Open or Closed Water Heater System - Which one is Better?

You bought a water heater, know what? One of the main decisions you should take when installing a new electric or gas water heater is either to install it in an open or closed water heater system.

This article talks about the pros and cons of both systems and how each setup can protect users, heaters, and your property.

The problem is that the water inside the heater tank, when exposed to the heat, increases the temperature and pressure and, therefore, the volume.

According to the study done by the manufacturer Rheem, the water pressure rises rapidly when the water temperature rises from 75 F to 100 F; it gets increased from 80 PSI to 520 PSI.

When the water pressure increases over 145 PSI, the TPR valve is the first element that reacts; it opens and releases some hot water while reducing the pressure. The working pressure inside the water heater tank is approximately 150 PSI, and the tanks are tested to withstand a pressure of 300 PSI.

If the pressure increases above safety level, the metal tank starts to deform, rupture, bulging, and eventually leak.

Open vs. closed water heater system

To protect the unit, you must consider an open or closed water heater system.

An open water heater system is when water that increased its volume and pressure due to the heating freely goes from the storage tank into the cold water supply line and eventually into the municipal water system if the pressure is high enough.

This is happening if there are no valves in a way (valves such as the shut-off valve, check valve, and similar). In order to prevent the water backup, some municipalities require installation per codes, which requires check valves, shut-off valves, and so on. Since nothing stops the water from expanding, the pressure in the open system is always equal to the supply pressure.

A closed water heater system means that the hot water cannot expand beyond the valves and mix with the municipal water. The valves usually found in the closed systems are the check valve, pressure-reducing valve, mixing valve, backflow preventer, and others. The problem with this system is that hot water during the thermal expansion cannot go anywhere; it is trapped, causing the pressure to increase.

The TPR valve will release some hot water if the water pressure goes beyond 145 PSI, but much higher pressures could develop if it doesn't operate properly.

As mentioned, extreme pressures can cause bulging, deformation, and eventually, breakage at the weld seams.

This is why the expansion tank installation in the closed water heater systems is required and often requested per codes. The expansion tanks are designed to receive increased water volume and reduce the pressure inside the tank.