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Inspecting Expansion Tanks on Hot Water Tanks

Most of the newer hot water tanks I see have expansion tanks installed. In past years it was rare to see one on a residential hot water tank. This Tech Tip will outline the reasons for installing an expansion tank, how to properly install one, and finally how to inspect them. Expansion tanks are being installed and in some instances required on hot water tanks because many municipalities are now installing back flow preventer valves after (house side) the main shut-off valve. Back flow valves prevent house water from backing up into the municipal water supply, possibly contaminating it. There may also be a pressure reducing valve installed. This valve will act as a back flow valve because it is a one way valve. Water expands 2% or more in a hot water tank when heated. If the pressure inside the tank does not exceed the capacity of the Pressure Temperature Relief Valve (PTRV), water will be backed up into the water supply pipe possibly contaminating the fresh water supply. A back flow preventer valve will not allow this to happen. This however will cause considerable pressure in the hot water tank and water pipe. The expansion tank will absorb the water and reduce the pressure in the tank and water line. It will also ensure the PTRV does not continually open when this condition is present. Also, if the water pressure coming into the house is near 80 PSI, an expansion tank will protect the tank and water pipes from excessive pressure. It will also prevent the PTRV from opening. When inspecting a hot water tank, here are some tips;

  • If a back flow preventer valve, or pressure reducing valve is installed, ensure an expansion tank is installed
  • If the water pressure to the house is near 80 PSI an expansion tank should be installed
  • The preferred method of installation is to install the expansion tank vertically, however it can be installed horizontally (see drawing)
  • The house water supply should be distributed, if possible, before the expansion tank
  • The expansion tank should be installed on the cold water supply pipe
  • Ensure the expansion tank and connections are not leaking
  • Ensure the expansion tank is properly supported, especially if installed

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