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 .