Short answer
Measure your house water pressure and set the tank’s air pre-charge to match before installing (Watts: never above 80 psi). Shut off the heat and the cold supply, relieve pressure, cut a tee into the cold line between the shutoff and the heater, thread the tank onto the tee with sealant, and support it with a strap or bracket. Restore water, check for leaks, then restore heat.
What you need
- Potable expansion tank About 2 gallon for most 40 to 50 gallon heaters at normal pressure; check the maker chartFind it
- Water pressure gauge Screws onto a hose bibFind it
- Tire gauge and bike pump To set the air pre-chargeFind it
- Tee fitting and connectors Push to connect, solder or threaded, to suit your pipeFind it
- Thread sealant or tapeFind it
Steps
Measure house pressure
Screw a pressure gauge onto a hose bib or the heater drain valve, with no water running. Write the number down.
Set the pre-charge
Check the tank’s air valve with a tire gauge before installing. Add air with a bike pump or release some until it matches your house pressure. Watts says never exceed 80 psi.
Shut off heat and water
Set the gas to pilot or off, or switch off the breaker. Close the main water shutoff or the heater’s cold supply valve, and open a hot tap to relieve pressure.
Install the tee
On the cold line between the shutoff and the heater, cut in a tee with the branch pointing where the tank will hang. Use fittings that suit your pipe: push to connect, sweat or threaded.
Thread on the tank
Seal the threads and screw the tank onto the tee branch. Do not twist the tank body against a tight joint; use a wrench on the fitting.
Support it
Strap or bracket the tank to the wall or a joist so its full weight does not hang on the pipe.
Restore water, then heat
Open the water slowly, bleed air at a hot tap, check every joint for drips, then restore gas or power.
Check it works
After the heater runs a full cycle, the relief valve should stay dry and house pressure should rise only slightly. A year on, tap the tank: the top half should sound hollow. If the air valve spits water, the diaphragm has failed and the tank needs replacing. Details in the expansion tank guide; dripping valve causes in the T&P valve guide.
Questions people ask
Where should an expansion tank be installed?
On the cold water supply to the water heater, between the cold shutoff valve and the heater, so it stays connected to the tank when the shutoff is open. Follow the tank maker’s orientation rules.
Can an expansion tank be installed horizontally or upside down?
Many makers allow vertical or horizontal mounting with support. Check the instructions for your tank; the support rule matters more as it fills.
Does an expansion tank need support?
Yes, typically. A full tank is heavy for a 3/4 inch pipe joint, so use a strap, bracket or support kit as the maker shows.
Why is my T&P valve still dripping after adding an expansion tank?
The pre-charge may not match the house pressure, the tank may be too small, the house pressure may be too high, or the relief valve itself may be worn. Check each in that order.
Sources
Last reviewed October 29, 2026. Models, codes and listings change; check your heater’s manual and your local code, and follow the maker’s instructions.