Guide to Expansion Tanks: What Homeowners Should Know

An expansion tank is a small safety device that gives heated water somewhere to go when it expands. Picture a tank about the size of a basketball mounted above or near your water heater. That's typically what an expansion tank looks like in most homes.

It's not a second water heater or a replacement for your main system. Instead, it's a protective add-on that works alongside your existing water heater to manage pressure changes that happen during normal operation.

Inside, an expansion tank has two chambers separated by a flexible rubber diaphragm. One side holds air that must be pressurized during installation to match your home's water pressure. The other side connects to your home's plumbing and can accept water when needed.

Water expands when it heats up. That's basic physics, but it creates a real problem in modern plumbing systems.

When water heats from 50°F to 120°F, it expands by about 2%. In a 50-gallon water heater, you're talking about an extra gallon of water that needs somewhere to go.

In older homes, this wasn't usually a problem. Expanded water could push back into the municipal water supply. Modern plumbing codes changed that. Most areas now require backflow preventers or check valves on water service lines. These devices protect public water supplies but create what plumbers call a "closed system." In a closed system, expanded water has nowhere to go. The pressure just keeps building every time your water heater cycles on. This causes several problems:

Your temperature and pressure relief valve starts dripping. This valve is designed for emergencies, but constant dripping means it's working overtime and will eventually fail. You're also wasting water and energy.

The pressure stresses your water heater's tank walls. Constant pressure spikes shorten the lifespan, and you might be replacing your water heater years earlier than necessary.

Pipes and fixtures throughout your home wear out faster. Faucets develop leaks, pipes spring pinhole leaks, and washing machine hoses fail more often.

This is why building codes in many areas now require expansion tanks whenever a home has a backflow preventer. The protection applies regardless of what type of water heater you have. Traditional tank models, tankless systems, and heat pump water heaters all create thermal expansion in closed systems.

Your water heater heats water, whether it's a 50-gallon tank keeping water hot all day, a tankless unit heating water on demand, or a heat pump system using ambient air efficiently. As that water heats up, it expands and needs more space.

In a closed system, that expanding water can't push back into the city's water supply. Instead, the excess pressure pushes some water into your expansion tank through the connection on your cold water line.

Inside the expansion tank, the water pushes against the flexible diaphragm, compressing the air on the other side. This compressed air acts like a spring, absorbing the pressure increase without letting pressure build throughout your plumbing system.

When water cools down or you use hot water and pressure drops, everything normalizes. The compressed air pushes the water back out of the expansion tank. The cycle repeats automatically every time your water heater operates.

There are no electronics, no moving parts to wear out, and no maintenance beyond an occasional pressure check. The expansion tank keeps your home's water pressure in the safe operating range—typically between 50 and 80 PSI—without your temperature and pressure relief valve constantly cycling open and closed.

You definitely need an expansion tank if your home has a backflow preventer or check valve on the main water line. Most municipalities now require these devices, which means most newer homes and many older homes with updated plumbing now have closed systems.

Local building codes might require expansion tanks for all new water heater installations. If you recently replaced your water heater, the inspector likely checked for an expansion tank.

Warning signs suggest you need one even if you're not sure about your plumbing configuration:

• If your temperature and pressure relief valve drips or leaks regularly, that's the clearest sign. This valve shouldn't activate during normal operation. Constant dripping means pressure is building higher than it should.
• Banging or knocking sounds in your pipes, called water hammer, can indicate pressure problems that an expansion tank would solve.
• Water pressure that feels inconsistent might be caused by thermal expansion affecting system pressure throughout the day.
• If your water heater tank shows signs of stress or is wearing out faster than expected, excess pressure could be the culprit. Frequent plumbing repairs throughout the house also point to system-wide pressure problems.

Not sure whether your home has a closed system? A licensed plumber can inspect your setup and give you a definitive answer.

Expansion tanks come in different sizes, and picking the right one matters for proper protection.

The size you need depends on your water heater's capacity, your home's water pressure, and your total system volume. A home with a 50-gallon water heater and 60 PSI water pressure needs a different size than a home with a 30-gallon heater and 80 PSI pressure.

Here's something that surprises many homeowners: the expansion tank doesn't need to match your water heater's size. A 50-gallon water heater might only need a 2-gallon or 4.5-gallon expansion tank. The expansion tank only needs to hold the extra volume created by thermal expansion.

A. O. Smith's TW Series expansion tanks offer properly sized options for residential water heating applications. These tanks must be pressurized during installation to match your home's water pressure.

Professional sizing takes the guesswork out of the equation. Contractors use calculations that account for your water heater type, capacity, and your home's operating pressure to recommend the right size. An undersized expansion tank can't handle all the thermal expansion, so pressure problems continue. An oversized tank costs more than necessary and takes up extra space.

Once a year, check the tank's air pressure using a standard tire pressure gauge. Keep in mind that the system must be drained down, and the tank must be completely empty of water before you can get an accurate reading. The valve is usually on top of the tank. The pressure should match your home's water pressure, typically 50-60 PSI for most residential systems. If it's low, you can add air with a bicycle pump or air compressor.

Do a visual inspection for corrosion, leaks, or damage. Tap on the tank and listen. It should sound hollow. If you hear water sloshing around or the tank sounds completely full, the internal diaphragm has probably failed. Time for replacement.

Most expansion tanks last 5-10 years, depending on your water quality and how often your water heater cycles. When an expansion tank fails, replacement is more cost-effective than trying to repair it.

Regular expansion tank maintenance helps your main water heater last longer, too. By preventing constant pressure stress, you're protecting the component that costs thousands of dollars to replace.

With over 80 years of water heating expertise, A. O. Smith combines proven reliability with innovative solutions for homes and businesses nationwide. Our TW Series expansion tanks work alongside our complete line of water heaters to protect your hot water system from pressure-related damage.

Ready to find out if your home needs an expansion tank? Connect with a certified A. O. Smith installer in your area who can evaluate your plumbing system and recommend the right protection for your water heater. Browse our TW Series expansion tanks to see options designed for residential applications, and explore our Info Center for more guidance on protecting your water heating investment.