Figure 1. The elevation in pressure with increasing temperature in an average plumbing system.
Throughout the history of the American plumbing industry, safeguards have been established and implemented to ensure the enduser has the highest quality water supply. The consumer has come to expect it, and has rarely thought of the ongoing efforts necessary to make it happen. The 1974 Safe Drinking Water Act, with subsequent revisions, has created a tremendous movement in safeguarding this quality by mandating that the ultimate protection belongs at the municipal water authority level. A development of this protectionary requirement is the goal of eliminating all cross-connections between potable and nonpotable waters.

Many authorities implementing programs have experienced the growing pains attributed to this requirement of cross-connection control. Water authorities embarking on newly implemented programs should learn from these experiences. Successful programs that have been executed include many stages of strategic planning.

Selection, installation, performance verification, and maintenance of the correct backflow preventer based on contamination potential, are outlined with careful documentation. Emergency response plans are written and emergency contamination drills rehearsed. This forward-thinking preparation ultimately results in saving lives, reducing property damage, and limiting the authority's liability when drinking water contamination occurs.

Figure 2. Expansion tank schematic.

Preventing Back Flow, But Increasing Pressure

As well thought out and comprehensive as these programs are, the water authority must not neglect the liability that may occur on the other side of the back flow preventer. The old adage that states "no good deed goes unpunished" is a good analogy of what happens when new safeguard programs are put in place. In Greek mythology, the dreaded Hydra was a serpent with a unique defense mechanism. When Hydra's menacing head was cut off, it sprouted two more in its place. A glowing example of this happening in real life is the addition of back flow prevention valves in plumbing systems. The valve that protects against potential cross-connection contamination has lead to major problems in buildings' plumbing systems.

Excessive water pressures build up and increasing water hammer occurrence are just two examples of what has happened by cutting off Hydra's head. The excessive pressure buildup is caused by thermal expansion of the building's plumbing system water. Water expands when it is heated. This occurs several times each day as the water heating equipment operates to heat water. This expanded water would normally back up into the water supply main, which traditionally absorbed the thermally expanded water. With a back flow preventer in place, water can no longer leave the building's plumbing system and increased pressure will result. Just how high the pressure rises depends on several factors.

Water is incompressible. When heated and expanded, pressure will increase suddenly and rapidly to dangerous levels in a closed piping system. Figure 1 shows the elevation in pressure with increasing temperature in an average plumbing system.

The pressure in the system will rise until a weak component in the plumbing system gives way and continually discharges water. This component is typically a faucet washer, although it could be a weak pipe, poor solder joint, water closet valve, etc.

Due to thermal expansion-related problems, water authorities have recorded a skyrocketing increase in home and building maintenance personnel calls. But for every call received, substantially more calls are never made.

Table 1. Thermal expansion tank sizing chart.

Common Symptoms

The most common symptoms that may indicate a thermal expansion problem exists include:

  • The water heater relief valve is operating to relieve excessive water pressure.
  • An excessive water pressure surge when a faucet is opened.
  • Increased frequency of faucet washer replacement.
  • Metallic expansion noises at the water heater.
  • Piping noises, cracking, or creaking with the water heater operating.
  • Premature failure of appliance solenoid valves and O-ring seals.
  • Severely reduced water heater life.
  • Flames periodically rolling out in gas-fire heaters.
  • Combustion products (flue gas) entering living spaces.

Major United States water heater manufacturers and the American National Standards Institute (ANSI) have become extremely concerned with potential liabilities stemming from thermal expansion. Manufacturers have developed mailers, for example, that state: "Your water heater and plumbing system are no longer adequate or safe!" Also, failure to rectify the thermal expansion problem may result in "leaks within the system, failure of the water heater, water damage, sooting, flame roll out (possible when relighting the water heater), and even the spread of deadly carbon monoxide gas!" Water heater failure, as explained by a second major water heater manufacturer, is caused by the sudden pressure-induced collapse of the center flue within a gas-fired water heater resulting in the very serious possibility of carbon monoxide poisoning. For electric water heaters, severe deformation of the water heater bottom dome results in the rapid reduction of heater life expectancy.

The solution to this potential liability problem must be accomplished in various stages. Stage I: inform and educate. The building owner must be informed of the conditions occurring within the plumbing system, the potential dangers, and the symptoms of the condition. Educate the building owner as to the acceptable solutions for properly controlling thermal expansion. Stage 2: repeat Stage 1. The success of any program depends on the continual barrage of the building owner with information on the installation and important maintenance requirements of the thermal expansion control equipment.

