What is a closed water supply system with back-flow preventer in the cold water supply how does work and how i now if in my house i have one i hear that when you have one the water heater has to have a thermal expansion tank but mind dosnt have it?


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If you have a closed loop system, which in your case could be a hot water circulating system, the back flow preventer keeps the water in the loop from back feeding into the system supply if the loop pressure exceeds that of the supply pressure.  It accomplishes this via a hinged flap of sorts that closes on a seat if the water reverses flow.  In you system the TMP VALVE (temperature pressure relief) will open up if the pressure with the hot water tank exceeds a preset level.  Without that TMP  you could end up with an explosion!  (Years ago in Oklahoma a hot water tank exploded due to a continuous burner fire.  The tank was missing a TMP VALVE.  This occurred at the STAR SPENCER ELEMENTARY SCHOOL killED several people.  The explosion ripped through a block wall which separated the lunch room from the mechanical room.)Expansion tanks in closed loop heating and chilled water systems are part of the design but usually are not found in residential systems ...not required.  (Look for design help at the engineers tool box)Call a professional plumber if the TMP IS TRIPPING ...(EXPELLING HOT WATER )

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