Most people never think about their water heater until something goes wrong. One of the most common culprits behind dripping relief valves, banging pipes and leaky fixtures is thermal expansion, and the fix is a small, inexpensive part: the water heater expansion tank. Below you’ll learn what an expansion tank does, whether your home needs one, what size to buy and how to tell if yours has failed.
Quick answer: A water heater expansion tank is a small tank on the cold water line that gives hot water room to expand, so pressure doesn’t build up in your plumbing. You need one if your home has a “closed” plumbing system, meaning there’s a check valve, backflow preventer or pressure-reducing valve on your main water line. Most plumbing codes require one in that case.
What is a water heater expansion tank?A water heater expansion tank, also called a thermal expansion tank, is a small tank, usually 2 to 5 gallons, installed on the cold water supply line next to your water heater. It absorbs the extra water volume created when your water heater heats water.
What is thermal expansion?
Water expands as it heats up. In a 50-gallon water heater, heating cold incoming water to shower temperature adds roughly half a gallon to almost a gallon of extra volume every heating cycle. Water can’t be squeezed into a smaller space, so if that extra water has nowhere to go, the pressure in your pipes and water heater climbs, sometimes well past 100 psi.
In older homes, that extra water simply pushed back into the city water main. Today, most homes have a device on the main line that stops water from flowing backward, which protects the public water supply but traps the expanding water inside your home.
Learn more: What is a backflow prevention device?
How does a water heater expansion tank work?Inside the tank, a rubber diaphragm divides the space in two. One side connects to your water line. The other side holds a cushion of pressurized air. As heated water expands, the extra water pushes into the tank and compresses the air. When you turn on a faucet and pressure drops, the air pushes that water back into your plumbing. The result is steady pressure instead of damaging spikes.
Do I need an expansion tank for my water heater?You need one if your home has a closed plumbing system. Your system is closed if any of these are installed on your main water line:
- A pressure-reducing valve (PRV)
- A check valve
- A backflow preventer
- A water meter with a built-in check valve (ask your water utility if you’re not sure)
Most U.S. plumbing codes require an expansion tank or other thermal expansion control in closed systems, and many inspectors require one whenever a water heater is installed or replaced. Check with your local building department for the rules in your area.
Signs your home needs an expansion tank
- Your water heater’s temperature and pressure (T&P) relief valve drips or discharges
- Water pressure surges when you first turn on a faucet after the heater has been running
- Toilets run or faucets drip, even after you replace their parts
- You hear banging or knocking in the pipes
What about homes on well water?
Homes on a private well usually don’t need a separate expansion tank, because the well’s pressure tank already absorbs the expanding water. The exception is when there’s a check valve or pressure-reducing valve between the well pressure tank and the water heater. That creates a closed system, so you’ll need an expansion tank.
What size water heater expansion tank do I need?The right size depends on your water heater’s capacity, your home’s water pressure and your water heater’s temperature setting. You’ll find the capacity on the label on your water heater, and you can check your water pressure with a pressure gauge on an outdoor hose bib. At about 60 psi and normal temperatures:
| Water heater size | Expansion tank size |
|---|---|
| 30 to 60 gallons | 2.1 gallons |
| 80 gallons and up | 4.8 gallons |
If your water pressure is higher than 60 psi or you keep your water heater set hot, size up. A tank that’s a little too big won’t hurt anything, but one that’s too small can leave your relief valve dripping. For an exact match, check the tank manufacturer’s sizing chart. Popular brands include Amtrol and Flexcon.
How much does a water heater expansion tank cost?The tank itself usually costs $25 to $60. If you hire a plumber, expect to pay roughly $100 to $300 more for installation, depending on your local rates and how easy it is to reach your water heater. Many homeowners install one themselves in under an hour.
Learn more: How to install a water heater expansion tank step by step
How long does a water heater expansion tank last?Most expansion tanks last about 5 to 10 years. The best way to help yours last is to keep its air charge matched to your home’s water pressure. Many homeowners replace the expansion tank whenever they replace the water heater.
Pros and cons of water heater expansion tanksPros:
- Prevent leaks and damage caused by pressure buildup
- Add a second layer of protection alongside the water heater’s relief valve
- Help your water heater, valves and fixtures last longer
- Inexpensive to buy and install
- Take up very little space
Cons:
- The diaphragm wears out and the tank needs replacing every 5 to 10 years
- The air charge needs a quick check once or twice a year
- Installation mistakes, like the wrong air pressure or no support for a horizontal tank, can cause problems
Even when an expansion tank fails, replacing it costs far less than repairing the plumbing damage high pressure can cause.
If something seems off, these quick checks will tell you whether your expansion tank is doing its job.
Tap the tank
Lightly tap both ends of the tank with a screwdriver handle. The air side (the end with the air valve) should sound hollow, and the water side (the end connected to the pipe) should sound duller. If both ends sound the same and solid, the tank may be waterlogged. This is a clue, not a final answer.
Watch for pressure surges
After your water heater has been heating for a while without anyone using water, turn on a hot water faucet. If water surges out hard and then settles down, pressure is building in your system and the expansion tank may not be working. A faulty pressure regulator can cause the same thing, so check further before replacing anything.
Check the air valve and air pressure
- Turn off the cold water supply to the water heater.
- Relieve the pressure by opening a hot water faucet until the flow stops. If you skip this, the gauge will read your water pressure instead of the tank’s air pressure.
- Find the air valve. It’s a tire-style (Schrader) valve under a plastic cap, on the end of the tank opposite the water connection.
- Press the valve pin for a second. If air comes out, that’s good. If water comes out, the diaphragm has ruptured and the tank must be replaced.
- Check the air pressure with a tire gauge. It should match your home’s water pressure. (New tanks usually ship pre-charged at about 40 psi, so they often need adjusting.)
- Adjust if needed. Use a hand tire pump to add air a little at a time, or press the valve pin to let some out. Recheck after each change.
- Turn the water back on and close the faucet.
Check the air pressure at least once a year.
Learn more: 7 water heater expansion tank problems and how to fix them
Water heater expansion tank FAQsWhere does a water heater expansion tank go?
On the cold water supply line to the water heater, between the water heater and your check valve, backflow preventer or pressure-reducing valve. Most installers put it within a few feet of the heater.
What happens if I don’t have an expansion tank in a closed system?
Pressure spikes every time your water heater runs. Over time, that can make your relief valve drip, wear out faucet and toilet parts, strain pipe joints and shorten the life of your water heater.
Does a tankless water heater need an expansion tank?
Usually not, because a tankless heater holds very little water at a time, so there’s very little expansion. Some local codes and manufacturers still call for one in closed systems, so check your water heater’s manual and local code.
If you have any additional questions, please do not hesitate to contact us.
