Expansion Tanks

Those of you who live in a home built in the last six or seven years may have noticed a large balloon-like tank connected to the piping near your hot water tank. This device is known as a thermal expansion tank. Its purpose is to protect your household plumbing in the event of a pressure increase brought about by the heating of the water in your hot water tank.

To understand why this might happen we need to first look at how the plumbing in your house connects to the water main in the street (See Figure 1). Water is delivered to your house through a service line. Before the service line enters your house it passes through a meter pit, which is
usually located in your front yard. The inside of the meter pit contains a small check valve that prevents water from moving from your house back into the water main. Such an occurrence might happen if the main were to break, causing a local depressurization of the distribution system. The check valve prevents any possible contaminates from entering the main where they might affect other users on our system. More information on this is available in the Backflow Prevention section of this site.

One of the physical properties of water is that the volume increases when it is heated, such as with your hot water tank. To appreciate how much it will expand; the water in a typical residential hot water tank holding 40 gallons will increase by about a half a gallon when heated 70 Fahrenheit degrees. However, if the water is restrained from expanding by the plumbing the result will be an increase in pressure. The resulting pressure can be enough to damage your hot water tank or plumbing.

Some people might have noticed that they don't have a thermal expansion tank and have never had any such problems. If there is no check valve in your meter pit then your plumbing is unrestrained. The volume expanse brought about by your hot water tank will simply expand back through the meter and into the water main. Because the water is unrestrained, there will be no increase in pressure.

For houses that do have a check valve in the meter pit the potential for trouble is much greater, but you may still go several years without experiencing any problems. There might be several reasons for this:
  1. While your hot water heater is running, the pressure in the tank and plumbing is increasing slowly. If at any time you open a faucet, such as to fill a glass with water, the pressure is instantly relieved.
  2. The increasing pressure might cause a small leak at a fixture, such as a toilet or faucet, causing it to run or drip until the pressure is relieved.
  3. If you stop using water before the hot water heater is done running the pressure may still increase, but not by enough to be noticed.
So what happens if the pressure is not relieved? . . . Hot water tanks contain a pressure relief valve to protect them from rupturing under such a scenario. The relief valve is typically set at 150 PSI, although this may vary among manufacturers. The valve opens when the tank pressure reaches the valve set point. After enough water has passed through the valve the pressure in the tank drops and the valve closes again. The concern of depending on these valves is that sometimes in older hotwater tanks they might be plugged with scale or corrosion and not function properly. For this reason, a thermal expansion tank is recommended. The basic objective behind a thermal expansion tank is to give the expanding water a place to go so the increase in pressure brought about by heating the water will be controlled. It is important to note that even with a properly selected and installed thermal expansion tank the plumbing pressure will still increase, but not by enough to cause a failure in the plumbing system.

Sizing a Thermal Expansion Tank

Boyles Law
To understand the inside of a thermal expansion tank a quick review of Boyle's Law is in order. The chemist Robert Boyle described the basic relationship between the pressure and volume of a confined gas in the seventeenth century.

The law states that the pressure times the volume of a fixed mass of gas at a constant temperature is a constant. This is expressed mathematically as:

PV = Constant    Where: P = Absolute Pressure    V = Volume

If the volume of that gas is reduced by compressing it without subtracting any gas there is a corresponding increase in pressure. The product of the pressure and the volume of a gas is the same after it is compressed as it was before it was compressed; This is express mathematically as:

P1V1 = P2V2 = Constant

Thermal expansion tanks have a rubber bladder that separates the interior into two parts. One side is filled with water and the other with air. Initially, the tank is pre-charged with air before it is installed on the cold water side of the hot water tank. After it is connected and the water is turned back on, water will flow into the tank pushing the bladder toward the air side until the air pressure equals the household water pressure. This is shown in Figure 2, which assumes the household pressure is 60 PSI. Please note: Your household pressure varies depending on where you live.

Boyle's Law is applied to the air side of the tank. When your water is heated it expands causing it to flow into the thermal expansion tank. This will further push the bladder toward the air side, reducing its volume even more and increasing its pressure according to Boyle's Law. At some point equilibrium will be obtained at a new higher pressure. In the example of Figure 3 this is shown as 105 PSI. Of course, all this assumes that the plumbing system is drip tight and no one uses any water during this time. If a fixture were to be opened the excess pressure would be relieved bringing it back to the normal household pressure of 60 PSI.
Using the Thermal Expansion Tank Calculator
To aid customers in understanding these principles and selecting a thermal expansion tank, DelCo has provided a tool that calculates the minimum size thermal expansion tank for your home. Click here to use the calculator.

