Open or Closed Water Heater System

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 installing an open or closed water heater system.

This article presents the pros and cons of both systems and how these setups can protect the users, heaters, and property.

The problem is that the water inside the heater tank, when exposed to the heat, increases its 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 increases 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. For the pressures above this value, the metal tank starts deforming, bulging, rupture 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 back up, some municipalities require installation per codes, which requires the check valves, shut-off valves, and so on. Since there is nothing that 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 that are usually found in the closed systems are the check valve, pressure reducing valve, mixing valve, backflow preventer and other. The problem with this system is that hot water during the thermal expansion cannot go anywhere; it is trapped, causing the pressure increase.

The TPR valve will release some hot water if the water pressure goes beyond 145 PSI, but if it doesn't operate properly, much higher pressure could develop. The extreme pressure values can cause bulging, deformation, and eventually, the breakage at the weld seams. This is the reason why the expansion tank installation in the closed water heater systems is required and per codes. The expansion tanks are designed to receive the increased water volume and therefore reduce the pressure inside the tank.

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