How to Size a Domestic Hot Water Expansion Tank

Updated February 21, 2017

You depend on your hot water supply in your home for cooking, cleaning and personal needs. A home's hot water system may use a gas or electric water heater, or use a water boiler. All hot water systems develop pressure when the water heats. Water leaks or failure of the hot water pressure relief valve can occur during this process. A domestic water expansion tank solves the problem of excess hot water pressure.

Look at the manufacturer's sticker on the side of your water heater. Determine the hot water tank capacity in gallons. Home hot water heaters can range from 20 to over 100 gallons. This capacity affects the size of the expansion tank, larger water heaters will require a larger expansion tank to allow for the increase in hot water volume.

Measure the water pressure of your home cold water system. Read the pressure gauge on your cold water supply if one is installed. Buy a pressure gauge that screws into a hose bib and read the pressure at that point. Your local water utility may provide a reading for you upon request.

Go to your plumbing supply dealer with the hot water tank capacity and pressure reading. They will consult a chart that sizes the expansion tank based on these two factors. Always choose a larger expansion tank, not a smaller one. There is no penalty or risk from an oversized expansion tank, but an undersized one may not protect your plumbing appropriately.

Turn off your water heater and the water supply. Install a tee fitting on the water heater cold water supply. Install the tee on a horizontal section of pipe and point the tee fitting up. Use flux and solder if the pipe is copper or primer and cement for plastic pipe. Install the expansion tank from the tee fitting, positioning it above the piping. Install pipe straps to support the weight of the expansion tank and to reduce stress on the water piping.

Turn on the water supply. Open faucets in the house and vent air from the plumbing before turning on the water heater. Turn on the water heater and check all connections for leaks.


If your hot water relief valve discharges or leaks after installing an expansion tank, you may need to install a larger tank capacity. Discuss any installation problems with your plumbing supply company or a licensed plumber.


Do not mix different types of piping in plumbing systems. Use the same pipe material for any renovation or addition. Different pipe materials, particularly different metals, can cause galvanic corrosion resulting in leaks and plumbing failures. Plastic pipe cement contains chemicals that may be harmful if inhaled or from skin contact. Maintain good air ventilation when cementing plastic pipe.

Things You'll Need

  • Expansion tank
  • Piping tee
  • Pipe, use the same material as your existing plumbing
  • Propane torch, solder and flux if using copper pipe
  • Plastic pipe primer and cement if using CPVC plastic pipe
  • Pipe straps
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About the Author

Paul Richard began writing in 2002 after a career in chemical processing, refrigerant alternatives and workplace safety. He has written articles for the "Cecil Whig" and "Air Conditioning, Heating and Refrigeration News." Richard holds a Bachelor of Science in chemical engineering from the University of Akron.