What Is The Purpose Of An Expansion Tank?

We all know about water heaters and how essential they are for our morning vibes. They give us warm water to bathe our bodies in, hot water for our dirty dishes, and even speed up the boiling process when cooking spaghetti.

But upon inspection of the tank itself, you notice another much smaller tank hooked up to the cold water supply line and question why it’s even there? Was it put there as sort of an add-on, like having additional gigabytes on your computer? Does it dispense some sort of liquid? What could possibly be the reason for it?

So what is the purpose of an expansion tank? Simply put, an expansion tank allows for the natural expansion of water as it heats up in your tank which is known as thermal expansion. The additional storage saves pipes, fittings and fixtures from any strain as the pressure increases when water gets hot. This will prevent failures occurring in your plumbing due to temperature change.

Why The Tank Was Created

There was a time when expansion tanks never even existed, but water heaters have been around for a long time, so what gives?

In the past, as water was heated up, the pressure would get relieved solely by being pushed outside of the home and into the city’s water line, which would dissipate the pressure. This system is called an open system.

Now of days, a lot homes will require a check valve due to health and safety concerns. A check valve contains a spring internally, followed by a rubber gasket.

The spring is on the side of water pressure flowing into the building, while the gasket is set on the other side. The spring will allow water to push upon it and leave a space between the gasket and the inner walls that allows water to pass through.

Because this is a one directional process, the gasket side will act as a seal when water wants to go in reverse or exit the building. This will cause the pressure to push upon the gasket and form a water tight seal.

The reason it restricts this backwards flow is because of harmful chemicals or substances that may come in contact with a fixture in your house and this contact could be made via a hose that is lying on the ground.

If a puncture occurred in the city’s water line, it would create a vacuum which would suck water out from all the surrounding area’s water supply’s. This would also suck up any chemical, substance or dirt into the stream, infecting the entire water supply. Big big mess, nobody wants to deal with that, thus the invention the check valve.

Another reason a check valve may be installed in your plumbing system is if you have a hot water re circulation system, that with the use of a pump, will push hot water back into the tank helping it recirculate and allowing for hot water to be accessible to even the furthest parts of a house or building.

The pump will push hot water into the cold water supply line, thus creating the need for a check valve to prevent this from occurring. Wherever you have a check valve, you create a closed system, and if you’re closing off the flow of water that is being pumped from heading back out in the city water line, then expansion IN the system will occur, and thus creating a use for an expansion tank to absorb that pressure.

When you have a check valve device or anything that restricts the flow from heading out in the city’s main water line anywhere in your plumbing, it is called a closed system.

How The Expansion Tank Works

Water is not compressible. When water gets heated up, it cannot compress so it tends to build up pressure as it expands, this is known as thermal expansion. An expansion tank has what’s known as a bladder on the interior of it.

This bladder is filled with air, air which is compressible. Once the water begins to get hot, the bladder gets pushed down, compressing the air inside and allowing for the change in water.

The bladder alternatively comes back to it’s normal position once the water begins to cool off.

The connection to the tank is made via your water supply lines (either copper, PEX, PVC or CPVC) which attaches to a F.I.P (Female Iron Pipe) fitting and screws on to the expansion tank. Add a little bit of teflon tape around the threads and voila, you’ve got a water tight seal.

I wrote another article about maintaining your reverse osmosis system, and in there is a step by step guide in diagnosing and re-pressurizing it’s tank when it begins to fail, and you can find it here. The reverse osmosis tank storage tank operates in the same manner as an expansion tank, but just serves a slightly different purpose.

When Do I Need An Expansion Tank?

On an open system

Running along the side of your hot water tank you’ll notice a pipe, either made from copper or CPVC. This pipe is hooked up to a device called a pressure relief valve.

Now the pressure relief valve is a device that’s found on all tanks. The purpose of this valve is to relieve any pressure that could possibly build up within the system by opening up the valve when needed, and dumping water into a drain or even in some cases just onto your basement floor.

These valves are mainly rated at 150 psi (pounds per square inch) but can be a little bit higher, such as 160 psi. If you notice that this valve is continuously opening up and letting water out, it means the city water pressure is too high in conjunction with thermal expansion for your water tank.

So if you see water coming out, or you notice random wet spots under your tank, it could mean that you need an expansion tank to help with the built up pressure.

Closed system

To expand on what I was saying before, in a closed system if thermal expansion occurred, it would build up the pressure in the tank and force the water to find relief for the pressure however it can. A pressure relief valve won’t always be enough.

Some things that can occur as a result of your closed system having too much pressure.

  1. Water pressure will become too harsh for rubber gaskets in your fixtures, allowing for drips to occur
  2. Flue of your hot water tank can fail, potentially causing harmful toxic gases to leak into your house
  3. Ruptured hot water tank
  4. Toilet tank ball cock begins to leak causing the tank to fill up and over time will overflow
  5. Pressure relief valve begins to drip out water

*Note – Expansion tanks should not be installed by anyone with little to no experience with plumbing. The result of incorrect installation could mean leaks or flooding which can quickly become costly. A licensed plumber is always the best route to go, as they deal with this on a regular basis and have experience in checking for leaks upon installation, along with proper installation techniques and tools.

Is An Expansion Tank Still Necessary If I Have A Pressure Reducing Valve?

A pressure reducing valve is designed to reduce high pressure in ones home. This is not a common device found in a house unless the pressure exceeds 80 psi. A normal pressure found in a house is around 40-75 psi.

Most of the time a pressure reducing valve is not even necessary at all, but if there happens to be one, you will likely still need an expansion tank.

Going back to the purpose of an expansion tank on a water heater…The tank will allow for thermal expansion, and thermal expansion can mean as much as 1-2 gallons difference from the size of the tank. On a closed system this would render city pressure a useless factor as the expansion would occur regardless of if the home had 30 psi or if it had 100 psi.

On an open system, where the relief is reliant upon water being pushed back out into the main water line, it would still be best to have an expansion tank as they won’t do your system harm. Water pressure is known to fluctuate over time, so you might test it at 70 psi one day, while another day it jumps up to 85.

If you need a tank, I would first recommend contacting a local plumber to help you answer any questions you still may have and for installations purposes, they are experts in the field and deal with installations such as this on a near daily basis. I also recommend picking up one of these (link to Amazon) as it is a dependable product made by a very dependable brand.





Tyler Takacs

My name is Tyler, I live in Ontario Canada and enjoy learning about common plumbing issues in the household. I have spent just over three years in the trades as a plumbers apprentice, but am now onto a less physical job. I still enjoy studying and learning about my own house's plumbing as well as finding ways to help others with their issues.

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