Does the Plumbing Code Require an Expansion Tank on a Water Heater?

If you’ve recently had plumbing work done or are troubleshooting water pressure issues, you might be asking: “Does the plumbing code require an expansion tank on a water heater?” You’re not alone. Many U.S. homeowners are surprised to learn that modern plumbing systems often do require an expansion tank—but only under specific conditions. In this guide, we’ll break down exactly when it’s mandatory, why it matters, and what could happen if you ignore it.

What Is a Thermal Expansion Tank and Why Does It Matter?

A thermal expansion tank is a small, pressurized vessel installed on the cold water supply line near your water heater. Its job? To absorb excess water pressure caused by thermal expansion—a natural phenomenon that occurs when water heats up and expands in a closed plumbing system.

Without a place for this expanded water to go, pressure can build to dangerous levels—potentially damaging pipes, faucets, your water heater, or even causing leaks or bursts.

According to the Uniform Plumbing Code (UPC) and International Plumbing Code (IPC)—the two primary plumbing codes adopted across the U.S.—thermal expansion control is required in closed systems. But what exactly is a “closed system”? That’s the key.

“Thermal expansion in a closed-loop system is one of the leading causes of premature water heater failure,” says plumbing engineer Mark Richardson, P.E., in a 2023 report by the American Society of Plumbing Engineers (ASPE).

When Does the Plumbing Code Actually Require an Expansion Tank?

Here’s the crucial distinction: an expansion tank is only required if your home has a “closed” plumbing system.

What Makes a System “Closed”?

A plumbing system becomes “closed” when a check valve, backflow preventer, or pressure-reducing valve (PRV) is installed on the main water supply line. These devices prevent water from flowing backward into the municipal supply—but they also trap expanded hot water inside your home’s pipes.

Most modern homes have one of these devices, especially in areas with:

• Municipal water systems that require backflow prevention (common in California, Texas, and Florida)
• High incoming water pressure (typically over 80 psi), which necessitates a PRV

If your system is open (no check valve or PRV), thermal expansion can safely flow back into the main water line—so an expansion tank isn’t required by code.

🔍 Pro Tip: Not sure if you have a closed system? Check your main water line near the water meter. If you see a bell-shaped or cylindrical device (often brass), that’s likely a PRV or backflow preventer—and you probably need an expansion tank.

Plumbing Code Requirements: UPC vs. IPC

Both major U.S. plumbing codes address thermal expansion—but with slightly different wording.

Most states and municipalities adopt either the UPC or IPC (or a modified version). For example:

• California, Arizona, and Washington follow the UPC
• New York, Texas, and Illinois follow the IPC

👉 Bottom line: If your local code follows either standard—and you have a closed system—an expansion tank is required by law during new water heater installation or major plumbing renovations.

What Happens If You Don’t Install an Expansion Tank (When Required)?

Ignoring the code isn’t just a paperwork issue—it can lead to real-world consequences:

• Voided water heater warranty: Most manufacturers (like Rheem and AO Smith) explicitly void warranties if thermal expansion damage occurs in a closed system without an expansion tank.
• Premature water heater failure: A study by the Plumbing-Heating-Cooling Contractors Association (PHCC) found that water heaters in closed systems without expansion tanks fail 2–3 years earlier on average.
• Leaks or pipe bursts: Pressure spikes from thermal expansion can exceed 150 psi—even if your home’s normal pressure is 60 psi.
• Failed plumbing inspections: Your new water heater installation may not pass inspection, delaying occupancy or requiring costly rework.

⚠️ Real-World Example: In 2024, a homeowner in Austin, TX had to replace both their water heater and drywall after a burst fitting caused $8,000 in water damage—all because a PRV was installed but no expansion tank followed.

How to Install a Thermal Expansion Tank: A Step-by-Step Guide

If you’ve confirmed your system is closed, here’s how a licensed plumber (or skilled DIYer) would install an expansion tank:

1. Turn off power/gas and water supply to the water heater.
2. Drain a few gallons from the tank to reduce pressure.
3. Install a tee fitting on the cold water inlet pipe, within 3–5 feet of the water heater.
4. Mount the expansion tank vertically (or per manufacturer instructions) using pipe straps.
   • Common size: 2-gallon tank for 40–50 gallon water heaters
   • Pre-charged pressure: 38–40 psi (must match your home’s water pressure)
5. Check air pressure in the tank’s Schrader valve with a tire gauge before installation. Adjust if needed.
6. Turn water and power back on, then test for leaks.

📌 Note: Expansion tanks typically last 5–10 years. Test them annually by pressing the Schrader valve—if water comes out, the bladder has failed and it needs replacement.

For more on how water pressure and thermal dynamics work, see this Wikipedia article on thermal expansion.

Expansion Tank: Pros and Cons

Despite the minor drawbacks, the safety and compliance benefits far outweigh the costs—especially in closed systems.

FAQ Section

Q1: Is an expansion tank required by code in all U.S. states?

A: No. It’s only required in homes with closed plumbing systems, regardless of state. However, most states that adopt the UPC or IPC enforce this rule during inspections. Always check with your local building department.

Q2: Can I install a water heater without an expansion tank if I don’t have a PRV?

A: Yes—if your system is open (no check valve, PRV, or backflow preventer), thermal expansion can safely dissipate into the main water line, and no expansion tank is needed per code.

Q3: How do I know if my expansion tank is working?

A: Press the Schrader valve (like a tire valve) on the tank. If air comes out, it’s functioning. If water comes out, the internal bladder has ruptured and the tank must be replaced.

Q4: Can I use another device instead of an expansion tank?

A: Yes. Codes allow alternative methods like pressure-relief valves or thermal expansion relief valves, but expansion tanks are the most common, reliable, and code-compliant solution for residential use.

Q5: Does a tankless water heater need an expansion tank?

A: Generally, no—because tankless heaters don’t store hot water, thermal expansion isn’t an issue. However, if a tankless system includes a recirculation loop with a storage tank, consult a plumber.

Q6: Will my home insurance cover damage from missing expansion tanks?

A: Possibly not. If damage results from non-compliance with plumbing codes, insurers may deny claims. Always verify code compliance during renovations.

Conclusion

So—does the plumbing code require an expansion tank on a water heater? The answer is: Yes, if you have a closed plumbing system (which most modern homes do). Skipping this small but critical component can lead to expensive repairs, voided warranties, and safety hazards.

Installing an expansion tank is a smart, code-compliant way to protect your plumbing system and extend the life of your water heater. If you’re unsure whether your home qualifies as “closed,” consult a licensed plumber or your local building authority.

💬 Found this guide helpful? Share it with a friend or on social media to help others avoid costly plumbing mistakes!

Stay safe, stay compliant, and keep your water flowing smoothly.