Water Heater Expansion Tank: What It Does and When You Need One

A dripping relief valve, a faucet that spits after the heater runs or a toilet that keeps refilling can all point to the same small part. If you have water around the tank, our leaking water heater repair in Newark can check pressure and find the source. This guide explains what an expansion tank does, when Delaware's plumbing code calls for one, and how to tell whether yours still works. It is general information, so your local inspector has the final word on code questions.

What does a water heater expansion tank do?

Water grows when it is heated. A 50-gallon tank warming from cold to 120°F adds a small but real amount of volume. In older homes that extra water simply pushed back into the street main.

Many homes now have a one-way part at the meter: a check valve, a backflow preventer or a pressure reducing valve. These protect the public water supply or tame high street pressure. They also stop heated water from flowing backward. The result is a "closed" system, and pressure climbs every time the burner or element runs. This is called thermal expansion.

A water heater expansion tank fixes that. It is a small steel tank, often about the size of a basketball, mounted on the cold line near the heater. Inside, a rubber bladder splits it into two parts:

• The water side connects to your plumbing.
• The air side holds a set charge of air, like a tire.

As water heats and expands, it pushes into the tank and squeezes the air. When you open a tap, the air pushes the water back out. House pressure stays steady instead of spiking.

Is an expansion tank required?

In most Delaware homes with a closed system, yes. Delaware adopted the 2021 International Plumbing Code (IPC) as the 2021 State of Delaware Plumbing Code, effective August 11, 2022. IPC section 607.3 says that when a storage water heater gets cold water through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device must be connected to the heater's cold supply. It must sit downstream of all those valves.

New Castle County, the City of Newark, Wilmington and Middletown each enforce the code through their own permit offices. Inspectors and installers often check for expansion control during a water heater replacement. Our guide to whether you need a permit to replace a water heater covers who to call.

Do I need an expansion tank? Signs your house has a closed system

Look for these clues:

• A pressure reducing valve near the main shutoff. It is a bell-shaped brass valve with an adjusting screw on top.
• A backflow preventer or check valve at or after the meter.
• A relief valve that drips a little after each heating cycle, then stops.
• Pressure that swings. A gauge on an outdoor spigot reads normal in the morning but climbs after the heater runs.
• A toilet that refills on its own, or faucets that spurt when first opened.

If you see one of these and no expansion tank, the heater is likely working under extra stress. Our guide to a water heater pressure relief valve leaking walks through the test.

Is an expansion tank necessary if nothing is leaking yet?

It still matters. Repeated pressure spikes strain the tank's welds, the relief valve, faucet cartridges, toilet fill valves and appliance hoses. Damage builds slowly and shows up later as a leak.

Tank makers are clear that the relief valve is not meant to be the fix. The temperature and pressure (T&P) relief valve is an emergency device. It opens at 150 psi or 210°F. If it is opening often, the system needs real expansion control.

What size expansion tank do I need?

Size depends on two numbers:

1. Your heater's capacity in gallons. It is printed on the label.
2. Your static water pressure. Read it with a hose-bib gauge on an outdoor spigot while no water is running.

Tank makers publish charts that cross these two numbers. AMTROL's residential chart and Watts' PLT chart both assume a 150 psi relief valve and an air charge equal to supply pressure. As a rule of thumb from those charts:

Always use the chart for the exact tank being installed. Two heaters, or a heater plus a storage tank, need a bigger unit.

How to install a water heater expansion tank

Installing one means cutting into a pressurized water line. Here is what a proper install looks like, so you know what to expect:

1. Measure static pressure with a gauge on a spigot.
2. Set the air charge before filling. Makers say the pre-charge must match house pressure. AMTROL and Watts tanks often ship at 40 or 20 psi, so most need adjusting. Zurn says not to exceed 80 psi.
3. Shut off the water and power or gas to the heater, and relieve pressure.
4. Install a tee on the cold supply, downstream of the check valve, PRV or backflow preventer.
5. Mount and support the tank. A full tank is heavy. Many installers add a strap or bracket instead of letting it hang on the pipe.
6. Restore water, check every joint, then restore heat.
7. Retest pressure after the heater has run a full cycle.

If your pressure is over 80 psi, the code and most makers call for a pressure reducing valve as well.

How to tell if your expansion tank has failed

A tank can look fine and still be dead inside. Try these checks:

• Tap test. Knock on the top and bottom. A good tank sounds hollow on top. A tank that sounds solid all the way up may be waterlogged.
• Air valve test. Briefly press the valve stem on top, like a tire valve. Air is good. Water means the bladder has failed.
• Weight test. With care, push up on the tank. A waterlogged tank feels much heavier.
• Gauge test. Watch a hose-bib gauge while the heater runs. A jump of more than a few psi means expansion is not being absorbed.

Makers suggest checking the air charge once a year. Many tanks last about as long as a water heater, so replacing both together is common.

Expansion tanks in Newark and Delaware homes

Delaware enforces the 2021 IPC through the Division of Public Health and local permit offices. Many newer homes in the area have a check valve or backflow device at the meter, which makes the plumbing a closed system.

• Check before replacing. If your old heater never had an expansion tank, ask about one when you replace it.
• Watch for PRVs. Homes on higher-pressure mains may already have a pressure reducing valve, which closes the system.
• Well homes in rural parts of New Castle County often have a pressure tank that can absorb some expansion. Ask a pro before skipping a thermal tank.

How to prevent expansion tank and pressure problems

• Check house pressure twice a year with an inexpensive hose-bib gauge. Aim for under 80 psi.
• Test the expansion tank's air valve once a year.
• Flip the relief valve test lever once a year, as your heater's manual directs.
• Replace the tank with the heater if it is the same age.
• Keep the heater at about 120°F, which our guide to water heater temperature setting explains.

Local note: Many townhomes in Bear and Middletown were built with backflow protection at the meter, so a missing or waterlogged expansion tank is a common cause of dripping relief valves. Older homes in Wilmington may be on an open system but can gain a check valve when the meter is replaced.

Related reading: Trace a drip with why is my water heater leaking?, see what a slow leak costs in why is my water bill so high?, and plan ahead with water heater replacement cost in Delaware.

When should you call a water heater professional?

Call a pro if:

• The relief valve drips, runs or has a wet, crusty discharge pipe
• Your house pressure reads over 80 psi
• The expansion tank sprays water from its air valve or sounds full
• You have a check valve, PRV or backflow preventer and no expansion tank
• You are replacing the heater and the system is closed

Expansion tank or pressure trouble in Newark? Call (302) 732-0874 for leaking water heater repair from Newark Heater Pros.