What is a Water Heater Expansion Tank?

Water expands when it is heated and, if it doesn’t have any place to go, this can create a dangerous situation.

Imagine the inside of your home’s water heater with too much pressure inside. 

In the past, pressurized water that built up would simply drain back into the city’s supply system. But this isn’t always possible. If something is there to prevent this release, you’re going to need another safeguard – which brings us to the water heater expansion tank.

A water heater expansion tank is an additional small thank that is attached to your water heater unit. It is designed to handle that excess water that builds up. So, when the water pressure gets too high, it will flow into your expansion tank instead of damaging your home’s plumbing valves, fixtures, and joints, and potentially causing a burst line with immeasurable damages and cost.

When your water thermally expands, a water heater expansion tank prevents unwanted increases in pressure. Water expands by roughly 2% as it heats up from 50° F to 120° F.

You install the expansion tank down the line from the inlet valve that sends hot water into your home. The tank has two sections that are separated by a rubber valve. Water flows through the top portion, and there is pressurized air in the bottom. 

It’s this air pressure that allows the tank to absorb the excess pressure from your water heater, which keeps it away from your home’s other systems. 

What is a water heater expansion tank

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Common questions about water heater expansion tanks include:

What is a check valve?

The check valve is the mechanism that prevents the backflow of thermally expanded water in the wrong direction. Without an expansion tank, the increased pressure can cause unnecessary stress on your plumbing fixtures, including the water heater, and cut down on their working life.

Does the expansion tank have to be installed on the cold side?

Although expansion tanks can be installed on the hot side, we strongly recommend they be installed on the cold line, downstream of the shutoff valve.

Where should an expansion tank be installed in relation to the hot water heater?

Your expansion tank can be anywhere on your plumbing system and does not need to be installed in close proximity to the water heater. It is most commonly installed using a “T” at the cold inlet to the water heater. But, functionally, it can be installed anywhere on the cold inlet line.

Can the expansion tank be installed at any angle? Pt. 1

The expansion tank can be installed at any angle. This will conflict with info that comes with any expansion tank you buy at a retail store. The installation instructions that come with retail expansion tanks say the tank must be installed in a hanging vertical position. When you choose a retail expansion tank, it comes with a “Saddle Fitting.” This allows you to avoid soldering and makes installing an expansion tank simpler for the average person. The saddle fitting simple clamps onto the pipe, and it comes with threading that allows you to attach the tank.

Can the expansion tank be installed at any angle? Pt. 2

First, you drill a small hole in the existing pipe. Next, the clamp is secured so that the hole aligns with the inlet for the expansion tank. While this is one option, we don’t recommend these fittings for several reasons. They only allow the tank to be placed in one position (vertical), and they are unreliable. Instead, we recommend soldering the proper fittings into the system or the use of galvanized fittings.

How to know if you need a water heater expansion tank replacement

While a traditional water heater might last 10-15 years, your expansion tank could fail before this point. First, you can extend the life of your expansion tank through proper installation that ensures the air pressure in the tank matches the water pressure in your home. You can prevent a costly accident by periodically inspecting your expansion tank. If you see dripping water under the tank or near the fittings, the valve might be blocked. This can happen due to debris, such as mineral deposits. You should also have a mechanism in place to check the pressure, such as a water pressure gauge, that can tell you if the tank is functioning properly or if the diaphragm has been damaged. A water heater expansion tank can often be replaced without having to replace your entire system.

Who needs a water heater expansion tank?

If you have a “closed system,” meaning there is a backflow preventer or other device that won’t’ permit your water to flow back into the main water supply, thermal expansion will lead to significant pressure increases in your home’s plumbing system. 

Not only should you have a thermal expansion tank under these circumstances, but some municipalities require it. Further, your water heater’s manufacturer might void your warranty if you have a closed system and fail to get this safeguard. 

You don’t need a water heater expansion tank if you have a tankless water heater since there is no tank and no buildup of pressure. Likewise, if your home runs on an open water supply system, where excess water goes back into the municipal water supply, it won’t place any strain on your home’s plumbing system.