Protecting Water Heating Systems with Expansion Tanks

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Water Heating Expansion Tank
Water Heating Expansion Tank
A solar expansion tank
A solar expansion tank (solar xtank; circled) is a critical component of a solar water heating system. The solar xtank pictured is sized to maintain system pressure during stagnation.
Pressure-relief valve
Manufacturers use a variety of pressure-relief valves for their pump stations. The rating for the valve (circled) in this PAW pump station is 87 psi.
Floor-mounted expansion tank
Some residential SWH systems may require the use of a larger, floor-mounted expansion tank (circled) to accommodate the steam expansion that occurs during stagnation.
A prevessel can protect the membrane from excessive temperatures
In systems with a large volume of HTF, a prevessel can protect the membrane from excessive temperatures by decreasing the temperature of the HTF in the expansion system.
Water Heating Expansion Tank
A solar expansion tank
Pressure-relief valve
Floor-mounted expansion tank
A prevessel can protect the membrane from excessive temperatures

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.

Solar Expansion Tanks for Closed-Loop SWH Systems

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.

Comments (4)

dave@atomicsolar.biz's picture

No solar water heater should ever stagnate, and I'm sory some designers produce systems that do under normal or power loss conditions. This leads to glycol degradation, and excessive pressure swings, and adding a large expansion tank may allow you to prevent the pressure relief from operating, it does nothing but save the boiling glycol from escaping, which indicates a problem, but is no longer apparent. If no problem is apparent, the glycol will turn acidic and eat the collector tubing, or lose it's freeze protection and burst. This is bad for the industry. Use DC pumps, PV, and properly sized expansion tanks for pressurized glycol systems, and heat dumps on larger space heating systems.

Vaughan Woodruff's picture

Dave,

Thanks for your comment.

Proper design can mitigate the concerns of acidic glycol due to overheating. As you are likely aware, the type of glycol makes a difference. Using glycol solutions with properly-rated inhibitors goes a long way.

The concept of "steam back" or "boilback" - when done correctly - can also minimize wear and tear on the glycol. If the system is only able to produce a fixed amount of steam when stagnation occurs (i.e. the steam stays trapped at the top of the system and no additional fluid is pushed into the collectors due to proper check valve location) and the collectors empty well, then the glycol really isn't exposed to excessive temperatures. The collectors empty immediately when a small amount of steam forms in the collector at the boiling temperature of the fluid (roughly 220-235F depending upon the system pressure) and as a result the glycol never sees temperatures above this.

A DC motor powered by PV has its issues as well - limited control options, the occasional need for overheating protection to protect the tank, limitations with certain systems, the potential for future shading of the PV module to cause issues. As with any SWH system, it really comes down to choosing the right approach for the application. Many are finding that the use of a "boilback" method with certain antifreeze systems is a robust solution that avoids the problems you mention. Others prefer PV with a DC pump.

Best,

Vaughan

Tom Lane's picture

I am glad to finally see HOMEPOWER Finally putting the expansion tank with the inlet facing up AND before the pump . It should be mentioned that it should be before the pump by a factor of 12 . That being 12 tines the diameter of the pipe . A tragic mistake in Home Power has been to show constantly the use of swing y or t check valves or even spring loaded check valves . As has been well documented in the American Literature and European literature now - these checks valves result in massive thermosyhon ing of heat back to the roof at night . At a recent IAMPO convention in Philadelphia this was discussed in detail . There are many examples from Bristol Stickey and other sources of exterior heat exchangers frozen and ruptured using mechanical check valves . It has be well documented by the Florida Solar Energy Center that only electrical solenoid check valves wired to the pump should be used on open loop direct or closed loop indirect systems to stop thermosyhon ing . IN POINT OF FACT A 50% glycol mixture will thermopyphon faster and easier than water . The steam back method of heat dissipation is degrading to the glycol solution as the glycol must go through a phase change to steam to Steam Back and not just vaporize out . A heat dissipation system or a vacation mode should be used in the control system . No American manufacturer I.e. AET , SunEarth , etc warrants their collectors to be used in steam back systems . The Europeans Schuco who are now out of the solar hot water business introduced this in to the USA with serpentine collectors . It is critical to understand that unless you especially design a interior bottom of the tank heat exchanger , that only electrical solenoid valves should be used as check valves . Solar Hot Water Lessons Learned 1977 to Today is an excellent book to learn about how to properly do glycol systems .

Vaughan Woodruff's picture

Tom,

Your statement about SunEarth is incorrect. If you go to the "Sizing Expansion Tanks" page on their website (http://sunearthinc.com/design-resou...) you will see that they specifically state that "the acceptance volume must be sufficient to accommodate expansion of the heat transfer fluid when the solar loop goes into stagnation."

Any antifreeze system with a differential controller is susceptible to stagnation. This is simply one way to mitigate the issue.

Vaughan

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