Did you know that every time your water heater runs, your plumbing system is under pressure—literally? That’s because heating water makes it expand, and in a closed system (one with a backflow preventer or pressure regulator), that extra volume needs somewhere to go. If it doesn’t, you risk leaks, damaged valves, or even a burst water heater. Enter the water heater expansion tank: a small, simple device that protects your pipes, your appliances, and your wallet. This guide covers everything you need to know about expansion tanks, the best models to buy, how to install one, and the top questions homeowners ask.
Quick Picks: Best Water Heater Expansion Tanks
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Best Overall:
Amtrol ST-5 Thermal Expansion Tank – Trusted, easy to install, and perfect for 40- to 80-gallon water heaters. -
Best for Large Homes:
Amtrol ST-12 Expansion Tank – Higher capacity for 80+ gallon tanks or high-pressure systems. -
Best Budget:
Watts DET-5 Thermal Expansion Tank – Solid performance at a wallet-friendly price.
What Is a Water Heater Expansion Tank?
- An expansion tank is a small, pressurized tank installed on your home’s cold water supply line, usually near the water heater.
- It has an air-filled bladder inside that absorbs excess pressure as heated water expands, preventing stress on your plumbing.
- Especially important in homes with “closed” plumbing systems—ones with a check valve, backflow preventer, or pressure reducing valve (PRV).
Why Do You Need an Expansion Tank?
- Protects Water Heater and Pipes: Reduces the risk of burst tanks, leaking valves, and other costly damage.
- Prevents T&P Valve Leaks: The temperature and pressure (T&P) relief valve won’t drip or open as often.
- May Be Required by Code: Many cities and inspectors require expansion tanks for all new water heater installs.
- Prevents Water Hammer: Stops sudden banging pipes caused by quick changes in pressure.
- Extends Appliance Life: Protects dishwashers, washing machines, and faucets from pressure spikes.
Buyer’s Guide: Choosing the Right Expansion Tank
- Size: Match the expansion tank to your water heater’s size (gallons) and home water pressure. Most homes use 2-5 gallon tanks for 40–80 gallon heaters.
- Pressure Rating: Ensure the tank is rated for your system’s pressure (typically 40–80 PSI).
- Pre-Charge Pressure: Tanks come pre-charged (usually 40 PSI) but can be adjusted with a standard tire gauge and pump to match your home’s pressure.
- Certifications: Look for NSF, ANSI, and UPC listings for safe potable water use.
- Build Quality: Choose heavy-duty tanks with butyl rubber bladders for long life and no odor or taste in your water.
- Warranty: Most reputable brands offer 5-year warranties or longer—buy from a trusted source.
How to Install a Water Heater Expansion Tank (DIY Tips)
- Turn Off Water: Shut off your home’s main water supply.
- Find the Right Spot: Locate the cold water line going into your water heater—this is where the expansion tank goes.
- Install a Tee Fitting: Cut the cold water line and add a tee or threaded fitting.
- Attach the Expansion Tank: Screw the tank onto the fitting (use Teflon tape for a watertight seal).
- Support the Tank: If it hangs sideways, use a bracket or strap to prevent pipe strain.
- Check Pressure: Before turning water back on, use a tire gauge to set the tank’s air pressure to match your house pressure.
- Turn Water On & Check for Leaks: Restore water supply and inspect all joints.
Tip: Most handy homeowners can install an expansion tank in under an hour. If you’re unsure, call a licensed plumber for help and peace of mind.
Detailed Reviews: Best Water Heater Expansion Tanks
Amtrol ST-5 Thermal Expansion Tank
- For 40–80 gallon water heaters (2-gallon tank size)
- Pre-charged to 40 PSI, adjustable with a hand pump
- Butyl rubber bladder, NSF/ANSI 61 certified
- 5-year warranty, made in the USA
Final Verdict: The best all-purpose expansion tank for most homes—durable, easy to install, and code-approved.
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Amtrol ST-12 Expansion Tank
- For 80+ gallon heaters, high pressure, or large homes (4.4-gallon size)
- Flexible and reliable—pre-charged, adjustable, and sturdy
- Long-lasting butyl bladder and steel construction
- 5-year warranty, trusted by pros nationwide
Final Verdict: The upgrade pick for big homes or commercial installs.
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Watts DET-5 Thermal Expansion Tank
- Fits most 40–50 gallon water heaters
- Easy to adjust, quality butyl diaphragm
- Affordable and reliable
- NSF and ANSI certified for potable water
Final Verdict: The best value for tight budgets and smaller homes.
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Comparison Table: Best Expansion Tanks for Water Heaters
| Model | Size | Fits Tank | Certifications | Warranty | Best For | Price Link |
|---|---|---|---|---|---|---|
| Amtrol ST-5 | 2 gal | 40–80 gal | NSF/ANSI 61 | 5 yr | Most homes | View |
| Amtrol ST-12 | 4.4 gal | 80+ gal | NSF/ANSI 61 | 5 yr | Large homes, high pressure | View |
| Watts DET-5 | 2 gal | 40–50 gal | NSF/ANSI | 1 yr | Budget/small home | View |
FAQ: Water Heater Expansion Tanks
- Do I really need an expansion tank?
If your home has a “closed” plumbing system (with a PRV or check valve), you need one to prevent dangerous pressure build-up. Many cities require them for all new water heater installs. - How do I know what size tank I need?
Most 40–80 gallon heaters use a 2-gallon expansion tank. Bigger tanks or higher pressure systems (over 80 PSI) need larger models. - Can I install an expansion tank myself?
Yes, if you’re comfortable with basic plumbing. Always set the air pressure to match your home’s water pressure for best results. - How long do expansion tanks last?
5–10 years is common. Replace if you hear water inside the air chamber, see leaks, or notice pressure problems. - Do they work with tankless water heaters?
Expansion tanks are usually needed for tank-type heaters, but some closed-loop tankless setups may benefit too—check your local code.
Conclusion
A water heater expansion tank is a small investment that protects your whole plumbing system, saves you money on repairs, and may be required by local code. Choose one of the top-rated models above, match the size to your water heater, and enjoy years of stress-free hot water. Ready to upgrade? Click through to the best prices and install with confidence!