How to Install an Expansion Tank
Updated February 20, 2026
Install a thermal expansion tank on your water heater to absorb pressure spikes, protect pipes, and meet code. A $30-50 part that prevents serious damage.
Overview
When water heats up, it expands. In a closed system (any home with a check valve, backflow preventer, or PRV on the main line), that expanded water has nowhere to go. Pressure builds inside the tank and pipes with every heating cycle. Over time: T&P valve dripping, water hammer, stressed fittings, shortened water heater lifespan. An expansion tank absorbs that extra volume. It is a small tank (usually 2-5 gallons) with an internal rubber bladder -- water pushes against the air-charged bladder instead of building pressure. Required by code in most jurisdictions with closed systems. Installs on the cold water line above the water heater.
What You'll Need
Safety First
- Turn off the water heater and let the water cool before working on the cold water line. Hot water under pressure can cause burns.
- Support the expansion tank with a bracket or strap. A full tank weighs 5-10 pounds hanging off a single threaded connection -- unsupported, it will stress and eventually crack the fitting.
Step-by-Step Instructions
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Check Your System Pressure and Pre-Charge the Tank
Use a pressure gauge on any hose bib to read your system pressure. Expansion tanks come pre-charged to a default (usually 40 PSI). The tank's air charge must match your system's incoming water pressure. If your pressure reads 60 PSI, use a bicycle pump or tire inflator on the tank's Schrader valve (looks like a tire valve on top) to inflate the bladder to 60 PSI. Check with a tire gauge.
Tip: This step is critical and most DIYers skip it. A tank pre-charged to 40 PSI on a 70 PSI system will waterlog quickly because the water pressure compresses the bladder flat. Match the charge to your actual pressure. -
Turn Off Water and Relieve Pressure
Turn off the cold water supply to the water heater. Open a hot water faucet nearby to relieve pressure in the system. Turn off the water heater (gas to pilot, electric breaker off).
Tip: If your cold water supply does not have a dedicated shutoff above the water heater, shut off the main. This is also a good time to install one if it is missing. -
Install a Tee Fitting on the Cold Water Line
The expansion tank connects to the cold water line above the water heater. You need a tee fitting. Cut into the cold water line or disconnect at a convenient union. Install a tee with the branch port facing up or sideways (the tank can mount vertically or horizontally, but hanging down from the tee is most common). Use the appropriate connection method for your pipe type -- threaded, soldered, push-fit, or PEX crimp.
Tip: Push-fit (SharkBite) tees make this job much easier if you are not comfortable soldering. Cut the pipe, deburr, push in the tee. Takes 2 minutes. -
Thread the Expansion Tank onto the Tee
Wrap the tank's threaded nipple with 4-5 wraps of Teflon tape. Thread the tank into the tee branch port by hand, then snug with a wrench. Do not overtighten -- the connection is 3/4 inch NPT and only needs to be hand-tight plus one turn with a wrench.
Tip: If the tank hangs below the tee (most common orientation), install a mounting bracket or plumber's strap to support it against the wall or ceiling joist. The weight of a waterlogged tank on an unsupported thread connection will crack the tee. -
Restore Water and Check for Leaks
Close the hot water faucet you opened earlier. Turn the cold water supply back on slowly. Listen for the tank filling -- you will hear a brief rush. Check every connection for leaks. Dry each fitting with a rag and watch for 2-3 minutes. Tighten slightly if needed.
Tip: A tiny weep at the tank thread connection usually means not enough Teflon tape. Shut off, disconnect, add more tape, reconnect. -
Restore the Water Heater and Verify
Turn the water heater back on. Let it go through a full heating cycle (30-60 minutes). Check the T&P valve -- if it was dripping before the expansion tank, it should stop now. Check the expansion tank -- it should feel warm on the bottom (water side) and room temperature on top (air side). If the entire tank is warm, the bladder may have failed and the tank is waterlogged.
Tip: Tap the tank with your knuckle. Bottom should sound solid (water). Top should sound hollow (air). If it all sounds solid, the bladder is not working -- the pre-charge was wrong or the bladder is defective.
Pro Tips
- Check the expansion tank's air pressure annually. Use a tire gauge on the Schrader valve. Recharge with a bicycle pump if it has lost pressure. A tank with no air charge is just a dead weight on your pipe.
- Expansion tanks have a lifespan of 5-10 years. The rubber bladder eventually fails. When the T&P valve starts dripping again, check the expansion tank first.
- Size the expansion tank to your water heater. A 2-gallon tank handles most 40-50 gallon residential heaters. A 75+ gallon heater or a system with very high pressure may need a 5-gallon tank.
- If you have a tankless water heater, you generally do not need an expansion tank -- tankless heaters do not store heated water. But if you have a recirculation loop with a tankless, check with the manufacturer.
- Some jurisdictions require a licensed plumber to install expansion tanks. Check your local code before starting.
When to Call a Pro
Call a plumber if you are not comfortable cutting into the water line, if your pipe material requires soldering (copper) and you do not have the equipment, if your T&P valve is discharging large amounts of water (may indicate a more serious pressure issue), or if you are unsure whether your system is open or closed.