Building inspectors will accept approved and listed thermal expansion control tanks as the means for controlling pressures. Some authorities still accept secondary relief valves, although this inexpensive "quick fix" is not considered acceptable by some water-conservation-minded authorities. A major dilemma occurs when new maintenance personnel are hired in a building that utilizes an auxiliary relief valve. Experience has shown that an uninformed maintenance person will plug the relief valve or remove it, restoring the system to its uncontrolled state. It is also regulated that a safety device, such as a secondary relief valve, cannot be used as an operating control to intermittently release water from the system. This is because any sediment collecting on the valve seat can cause continuous water flow conditions, or worse, a totally blocked relief valve.

Thermal Expansion Tanks

All national cross-connection control sections of the plumbing codes, including those maintained by the Building Officials and Code Administrators (BOCA), International Association of Plumbing and Mechanical Officials (IAPMO), Southern Building Code Congress International (SBCCL), and the Plumbing-Heating-Cooling Contractors - National Association (PHCC), carry mandatory requirements for the provision of an approved thermal expansion control device. The preferred specified device is an approved thermal expansion tank. In fact, most relief valve manufacturers have acknowledged the advantages of the thermal expansion tank and have begun offering this alternative.

The thermal expansion tank acts like a "lung" on the piping system, accepting expanded water as the water heater functions. As water is used, it is pushed back into the piping system by the tank's compressed air cushion. Typically, expansion tanks utilize a flexible butyl diaphragm or bladder to separate a sealed compressible air cushion from the incoming expanded water. The air cushion is "pre-charged" to approximately a 40 psig. (Department of Transportation shipping limitations) and retained by the flexible bladder. The expanded water enters the bladder, which protects the tank's steel from the potentially corrosive fresh water. (Figure 2.)

The proper thermal expansion tank size is critical in limiting the pressure fluctuations in a piping system. Too small of a tank leads to high-pressure fluctuations; too large of a tank reduces pressure fluctuations but creates cost constraints that may prove prohibitive.

The critical size requirement for any thermal expansion tank is based on the physical properties of water and the subsequent required air cushion needed to absorb the water within a specific pressure range. The tank size is calculated by:

TX = Expanded water/(P1¿P2-P1¿P3)


TX = Thermal expansion tank volume (gal);

P1 = Pre-charge pressure (psia);

P2 = Static line pressure (psia); and

P3 = Maximum allowable tine pressure (psia).

The amount of expanded water depends on temperature rise and water heater volume. Industry practice is to protect against the potential condition of extreme temperature increase (40¿F to 160¿F) for the entire water heater volume. This extreme condition may occur only five to 10 times per year, compared to the thousands of smaller expanded water conditions occurring yearly, yet protection against this condition must be achieved. The typical air cushion pre-charge pressure for a thermal expansion tank is 40 psig (Department of Transportation shipping limitations) and the maximum allowable line pressure is 135 psig (10% below safety temperature and pressure (T&P) relief valve settings).

The equation then simplifies to:

TX = .0209 x Water heater volume¿(54.7¿P2-.365)

To help aid the water authority personnel, code officials, and plumbing contractors in sizing based on the above equation, a quick sizing table has been developed for use. (Table 1.) To use the table, simply check the water heater volume, on the left hand column, and supply-line pressure, along the top row. Trace down from the static pressure and over from water heater volume until the two lines meet.

The total critical volume (in gal) required to absorb the thermally expanded water within the specified pressure range is displayed. For example, a 500-gal water heater with 60-psig static line pressure requires a 28-gal thermal expansion tank to control pressures. The installed tank can be verified by the authority having jurisdiction to meet or exceed this 28-gal requirement.

Manufacturers will typically create sizing tables reflecting exact model number requirements to satisfy this critical sizing. Utilizing a chart of this type ensures the sizing is based on requirements similar to those previously discussed.

The figures in Table 1 indicate the tank size required without changing the factory precharge setting of 40 psig. The knowledgeable engineer and contractor will size the tank with the precharge air cushion pressure equal to the line pressure at the point the tank is installed in the system. This will allow a smaller tank to be used to handle the expanded water while controlling the supply line pressures to remain well below the maximum design pressure. The governing equation reduces to:

TX = .0209 x Water heater volume ¿ (1-P2¿149.7)

Using this equation for the above stated example results in a tank requirement of 20.9 gal. This represents a 25% reduction in tank size requirement for this example. Care should be made to ensure the tank is then properly pre-charged at the job site.

The potential for public harm far outweighs the necessary effort to educate the building owner on the newly implemented or ongoing cross-connection control policies and programs. The cause and effect of thermal expansion create even higher potentially dangerous conditions and must be included in all educational information programs. Protection of life, property, and the eradication of liabilities are all at stake when thermal expansion is neglected. ES