Sizing a Thermal Expansion TankĀ 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.
Tankless Hot Water Heaters
If you have a tankless hot water heater there is still a small amount of heated water stored within your plumbing system. Therefore it is the recommendation of Del-Co Water that you have a thermal expansion tank installed inside your home.