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The return piping shall be sized for a maximum temperature drop of 20°F (6.67°C) for forced circulation systems and 40°F (4.44°C) for gravity circulation systems. Such piping may be of the upfeed system, downfeed system or the combination upfeed and downfeed systems and shall be installed as follows:
1. For upfeed systems, the return circulation pipe shall connect so the supply risers below the top-most supply branch to fixtures and not more than 12 inches (300 mm) below.
2. For downfeed systems and combination upfeed and downfeed systems, the return circulation pipe shall connect to the base of each downfeed riser.
3. For inverted downfeed systems, the return circulation pipe shall connect to the supply risers above the lowest supply branch to fixtures and not more than 12 inches (300 mm) above.
4. For inverted upfeed systems and inverted combination downfeed and upfeed systems, the return circulation pipe shall connect to the top of the riser above the highest fixture.
5. Return circulation pipes shall connect to the water heater or storage tank. Each return circulation riser branch shall be provided with a shutoff valve and check valve. Where more than one return circulation riser connects to a return main, each return circulation riser branch shall be provided with a shutoff valve, check valve and balancing valve or a check valve and combination shutoff balancing valve. All trapped piping shall be provided with an automatic air vent at the highest point.