A thermal expansion tank is needed anywhere when certain conditions exist. First, you need to understand a couple of basic principals of physics. Heated water expands and water does not compress. When a water heater heats water, the water expands and has to go somewhere. If the water supply pressure does not come through a pressure regulator valve (PRV) to reduce overly high pressure, the expansion simply is absorbed by the city water main. This is called an "open system". However, many times the incoming pressure is too high and a PRV is installed to lower the pressure. Within the PRV, there is a check valve that prevents the expanded water from passing through and back into the water main. This is called a "closed system". Since water will not compress, what happens to the expansion? It builds up pressure in the water heater until the safety limits of the temperature/pressure valve on the water heater is reached. At that point the T/P valve opens and relieves the pressure. If working properly, a fairly small amount of water is released, the pressure is reduced, and the valve closes. A bit of a puddle of water, but not a flood. Sometimes the T/P will stick and not close and you do get a flood. Now enter the thermal expansion tank. This is placed in the water supply between the water heater and the PRV. It's internal air pressure is adjusted to match the PRV's pressure setting, and when the water expands it is absorbed by the expansion tank. When the heating stops, the pressure lowers and the expansion tank releases the collected water back into the system. Some newer water meters being installed by cities today have a check valve built into them so even without a PRV, you would have a closed system. In addition, some PRVs have a bypass which allows the pressure build up to pass. First thing to do is to get a decent pressure gauge and determine what your incoming water pressure is. The best kind has two needles. One to record the highest pressure of the day or night, and the other the current pressure. Sometimes pressure is is considerably higher at night than in the daytime. If you do need a PRV and expansion tank, you will need a gauge to set the PRV and expansion tank pressure. It's kind of nice to know what your pressure is anyway.