In a closed water-distribution system — one isolated by a check valve or backflow preventer — what does a water heater's thermal expansion require?
Not quite. The answer is C: A thermal expansion tank or other approved expansion-control device
Once the system is closed off from the street main, heated water has nowhere to expand into except back through the piping itself, spiking pressure with every heating cycle — an expansion tank gives that water somewhere to go, rather than relying on the relief valve to open repeatedly under normal operation.
Checked against the 2021 UPC text. Washington's exam is written against the 2015 edition, which we have not read for this section, so confirm it against your 2015 book.
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