To install a water heater expansion tank, first turn off the water supply and drain the water heater, then connect the expansion tank to the cold water line using a T-fitting, ensuring proper alignment and sealing with Teflon tape, and finally, restore the water supply and check for leaks.
A water heater expansion tank is a small but crucial component that protects your plumbing system from excessive pressure. When water heats up, it expands, and without a proper outlet, this can strain pipes, valves, and even the water heater itself. This guide will walk you through the complete installation process.
Why You Need an Expansion Tank
In closed plumbing systems (where a check valve or backflow preventer exists), thermal expansion has nowhere to go. This creates dangerous pressure spikes that can:
- Damage water heater components
- Cause leaks at pipe joints
- Trigger the temperature-pressure relief (TPR) valve unnecessarily
Many building codes now require expansion tanks, especially for tankless systems. Even if not mandated, they’re recommended for all water heaters. For more on system types, see our guide on direct vent vs power vent water heaters.
Tools and Materials Needed
| Tools | Materials |
|---|---|
| Pipe wrench | Expansion tank (properly sized) |
| Channel-lock pliers | 3/4″ copper tee fitting |
| Teflon tape | Copper pipe (short sections) |
| Drill with bits | Mounting straps/brackets |
| Soldering equipment (if not using push-fit) | Dielectric union (if connecting dissimilar metals) |
Step-by-Step Installation Guide
1. Shut Off Water and Power
Turn off the main water supply to your home. For electric water heaters, shut off power at the breaker. For gas units, turn the gas control valve to “pilot” or shut off the gas supply line.
2. Drain Some Water
Open a hot water faucet somewhere in the house to relieve pressure. Drain about 2-3 gallons from the water heater using the drain valve.
3. Choose Installation Location
The ideal spot is on the cold water supply line, within 6 feet of the water heater inlet. It can be mounted vertically or horizontally, but vertical is preferred when possible.
4. Install Tee Fitting
Cut into the cold water line and install a 3/4″ copper tee. Use either:
- Sweat-soldered connections (most permanent)
- Push-fit fittings (easiest for DIY)
- Threaded fittings (if pipes are already threaded)
5. Mount the Expansion Tank
Secure the tank to wall studs or blocking using manufacturer-supplied straps. Position it so the connection points align with your tee fitting. For more on proper water heater setup, see our parallel water heater installation guide.
6. Make the Connections
Wrap male threads with Teflon tape (4-6 wraps). Connect the tank to the tee using a short section of pipe. Install an isolation valve if desired for future maintenance.
7. Restore Water and Test
Slowly turn water back on and check for leaks. Open hot water faucets to purge air. Turn power/gas back on. Monitor system pressure – it should stabilize between 40-80 psi.
Expansion Tank Maintenance
To ensure proper operation:
- Check air pressure annually (should match incoming water pressure)
- Listen for waterlogged tanks (should sound hollow when tapped)
- Watch for leaks at fittings
- Replace every 5-7 years or if diaphragm fails
For more on water heater maintenance, check our article on anode rod replacement.
Common Installation Mistakes
Incorrect Sizing
Tanks that are too small won’t handle expansion volume. Follow manufacturer sizing charts based on:
- Water heater capacity
- Incoming water pressure
- Maximum temperature setting
Poor Location
Avoid installing:
- Too far from water heater (reduces effectiveness)
- In inaccessible spots (hinders maintenance)
- Without proper support (can stress pipes)
Improper Pressure
Pre-charge pressure must match household water pressure. Use a tire gauge to check and adjust (via Schrader valve) before installation. According to This Old House, mismatched pressure is the leading cause of premature failure.
When to Call a Professional
Consider hiring a plumber if:
- Your plumbing system uses uncommon materials (PEX, CPVC, etc.)
- You’re uncomfortable soldering copper pipes
- The installation requires complex rerouting of pipes
- You need to adjust household water pressure
For more complex systems like tankless water heaters with recirculation pumps, professional installation is often recommended.
