NOVELTY - A system comprises a piping loop; and a heat transfer fluid (214) that circulates within the piping loop such that heat is transferred into or out of the heat transfer fluid through the walls of the piping loop, where the heat transfer fluid is a nanofluid containing nanoparticles, and the nanofluid having enhanced thermal conductivity as compared with a similar fluid not containing the nanoparticles.    USE - As system for heat transfer or heating water (claimed).    ADVANTAGE - The nanoparticles in the nanofluids increase the thermal conductivity of the fluid, as compared to a similar fluid without nanoparticles, and the enhanced thermal conductivity enables more efficient heat transfer into and out of the fluid. The nano fluids enhance heat transfer in thermal systems. The system has the ability to use a shorter piping loop, which enable installation of a ground coupled system in a smaller area than was previously possible. Accordingly, homeowners or others who would otherwise be constrained by property size limitations can be able to take advantage of a ground coupled system. The presence of nanoparticles in heat transfer fluid e.g. copper and silver ions, increases the thermal conductivity of heat transfer fluid, and also improves the effectiveness of convective heat transfer between heat transfer fluid and other structures such as heat exchanger and internal parts of collector. This improved heat transfer effectiveness can be exploited by a system designer to improve the performance of the system, to reduce the cost of the system, to adjust the cost/performance balance, or to provide other benefits. Because the improved heat transfer characteristics of heat transfer fluid improve the performance of heat exchanger, the size of heat exchanger can be reduced, because less surface area in contact with water in tank and with heat transfer fluid can be needed to maintain the performance of the system as compared with a system that does not utilize a nanofluid. Thus, the cost of heat exchanger can be reduced. The number of tubes within collector can be reduced, resulting in cost savings. The nanofluid improves the transfer of heat between tubes and heat transfer fluid. This improvement in performance enables material savings in collector. The improved heat transfer characteristics enable use of materials with poorer heat transfer characteristics but other desirable qualities, while maintaining acceptable or even improved system performance. Additionally, the lighter weight of collector can result in savings in shipping costs, and improvements in the ease of installation of collector. The use of nanofluid in ground coupled piping loop, enable flexibility in design, and can result in improved performance of an environmental control system and/or reduced cost, as compared with a system that does not use nanofluid.    DETAILED DESCRIPTION - A system comprises a piping loop; and a heat transfer fluid (214) that circulates within the piping loop such that heat is transferred into or out of the heat transfer fluid through the walls of the piping loop, where the heat transfer fluid is a nanofluid containing nanoparticles, and the nanofluid having enhanced thermal conductivity as compared with a similar fluid not containing the nanoparticles. The heat transfer fluid contains ions of copper, and/or silver. The system further comprises an ion generator (208), including: a pair of spaced apart electrodes (201, 202) in contact with the heat transfer fluid; and circuitry arranged to impress an alternating voltage between the two electrodes such that ions of material from the electrodes are generated within the heat transfer fluid by electrolysis; a solar collector; a tank of water to be heated; and a heat exchanger within the tank. INDEPENDENT CLAIMS are included for the following:    (1) a system for heating water comprising a solar collector, a tank of water to be heated, a heat exchanger within the tank, a fluid circuit coupling the solar collector with the heat exchanger, and the heat transfer fluid that circulates within the fluid circuit;    (2) a thermal system comprising an environmental control system, a ground coupled piping loop coupled to the environmental control system, and the heat transfer fluid that circulates within the ground coupled piping loop; and    (3) an underfloor environmental control system comprising a piping loop embedded within a floor, the heat transfer fluid that circulates within the piping loop, and a device coupled to the piping loop, such that the device imparts heat to the heat transfer fluid or removes heat from the heat transfer fluid.    DESCRIPTION OF DRAWING(S) - The figure shows a view of operation of ion generator.    Fluid circuit (103)    Ion generator (108)    Electrodes (201, 202)    Oscillator (203)    Transistors (211, 212)    Heat transfer fluid (214)