hydronic or other non-ducted distribution systems. Ductless split systems combine the zoning flexibility of conventional room units with the whole house cooling potential of a central system.
   Mini split systems are easy and quick to install. They operate quietly. They are versatile and efficient and can be installed in variety of zones. They are far more advanced and provide added security (against intruders) as compared to window and wall type units. They are stylish. Most importantly, they are efficient and economical. For these and other reasons, Mini Splits often are preferred over central air-conditioners. 
 http://www.ductlessdepot.net/   718-894-8474

With SEER ratings above 18 the inverter style split cooling systems are
the most efficient, with no duct losses, they are also the simplest, least expensive,
most comfortable and cleanest cooling systems available today.

   Most contractors today have not adjusted their cooling or heating equipment to the actual reduced requirements of the homes being built today. Most new heating and cooling systems being installed today are oversized considerably, often two or three times the actual need. This leads to  short cycling and inefficiency. Heating Btu requirements normally are running below 15 Btu's per Sq. Ft. and in very warm climates with good construction practices 15 Btu's per Sq. Ft. will be more than sufficient for cooling.       

   Cooling a radiant heated house can be often be done simply and inexpensively. Here are several methods you can consider. These can also be combined with a conventional cooling system to save energy. Cooling requirements are often much lower than most contractors are recommending for new highly insulated homes. Although high insulation values will help with heating and cooling there are some things that will help the cooling much more. Very white roofs, light colored exterior walls.  Attic fans need temperature controls on them to prevent hotter air from being brought in at the wrong time. Insulated overhanging roof eaves are very important. High vaulted ceilings are a No No. The amount and the type of glass is very important. If you use drapes or curtains inside be sure they are insulated to prevent sun from entering. Glass in vaulted areas should be protected by automatic shades. You want to prevent the heat from coming in not try to remove it after it is in.
Cool air cooling: If you are in a climate that cools down at night you can use fans to bring cool air in as soon as the outside temperature drops below the inside temperature. A fan that takes the air out of the house and exits it up through the attic works best. 
Cool water cooling:
If you are irrigating your lawn or your garden you can simply run the cool water through a coil in your furnace before it goes out to the lawn. This can be run with a solenoid valve and a thermostat. A nearby stream, lake or pond can do the same thing.
Room by room cooling: Cooling one or two rooms and ignoring others can save a lot of energy. Or just cooling one side of the house where the most heat gain is will help. Use wall mounted air conditioners in rooms where they are needed.
Evaporative cooling: In dry climates evaporative cooling systems are very economical to operate.
Evaporative cooling towers:
There are some companies that make small closed evaporative systems some are coupled with a Freon compressor. These are very efficient.
Radiant cooling systems: This requires exacting design and, possibly, added equipment and controls to assure that indoor environments are comfortable and free from excess moisture that can result in surface condensation.          MORE COOLING