National Oilheat Research Alliance 11 tion of hot water when the thermostat is not calling for heat. Without a flo-control valve, gravity circulation occurs because hot water tends to rise through a system’s piping simply because it weighs less than the colder water in the piping. Flo-conrol valves open, allowing water to move through the system when a circulator operates and they close when the circulator stops running. Flo-control valves can be used in systems with multiple zones to allow heat to flow only into the zones that are calling for heat. Most can be opened manually to allow for temporary gravity flow if the circulator malfunctions. While they are normally installed in the supply piping, there are situations in which an additional flo-control valve is required on a return line. Flo-control valves are also used in primary-secondary systems on both the feed and return lines of all secondary circuits except those that are below the primary circuit’s piping. 13Indirect water heater—Indirect water heaters are storage tanks with internal heat exchangers. They are equipped with an aquastat that controls a circulator to move hot boiler water through the inside of the heat exchanger, heating the domestic water in the tank. 14Thermal expansion tank —Domestic water expands when it is heated and 50 gallons of cold water becomes about 52 gallons of hot water. Thermal expansion tanks will take-up the excess water, preventing continual discharges from the relief valve. These tanks are necessary when back flow devices are installed; back flow devices prevent the expanding water from flowing back into the water main. 15 Temperature & Pressure relief valve (T&P)—The T&P valve provides automatic temperature and pressure protection for the water heater. Typically, PRVs open and release water when the water temperature reaches 210 º F and/or the water pressure reaches 150 psi. Figure 12
NORA Advanced Hydronics
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