Replace Thermal Expansion Tank on Water Heater When performing the water heater flush, I noticed that the thermal expansion tank was saturated. The thermal expansion tank gives water, and the increased pressure associated with heating, a place to go. It is a safety device put in place to help maintain a suitable pressure for your home. I arrived today to flush a tanked style water heater that is located in the crawlspace. Previously, Nathan has came out here to perform work that was needed for the homeowners. They moved in here last September. It was noted that the water heater was in great condition, and up to code. I explained to the homeowner I would flush the heater, and also check to make sure that it was still operating properly. I began by turning the gas off to the unit using the gas valve in the water heater. I then proceeded to connect the hose to the drain, and started to drain the heater with water running through it. I left the water running until I heard a good flow run