answered
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?
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 … Read more