What Is A Water Heater Expansion Tank?


What if you kept heating water in a pressure cooker without relieving the pressure? It will eventually blow up because there would be too much pressure inside. Just like a pressure cooker, inside a water heater, you are heating water in an enclosed space. If the pressure gets damagingly high, it must be relieved or it will damage your plumbing system. This is where expansion tanks come in. They are safety devices that protect your plumbing system from damage due to thermal expansion. They relieve damaging pressure by providing extra volume for hot water and steam to expand into. When the water pressure gets too high, steam will flow into your expansion tank, relieving the pressure inside your home's plumbing. The water heater expansion tank is a small tank located right above the water heating unit.

Not everyone needs an expansion tank. If water entering your home is allowed to go back into the city line, you won't need an expansion tank. This is called an open plumbing system. An open plumbing system gives the hot water much room to expand, relieving the pressure inside the water heater tank. If you have a closed plumbing system, however, you will need an expansion tank. A closed plumbing system doesn't allow water to flow back into the city lines once it enters the home. The water has no room to expand.

How do I know if I have a “closed” plumbing system? The bottom line is if your home has any kind of backflow prevention device installed on your home's main water line, your home has a closed plumbing system.

Look for a check valve or pressure-reducing valve connected to your main water shutoff valve. Your main water shutoff valve is usually in the basement or on an outside wall of the house.

Most houses have a closed plumbing system which doesn't give hot water much room to expand into. When the water volume expands, it puts extra pressure on the tank. Over time this takes a toll on your tank. If you install a thermal expansion tank, the extra water will rush into it, relieving the pressure inside the water heater.

Picking The Right Thermal Expansion Tank

Now, If you have realized that you do need to buy an expansion tank. The question arises — which one should you buy? The answer depends on several factors.

Expansion Tank Sizing

To pick the right one, you need to know two things:

  1. The Size Of Your Water Heater:

The bigger the water heater capacity, the bigger the expansion tank you need. Your water heater comes with a factory label stuck to it. Look at it.

  1. Water Pressure In Your House:

Higher the incoming water pressure, the bigger the tank you need. If you are not sure, you can choose an expansion tank that is one size larger than the required size. You can measure the pressure with a gauge—a tiny clock-looking thing.

  1. Screw the gauge in a faucet or a hose.
  2. Turn on the water.
  3. Take the reading, which should be around 30-80 psi. The ideal water pressure would be 50-60 psi.

(Tip: If your water pressure is more than 80 psi, you need to install a pressure-reducing valve where your water main water line comes into your house)

Which Size Water Heater Expansion Tank Should I Buy?

You can use an online expansion tank calculator website to calculate the size you require.

  1. Type in the size of your water heater in gallons.
  2. Type in the approximate temperature setting of your water heater.
  3. Type in the incoming water pressure into your house from the city lines.

Generally, 50 gallons of water will expand to 52 gallons when it heats up. Therefore, a 2-gallon expansion tank is usually enough for a 50-gallon water heater. If you are not sure, you can buy one size bigger than the calculated size. Oversizing is better than undersizing.

Steps To Install A Water Heater Expansion Tank

How To Install An Expansion Tank

A water heater expansion tank will be installed right above your water heater. It will intercept the connection between your water supply line and your water heater.

Supplies You Will Need

  • Thermal expansion tank
  • Dielectric water heater nipple
  • Pipe wrench
  • Pipe-sealing tape
  • Two 3/4-inch copper female-threaded unions
  • 3/4-inch copper tee-fitting
  • 3/4-inch copper pipe

Step 1: Cut Off The Water And Power Connection

Rotate the valve close to cut off the water supply to your heater. Turn off the power supply. If your water heater uses electricity, turn off the circuit breaker that supplies electricity, or cut the gas supply to the heater.

Step 2: Drain The Water Heater

If you have a water heater that you are currently using, you must drain the water heater. Connect a hose to the drain valve at the bottom of the heater and drain the water heater. After a few minutes of draining, carefully open the T&P valve to check for pressure. If there is still pressure, wait a few more minutes. If there is none, turn off the drain valve.

