With a water heater when it is heating the water there will be some expansion, which in a closed system will increase the pressure and cause the T&P Valve on the water heater to discharge. One of the functions of the T&P Valve is to open as a safety mechanism when the pressure in the water heater tank reaches 150-psi. The amount of expansion that occurs is determined by how much water is in the tank and the difference in the temperature of the water at the start of the heating process and at the finish of the heating process. The larger the volume of water is being heated and the larger the temperature differential the more expansion you will have. As an example if a new 40-gallon water heater was installed and filled with 40-degree F water, which was then heated to 125-degrees F the amount of expansion would equal about .44-gallon. If we increased the temperature to 140-degrees F the amount of expansion would equal .6-gallon. This scenario would result in the expansion tank holding from 1 to 1.5 gallons of water when properly installed. While the actual volume change is very small it is easily masked by dripping faucets and leaking toilets often causing that you fixed my toilet the other day and now my water heater leaks type of call. The effects of thermal expansion can have a detrimental effect on the service life expectancy of a water heater. The water heater is a pressure vessel made in most cases from steel with a glass lining applied to the inside of the tank to prevent corrosion. As the pressure increases in the tank from the supplied water pressure to the 150-psi where the relief valve opens the tank will start to bulge from the added pressure. Then when the added pressure is released the tank will spring back to its original shape. These two events combine to become a pressure cycle and contribute to cracking of the glass lining exposing the steel to water causing corrosion and also metal fatigue which will eventually cause the metal to crack at points such a seams where the top and bottom of the tank are welded on. In a worst-case scenario with a failed T&P valve not opening the pressure increase may cause the tank to permanently bulge, which would result in the tank having to be replaced. Installing a thermal expansion tank will eliminate pressure cycling greatly increasing the service life expectancy of your water heater.
The thermal expansion tanks used on water heaters should be listed as “Potable Water Expansion Tanks” which means they are constructed from materials that are safe to use on drinking water systems. Expansion tanks are also used on hot water boilers but the water in those systems are never consumed so the materials used are different. Make sure the right type of expansion tank is used on your potable water system. The expansion tank should be connected on the inlet side of the water heater and connected by a tee between the inlet to the water heater and the shut off valve on the inlet of the water heater. There should be no valve between the water heater and the expansion tank, which would defeat the protection provided by the expansion tank. The water in the tank will add considerable weight to the expansion tank and it should be adequately supported to avoid placing stress on the pipes. The tank may have a longer line plumbed to it allowing the tank to be remotely mounted. it does not have to be directly on top of the water heater.
Potable Water Thermal Expansion Tanks are made by a few different companies, Watts with their model PLT-5, PLT-12, PLT-20, and PLT-35 tanks used in residential and light commercial, and Amtrol with their Therm-X-Trol model ST-5, ST-8, ST-12, and ST-25V tanks also used in residential and light commercial are very popular. Larger commercial water heating systems would require using an ASME Rated expansion tank. The tanks are made of drawn steel that has a rubber bladder dividing the tank into two sections. On one side there is a threaded connection that allows water to enter the tank. On the other side there is an air valve similar to the one on a tire through which the air pressure of the pre-charge on the air side of the bladder can be adjusted and checked. The pre-charge should match the typical pressure on your water supply system. The air is able to compress which allows the expanding water to enter the expansion tank with only a slight increase in system pressure preventing the water heater tank from experiencing pressure cycles and providing a substantial increase of the service life expectancy of your water heater.