Radiant floor heating is ideal for liquid solar systems because it performs well at relatively low temperatures. In a radiant floor system, solar-heated liquid circulates through pipes embedded in a thin concrete slab floor, which then radiates heat to the room. You can use a radiant floor, hot water baseboards or radiators, or a central forced-air system to distribute the solar heat. They meet building codes for pressure vessel requirements, are lined to inhibit corrosion, and are easy to install. The simplest storage system option is to use standard domestic water heaters. It may be more practical to use several smaller tanks rather than one large one. Each type of tank has its advantages and disadvantages, and all types require careful placement because of their size and weight. Concrete and wood (hot tub) tanks are also options. They are usually stainless steel, fiberglass, or high temperature plastic. Specialty or custom tanks may be necessary in systems with very large storage requirements. You should also note how much insulation is necessary to prevent excessive heat loss, and what kind of protective coating or sealing is necessary to avoid corrosion or leaks. Tanks also have limits for temperature and pressure, and must meet local building, plumbing, and mechanical codes. You may need to construct a tank on-site if a tank of the necessary size will not fit through existing doorways. Before choosing a storage tank, consider cost, size, durability, where to place it (in the basement or outside), and how to install it. Tanks are pressurized or unpressurized, depending on overall system design. In tank type storage systems, heat from the working fluid transfers to a distribution fluid in a heat exchanger exterior to or within the tank. Liquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. To learn more about types of liquid solar collectors, their sizing, maintenance, and other issues, see solar water heating. The flow rate depends on the heat transfer fluid. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls. The liquid flows to either a storage tank or a heat exchanger for immediate use. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows rapidly, so its temperature only increases 10° to 20☏ (5.6° to 11☌ ) as it moves through the collector. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. They are the same as those used in solar domestic water heating systems. Solar liquid collectors are most appropriate for central heating.
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