Recently, SnEarth was in reciept of the question, "What is a Potable Expansion Tank?" Read along, as Bob Danielson, SunEarth Technical Sales, provides us with an answer.

The pressure relief valve is a Safety device and is designed to open, release pressure above its rating tag, when the water or fluid pressure exceeds safe limits based on design of the system .  ideally it should never open .

The expansion  tank, potable or non-potable, allows the water as it is heated to expand into the air chamber of the expansion tank , thus maintaining the water pressure within a few PSI of the normal potable pressure. This provides protection for any fixture on the  water heating piping including water heaters, faucets, water filters and toilets.

 In the last 10 -15 years the potable water supplies  have added back flow preventers  to protect the supply from contaminated water being able the “ flow back” into the potable water system . Many water suppliers have installed new water meters on homes with built-in check valves / backflow units  to prevent  a home’s water from entering the main supply .

The backflow preventer/ water meter   or a check valve installed  on the cold feed to the home or just before the water heater  will prevent  the pressure that develops  when the water is heated ( ie. pressure cooker )   from  equalizing, therefore the pressure in a water heater rapidly increases until the pressure relief valve starts to open ; it  may not open due to calcification and the water heater or other fixtures can fail.

Bob Danielson Note : T & P valves  have instructions that recommend the valve be opened manually at least once a year ( I like  6 months ) to ensure they are not clogged  and will open if needed .

 A potable expansion tank should be air charged to the measured incoming pressure to the home ; if necessary   adjust the home’s pressure regulator to 50 -65 psi   or replace the regulator.

The Expansion tank’s piping has to have a direct  flow path  to the  water heater  or solar tank  or any tank type of water  heating unit ( The CopperHeart  )  in order to “absorb’ the expanding water when the water is heated by any source . 

Having a pressure relief valve only allows wide swings in the piping system’s pressure which decrease the life of all fixtures and can results in  leaks ; the EX tank  “ smooths the ride” in the plumbing system .