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You can learn more about your hot water system in the free lessons at
http://www.hydronic.net
Your pvc pipe may be in violation of codes as the relief pipe should have no fittings attached to the end just off the floor. This is so to prevent people from leaving the repair of their system and relief valve off to a later time as it is a prime safety device.
The running may be caused by a water-logged or improperly sized expansion tank. The hydronic site explains the sizing and function of expansion tanks along with everything else.
