Many homes have a closed system which is a water system with a one way valve such as a backflow valve, check valve, or pressure reducing valve. In a closed system this extra water pressure from thermal expansion can cause the most damage because the extra water pressure has nowhere to go since it cannot push back into the city water. Most cities require that expansion tanks be installed in closed systems. If you have some form of backflow preventer installed on the main water line you may be required to also have a water heater expansion tank installed. Check the local building code or call the building and safety department to find out.
If you do not have a one way valve installed on the main water line, and therefore do not have a closed water system, you may still benefit from a water heater expansion tank. Expansion tanks are normally installed above the water heater on the cold side before the water comes into the water heater. The extra hot water from the thermal expansion can be stored in the expansion tank and when hot water is demanded it is put back into the water heater. Having an expansion tank helps prevent dripping faucet and running toilets by keeping the extra built up pressure from reaching the fixtures and prematurely damaging them.
Water heater expansion tank prices start at around $40 and the tank is easy to install with a normal 3/4” threaded connection. A tee will have to be installed above the water heater if one is not there already. If you are hiring a plumber to have a water heater installed it’s a good idea to get a price for the installation of an expansion tank also. It may not be much more than you are already paying if it is done at the same time. If you are installing a water heater yourself consider installing an expansion tank even if it is not required since the benefits far outweigh the work and cost involved.