Water is also hydrolic, and can not be compressed. Since hot water will take up more space than cold, This water WILL go some where, the expansion can happen on your tank causing premature tank failure, or if you have an old water meter with out a check valve the water may actually push back down the line causing the meter to spin backwards. With reclaimed water systems being installed all over down here, they (the city) are installing meters with double check valves, they will then require a means of thermal expansion in the system. This may be done with 2 devices that I am familiar with. 1- Expansion tank- see manufactureer recomendations for appropriate size to heater and installation locations ( usually the inlet side between the heater and the water service but be sure there arent any other check valves in the sysem ie circ line etc. and then the blow off device, usually installed on a tee at the first hose bibb above the water service or homes main shut off. If there is not a check valve on the meter then the inspectors here will allow the meter as a meens of thermal expansion. We have some great inspectors here, who use there heads!! I know you don't hear that very often.