Step 3: Find The Cold Water Supply Line

Presently, your water heater should be connected to your cold water supply line, which supplies water to your water heater.

By adding an expansion tank, you will be intercepting your water heater supply line. The cold water supply line should be horizontal to the water heater, connected to the top of the water heater.

Step 4: Install A Tee

Using a wrench, remove the copper flex line from the nipple at the top of the water heater and install a tee where the flex line was connected. Tighten the tee so that one outlet is pointing up and the other is facing the direction where you want the expansion tank.

Step 5: Install The Expansion Tank

Screw the expansion tank to the tee tightly. Also, don't forget to wrap the Teflon tape and apply the pipe joint compound (liquid Teflon) to the connecting points of your construction — the water heater nipple, both ends of the tee, and the expansion tank nipple.

Expansion tanks need to be installed vertically and be properly supported because as they fill with water they’ll get much heavier.

Frequently Asked Questions

Can I Install An Expansion Tank Myself?

If you have the tools and some basic plumbing skills necessary, then you can install an expansion tank yourself. You can use the instructions mentioned above or follow the installation manual that comes with an expansion tank.

What Is The Price Of Water Heater Expansion Tanks?

The price of Water Heater Expansion Tanks ranges from $20 to $400. The price of an expansion tank mainly depends on its quality and size. Should I install the expansion tank on the hot side or the cold side of my water system? Although you can install your expansion tank on the hot side of the water system, we highly recommended installing it on the cold side. While there are some instances where it’s OK to install an expansion tank on the hot water side, it should always be installed on the cold side if possible.

How Often Do I Need To Change My Water Heater Expansion Tank?

Water heater expansion tanks last anywhere between 5-10 years. If your water heater or your expansion tank is not leaking, expect to change the expansion tank after 7 years or so.

How Do I Check If My Expansion Tank Needs To Be Replaced?

There are two simple ways: Knock on the tank. If it sounds hollow, your tank is working well. If it makes a thud sound, you need to replace it. Check the Temperature/pressure relief valve located on the upper side of the water heater with a tube that goes to the floor. If the relief valve is dripping, you probably need to replace your expansion tank.

Why Is My Expansion Tank Leaking?

Caused by corrosion, leaks usually occur at the pipe fitting that connects the expansion tank and the water heater. This usually occurs when the pipe fitting is of terrible quality or you didn't install the water heater properly—the connections aren't screwed tight or the lack of Teflon tapes.

If the tank itself is leaking, the leak is probably because the rubber bladder has been worn out. In that case, you need to buy a new expansion tank. These are the reasons why you need to buy a decent quality expansion tank and install it properly, paying special attention to the connection, so that your expansion tank doesn't start leaking.

Is An Expansion Tank Required For A Water Heater?

When water is heated, it expands. If it doesn't have space to expand, it starts to apply pressure to the walls of the tank. If the pressure gets high enough, it can damage the plumbing system, causing a leak. To relieve the pressure, an expansion tank is installed. It provides space for expansion.

You don't need an expansion tank if the water is allowed to expand back into the city lines. In this case, the city lines provide the space for expansion instead of the expansion tank.

Does An Expansion Tank Affect My Water Pressure?

It's a common misconception that installing a water tank will decrease your water pressure.

Although water pressure coming into your house does affect your expansion tank, your expansion tank does not affect your water pressure.

What Are Some Common Issues With Expansion Tanks?

The most common problem with expansion tanks is that they may leak. Leaks are inevitable due to degradation. However, it may be accelerated if the expansion tanks are not installed properly, or if a smaller tank is installed than required.

What Is The Difference Between A Thermal Expansion Relief Valve And An Expansion Tank

Thermal expansion relief valves and expansion tanks offer an outlet for steam when hot water heats up, reducing the pressure inside the tank. Although they perform the same function they do it differently. The relief valve allows water and steam to leak out into your home if the pressure inside the water heater tank reaches a certain level. Whereas if you have an expansion tank, the steam rises from the water heater tank into the expansion tank, relieving the pressure. Steam and hot water do not leak into your house.