Most homeowners do not know whether their plumbing system is "open" or "closed" — but the distinction matters enormously for water heater longevity and safety.

An open system allows expanded water to flow back toward the street main when the water heater heats up. A closed system has a one-way device — a backflow preventer, pressure reducing valve (PRV), or check valve — that prevents water from flowing back. Most homes in Calvert County and Anne Arundel County have PRVs installed on the main water line, which means they have closed systems.

In a closed system without an expansion tank, here is what happens every time your water heater fires: the water in the tank heats up and expands. That expanded water has nowhere to go. System pressure rises — sometimes to 150 PSI or more. The temperature and pressure relief valve (T&P valve) opens to release the pressure. This is why T&P valves drip on closed systems without expansion tanks.

The T&P valve is designed for emergency use — not daily pressure relief. Repeatedly discharging the valve causes it to wear out and eventually fail to reseat properly, leading to a constant drip. More importantly, the repeated pressure spikes stress the water heater tank, the anode rod, and all the plumbing connections throughout the home. Water heaters on closed systems without expansion tanks typically fail 3–5 years earlier than they should.

Maryland plumbing code requires expansion tanks on all new water heater installations in closed systems. Without an expansion tank, thermal expansion causes: T&P valve failure, water heater damage, pipe stress, and potential flooding. The T&P valve is a safety device — if it's dripping regularly, it's doing its job but shouldn't have to.