You may have a thermal expansion tank in your home, or you may not have one but should. On this page I want to describe and explain the function of a thermal expansion thank. You should then know if you need one, and how to know when you must replace one.
The first order of business must be to help you to understand thermal expansion. For this I will borrow from my page on water heater dip tubes.
All matter is made up of atoms and molecules. These particles are constantly vibrating and this vibration creates space between them. As molecules absorb heat they vibrate faster, this expands the space between the molecules and so expands the space they fill as a mass.
Consider the water that enters your house via the cold water supply system. Some of it will go into the water heater to be heated, when it does it's volume will increase. It's mass, (atomic weight), will not increase, but it's volume will and since water is not compressible, nothing will stop it. This increase in volume is what we are referring to as thermal expansion.
There was a time when thermal expansion simply pushed back into the gravity fed, (by virtue of water towers), municipal water system. The modern push to create separation between the municipal water system and the individual systems of it's end use consumers has driven municipalities to insert various kinds of check valves at each meter head. These check valves are in place to protect the municipal water distribution system and one must be sympathetic to this modern requirement. They do however create "Closed Systems" down stream of each check valve. Practically speaking, in each home. Now that most homes are on closed water supply systems the thermal expansion we never had to consider has become a problem to resolve.
Now, in many homes, thermal expansion must be absorbed within the system or burst out of it. Fortunately water heaters have long been required to have "Temperature and pressure relief valves", (PTRVs). Also sometimes called T&P valves.These valves are intended to protect the home from the cataclysmic eruption of a water heater if the thermostat fails to stop the heating process once the target temperature has been reached. Since they tend to be the weakest link in a closed residential water system, TPRVs may open to relieve thermal expansion.
Here is what you may experience where thermal expansion tanks are either not employed or are not functioning properly. Some amount of water will flow from the "Drop tube" that is attached to the TPRV. The amount varies depending upon the condition of the TPRV, the size of the water heater, the amount of hot water that has been used, and the difference between the temperature of the cold water coming into the heater and the target temperature the heater is set to. Because of this last factor, thermal expansion may be greater during the winter. If you regularly see water emanating from the TPRV's drop tube or you must often replace your TPRV, it could be the result of uncontrolled thermal expansion.
Other factors that are at play and may be confusing your problem solving diagnostic are:
A) At times thermal expansion may occur during a period of use. When a large quantity of hot water has been consumed, and the resultant thermal expansion occurs, while the system is open because cold water is being consumed during the water heater's recovery cycle, the usual spillage may not occur.
B) Thermal expansion is compounded during winter not only because the incoming water is colder and therefore smaller then, but because showers run longer and hotter in that season as consumers want to be warmer.
A thermal expansion tank is a miniature version of the pressure tank one installs into a well water system. It's main function is also the same, that is to create a reservoir of compressible air in a system that is otherwise filled with only non-compressible water. The tank has two chambers that are separated by a stretching membrane. One chamber is filled with water and the other is filled with air. The membrane is meant to be relaxed in use so the air chamber must be pumped till it matches the static pressure of the system it will be installed into. Since water will not compress the air side of the tank must be pressured up before the water side is. Once installed and under pressure the tank should be half filled with air and half filled with water.
There are large and small thermal expansion tanks, properly sized and pressurized a tank should absorb the thermal expansion created by the consumption of all of the available hot water at the lowest possible inlet water temperature. Larger water heaters and larger systems require lager tanks.
When thermal expansion occurs during a non-use period the membrane in the tank pushes toward the air side, compressing the air. At the next usage of water, be it hot or cold, the membrane relaxes and the accumulated pressure is run out of the system.
Typically there are two water systems in a home, one is the cold water supply and the other is the hot water supply. The main point of separation is not the water heater as you might suppose, it is the supply valve that is just upstream of the water heater, usually leading to the cold side flex connector. A second valve on the downstream side, (after the hot side flex connector), is not a point of separation since the water heater is part of the hot water supply system.
Your thermal expansion tank must be filled with cold water but must be installed into the hot water system.
The proper place to install your thermal expansion tank is downstream of the water heater's inlet control valve and upstream of the water heater. By placing it there you keep it filled with cold water and the water heater's supply valve which is used to isolate the water heater and the hot water system will also isolate the thermal expansion tank. For full isolation install a second valve down stream of the T-E tank.
Your thermal expansion tank is not a pass-through fixture, it uses a single port for both inlet and outlet flows. Install it by adding a single tee fitting to the water heater inlet pipe. It does not have an attitude requirement so it can be installed at any angle that is convenient. It does require support though as it will get heavy in use and heavier yet if it's membrane fails. It may hang vertically from or sit vertically on the pipe it is joined to if that pipe has proper rigidity and support. Otherwise it should be independently supported. It may be joined via a pipe nipple, a section of pipe, or a flexible supply as long as it is properly supported.
Do read and follow the instructions! As many of these tanks and other fixtures I have installed over the decades I continue to peruse the instructions and manufacturers recommendations.
Unfortunately your thermal expansion tank is not going to last forever. I installed one in my home when I began to experience spillage at my TPRVs drop tube. It solve the problem but a mere five years later the spillage recurred. When I depressed the air valve stem on the air side of the tank to check the air pressure water came out instead of air. I knew then that my tank's membrane had failed. When that happens the water in the tank absorbs the air it then has
access to. Once the air is fully dissolved into the water the tank is completely filled with water and can no longer function unless air is added again. Because my tank hangs from the supply pipe the air valve points downward and any air I pump into the tank will flow into the system and not remain in the tank. If I had set the tank so that the air valve was up I could have added air but I would have had to make that a regimen of maintenance till I replaced the tank. If I had installed my tank facing downward, and had wanted to add air, I would have closed the service valve, opened the downstream system, and let all of the water drain from the tank. To do this I would have opened the air valve at the top of the tank. Once empty I would have closed the air valve and the downstream system, then opened the service valve. The air to water ratio may not come out 50/50 but it will work till the air is reabsorbed.