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The expansion tank allows built up pressure created from the heating of the water in the water heater to expand into the tank instead of the plumbing system. Therefore it would not be wise to remove and plug. Problems that could arise include, temperature/ pressure relief valve leaking, hot water into the cold service, premature water heater failure and pipe noise.
You see when the city sends water to the house it can sometimes exceed 70# in pressure. To reduce this pressure a pressure reducer in installed at the entry to the house, to reduce incoming pressure to the 50# range. This reduces wear and tear on fixtures, pipes, etc.
When heated water expands it creates increased pressure at the water heater and if no relief, ie: someone uses the hot water,it builds up to sometimes 150-200# and with no where to go it makes the Temperature Pressure relief valve leak water as it is intended to do when pressure exceeds 125#.An expansion tank allows this excess pressure room to escape. This is the reason the latest version of the UPC code book requires an expansion tank at each new water heater installation. To remove it would violate the code requirement, not to mention cause you problems in the future.
Good Luck,
The Local Plumber
Tustin, California

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