Water expands when it is heated and it is nearly incompressible. These two properties can create a powerful force in a water heater. The water in a standard 80-gallon solar storage tank may expand by as much as 2.5 gallons as it is heated, and this thermal expansion will cause a significant increase in the pressure in the plumbing system unless the extra fluid volume is accommodated. In a now-infamous episode of “Mythbusters,” a residential electric water heater was turned into a rocket by disabling its high-limit thermostat and temperature and pressure (T&P) relief valve.
In some homes, this thermal expansion is absorbed by pushing pressure back to the public water supply. But in homes where check valves are required to either maintain the water pressure from a well pump or to protect the public water supply from possible contamination, a device is needed to absorb this expansion. For some water systems, like those that are supplied from a well, the pressure tank often serves this purpose. For homes that have a check valve installed at the public water supply, a potable water expansion tank (aka xtank) is required.
A potable water expansion tank is typically a 2- to 5-gallon steel tank containing a flexible membrane (either a bladder-type or diaphragm-type) that divides the tank. One side of the tank, connected to the pipes of the hot water system, contains water. The other side contains air. The membrane in a bladder-type expansion tank resembles a balloon, while the membrane in a diaphragm-type tank is sealed to the wall of the tank. Since the potable water in a diaphragm-type tank could otherwise be in contact with the steel tank, a liner made from a material such as polypropylene is required to resist corrosion.
Air is compressible and, as the volume of water in the storage tank expands, the volume of air in the expansion tank will decrease. This leads to increases in the air pressure and—as a result—increases the potable water pressure. That’s where proper selection and proper sizing of the expansion tank come into play. If the expansion tank is undersized and cannot absorb the additional pressure, the potable water pressure may exceed allowable limits and activate the storage tank’s T&P (aka relief) valve. This is a problem since the relief valve is a safety device; constant operation of the valve can lead to its failure, and to potentially dangerous consequences.
Selecting an expansion tank for an SWH system is similar to selecting one for a conventional water heating application, but with two distinct exceptions: SWH storage tanks tend to be larger than conventional water heaters and the maximum temperatures in a SWH tank may be 50°F to 60°F higher. This can translate into thermal expansion that is four to 10 times greater than the expansion in a conventional water heater. This needs to be considered when sizing the expansion tank, along with the system’s normal operating temperatures, standard water pressure, and the water heater’s volume. Expansion tank manufacturers typically provide sizing charts that account for these variables.
In closed-loop antifreeze-based (aka glycol) SWH systems, solar expansion tanks (aka solar xtanks) are required to absorb the expansion of fluid in the solar loop. The solar collectors, piping, and heat exchanger are filled with a glycol solution that—like water—expands as it is heated. Though the volume of glycol in the solar loop is far less than the volume of water in a solar storage tank, solar expansion tanks tend to be the same size or larger than the expansion tanks used on the system’s potable water side to handle the normal expansion of fluid and steam expansion, which can occur if a system stagnates.
Stagnation occurs when there is sufficient solar radiation available but the heat-transfer fluid is not flowing through the collectors. This can occur when a component fails, such as a controller or a pump; or during a power outage, since the circulator pump will no longer have power; or under standard operating conditions, when the system has heated the water to the storage tank’s maximum temperature limit and the controller shuts off the pump to prevent overheating of that tank. All antifreeze-based systems must consider stagnation in their designs.