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A thermal expansion tank for a 40 gallon water heater is pretty small and by far the most reliable option. Since pressure equalizes throughout the water piping you can install it just about anywhere on a cold water line. But closer to the tank is better.
Some jurisdictions allow an expansion valve but they are not reliable over the long haul. They tend to leak after a while under normal pressure. You also need some place for the discharge to drain. Unless you're in an area that never sees freezing temps that means a properly trapped drain standpipe or sink. Expansion valves are far more expensive to install.
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