What does an inspector report about a water heater with no visible expansion tank on a closed system?

Water expands as it is heated, and in an open system that expansion pushes harmlessly back into the supply main. A closed system — one with a backflow preventer, check valve or pressure reducing valve at the service — removes that escape route, so the expanding water has nowhere to go and system pressure rises sharply every heating cycle. The pressure spikes stress fixtures, supply connections and the water heater itself, and the most visible symptom is the relief valve weeping or discharging periodically, which people often misread as a faulty valve when the valve is doing exactly its job. A thermal expansion tank absorbs the volume change with an air cushion. So the finding is not that the heater malfunctions but that a closed system without expansion control subjects the whole plumbing system to repeated pressure cycling.

On a closed system the expansion produced by heating cannot push back into the main, so pressure rises with every cycle. An expansion tank absorbs that volume, and without one the system experiences repeated pressure spikes.

Backflow preventers, check valves and pressure reducing valves create closed plumbing systems in which thermal expansion cannot return to the main. A thermal expansion tank provides the compressible volume needed, and its absence causes pressure cycling and relief valve discharge.

Closed System Needs Somewhere to Go. Heat the water, it expands. Block the way back to the street and the pressure has to land on something.

A relief valve that discharges periodically on a closed system is usually reporting thermal expansion, not failing. Replacing the valve does not fix it.

• •Diagnosing a weeping relief valve as defective on a closed system
• •Failing to establish whether the system is closed before assessing the need
• •Treating the issue as a water heater fault rather than a system one