Back to Basics: Expansion Tanks

By Roy Collver

Water expands when it is heated. Inside of a closed container or piping system, the internal pressure will increase rapidly because water is incompressible in real-world conditions. My first illustration gives one pause to consider the effect on hydronic system components if water volume increase is not accounted for. The need to add a pressure safety relief valve to any closed-loop piping system is clear from this chart.

To eliminate excess water volume, the use of a pressure relief valve is an option, but the system would then lose fluid on every heat/cool cycle, which would require the automatic addition of fresh makeup water. This a waste of water and the constant addition of corrosive oxygen.
Air is compressible, however, and experts realized that an air pocket of sufficient size within a closed-loop system would squeeze down in volume, providing room for the extra water. It is not a good idea to have air pockets running around inside boilers or distribution piping, therefore the hydronic expansion tank was developed as a temporary “parking lot” for excess water volume when the system is hot. This allows it to be recycled back into the piping system each time it cools down again.

Required pressure
The minimum pressure in an operating hydronic system is established by the cold-fill pressure. This is normally the pressure required to push the water to the top of the heating loops plus enough additional pressure (typically, five psi) for air vents at the top to work properly. Each one psi pressure increase will elevate a water column 2.31 ft. To calculate the cold-fill pressure of your system, multiply the height from top to bottom by 0.433 to get psi at the bottom, and then add five psi.

“Keep in mind that many methods make assumptions regarding some of the above parameters and will have built-in factors that will end up with a tank one size larger than the math suggests.”

The maximum pressure in the system is established by the pressure relief valve selection. Choose a valve set to start dumping fluid before pressure can increase to damaging levels. Thirty psi is a common setting in residential and low-rise commercial buildings; high enough to avoid releasing under normal operating conditions and low enough to avoid component damage.

Early systems were open to the atmosphere and allowed heated water to expand into an open tank at the high spot in the system. This was an elegant solution, but unfortunately provided a way for air to enter the system and contribute to oxygen corrosion. The invention of the fully closed system, by adding an “air cushion” tank, mostly solved the oxygen corrosion issue, but very careful design, special fittings and components, and operating parameters were needed to keep these tanks from having their air dissolve into the water.

Corrosion would result and flooding of the tank would render it useless. They needed to be filled with enough water to compress the air to the cold fill pressure and still have enough space to be able to park the heated water as it increased in volume. These tanks could also become large and unwieldy and needed to be installed above the boiler—usually tucked up between the floor joists. You find them still in service today and if you find one and don’t know how to deal with it, ask an old-timer like me.

Doing it old-school
In the 1950s, the diaphragm expansion tank made its way onto wholesalers’ shelves and quickly became the popular choice. The heating fluid was contained by a flexible rubber barrier (diaphragm) in one half of the steel tank and the air was no longer in contact with the heating fluid, almost eliminating oxygen corrosion. The air side of the tank was to be pre-charged with air at the desired system fill pressure, making them much smaller in size than the original cushion tanks. Bladder-type expansion tanks were also developed. They completely separated the heating fluid from the steel of the tank. The flexible bladder was like a heavy-duty balloon containing the heating fluid. Pre-charging the air side of the tank collapsed the balloon while heating the fluid would fill and stretch the bladder as needed. Refer to the many illustrations accompanying this article for more information on how these devices work and some rules for properly applying them.

Size matters
Sizing expansion tanks is not particularly difficult but does require some effort on the part of the designer or service technician to gather the correct information.
This should include:
1. The fluid volume of the system including the boiler, piping, and other components—this is the tricky part and takes some time.
2. The boiler documentation or rating plate should list the water content, and you can find charts that will illustrate the volume per foot of different sizes and types of pipes.
3. You may have to estimate the volume of some of the other components like air separators and low loss headers, but they should also have documentation with that information.
Once you have it all added up, you will be ready to plug in the number in addition to fluid type (percentage of glycol—if any), cold-fill temperature, highest temperature developed, initial cold-fill pressure, and final pressure.
With this information compiled, there are several selection methods available. Keep in mind that many methods make assumptions regarding some of the above parameters and will have built-in factors that will end up with a tank one size larger than the math suggests. This is not necessarily a problem, as oversizing an expansion tank slightly will have no negative effect on system operation and can be considered “good insurance” for those of us who may be mathematically challenged.
Many manufacturers will have online sizing calculators on their website—use them. There are some really good ones for free, but keep in mind the program will select one of their own products. Do not assume their tank is a match for another brand of the same size.
Manufacturers’ representatives and local wholesalers are other effective resources for determining how to size an expansion tank.