The inputs to the program and their explanation are given below:

Cold Water Temperature:
This is the temperature of the water inside the hot water tank before it starts heating. This may be the most difficult parameter to estimate. Normally, hot water is being removed from your tank at the same rate cold water is entering so the temperature does not get low. However, under certain conditions it may approach the temperature of the water entering your house. If you use a large amount of hot water at one time, or if the tank heating element does not function for an extended period, the temperature of the incoming water could be very low.

Hot Water Temperature:
This is the temperature to which your water is heated. It is determined by the thermostat setting. Many people keep their residential hot water heaters set between 130 and 140 degrees Fahrenheit.

Thermal Expansion Tank Pre Pressure:
Thermal Expansion tanks usually come from the store pre-charged to about 40 PSI. Ideally, you will want to increase the pressure to match that of the Del-Co System Water Pressure. This will result in the lowest size expansion tank after heating. CAUTION! Some tank manufactures recommend maximum pre-charge pressures for their tanks. Do not exceed their recommendations.

Hot Water Tank Volume:
This is the size of your hot water tank. A common size for residential hot water tanks is 40 gallons.

Del-Co System Water Pressure:
This is the pressure of the distribution system where it enters your home. It is also your household pressure assuming you do not have a pressure reducing valve on your service line where it enters your house. The best way to obtain this pressure is to put a pressure gage on one of your fixtures.

Maximum Allowable Pressure in Plumbing:
As the water volume increases due to thermal expansion and flows into the thermal expansion tank, it will compress the air and increase the pressure in your plumbing. The size of the expansion tank, along with the other parameters described above, determine how high the pressure will rise. Enter the maximum pressure you want to allow in your plumbing. Generally, you will want this pressure to be less than the setting on the pressure relief valve on your hot water heater so that it will not open.

Use of the calculator can be best understood by going through some examples.

Example 1
Assume the following input parameters:
  • Cold Water Temp = 45 degrees Fahrenheit
  • Hot Water Temp = 135 degrees Fahrenheit
  • Hot Water Tank Volume = 40 gallons
  • Tank Pre Pressure = 40 PSI
  • Del-Co System Water Pressure = 70 PSI
  • Maximum Allowable Pressure in Plumbing = 130 PSI
Press "Calculate" and the results display:
Minimum Thermal Expansion Tank Size = 2.18 Gallons
Under these conditions, you would want to select a thermal expansion that is the next standard size larger than 2.18 gallons.

Example 2
In this example, use the same input parameters as before, only change the size of the hot water tank to 80 gallons. The input parameters are as follows:
  • Cold Water Temp = 45 degrees Fahrenheit
  • Hot Water Temp = 135 degrees Fahrenheit
  • Hot Water Tank Volume = 80 gallons
  • Tank Pre Pressure = 40 PSI
  • Del-Co System Water Pressure = 70 PSI
  • Maximum Allowable Pressure in Plumbing = 130 PSI
Press "Calculate" and the results display:
Minimum Thermal Expansion Tank Size = 4.36 Gallons
Larger hot water tanks require larger thermal expansion tanks.

Example 3
In this example, see what the effect is of adding air to pre-charge the thermal expansion tank. Use the input parameters from Example 2, only change the Tank Pre-charge Pressure to match the Del-Co System Water Pressure of 70 PSI. The input parameters are as follows:
  • Cold Water Temp = 45 degrees Fahrenheit
  • Hot Water Temp = 135 degrees Fahrenheit
  • Hot Water Tank Volume = 80 gallons
  • Tank Pre Pressure = 70 PSI
  • Del-Co System Water Pressure = 70 PSI
  • Maximum Allowable Pressure in Plumbing = 130 PSI
Press "Calculate" and the results display:
Minimum Thermal Expansion Tank Size = 2.82 Gallons
As you can see, adding air to the thermal expansion tank results in a lower size tank. The optimum pre-charge pressure for the tank is equal to the system pressure. CAUTION! The thermal expansion tank should be pre-charged before it is installed on your plumbing. Adding air after it is installed will not have exactly the same effect.

Example 4
For a final example, assume you have an older house and are concerned about the letting the pressure in your plumbing get too high. Use the input parameters from the previous example, only reduce the Maximum Allowable Pressure in Plumbing to 100 PSI. The input parameters are as follows:
  • Cold Water Temp = 45 degrees Fahrenheit
  • Hot Water Temp = 135 degrees Fahrenheit
  • Hot Water Tank Volume = 80 gallons
  • Tank Pre Pressure = 70 PSI
  • Del-Co System Water Pressure = 70 PSI
  • Maximum Allowable Pressure in Plumbing = 100 PSI
Press "Calculate" and the results display:
Minimum Thermal Expansion Tank Size = 4.46 Gallons
From this example you can see that in order to reduce the allowable pressure you must provide a larger thermal expansion tank.