Water heater expansion tank is a safeguard system that reduces the amount of pressure in your plumbing. These tanks are small tanks that are installed on the water supply pipe leading to your water heater.
Water heater expansion tanks prevent excess pressure from damaging or overworking your plumbing and plumbing fixtures. They do this by accepting any excess water from a water heater and compressing air inside the tank.
It’s easier to understand how a water heater expansion tank works if you know the basics of how a water heater works.
How a Water Heater Expansion Tank Works
A water heater expansion tank is a simple but effective backup system that protects the plumbing in a home from excess pressure. Where would excess pressure come from?
It all starts with the water heater, which has the simple job of heating water for use in your home. As the water in your water heater gets hotter, it naturally expands.
This process is called thermal expansion. A 50-gallon water heater is built to hold 50 gallons of cold water, but once that water heats up, the water volume expands to 52 gallons.
With two additional gallons of water volume that the water heater can’t handle, the excess water has nowhere else to go but your plumbing. It pushes against the walls of pipes, joints, and fixtures, weakening them over time.
Because the water heater is now dealing with more water volume than it started with, the pressure inside your plumbing rises.
That pressure can damage your plumbing and plumbing fixtures over time, just like high blood pressure strains your body. It can cause your T&P (Temperature and Pressure relief valve) to start dripping and it can even rise high enough to cause a water heater to burst.
Pressure Released Into Tank
Here’s how the water heater expansion tank works to prevent that kind of damage: Half of the tank is filled with water from your home’s water system.
The other half is filled with compressed air. In the middle is a butyl rubber bladder that allows the water in the tank to expand when it gets hot.
This excess, expanded water from the water heater flows into the expansion tank and pushes the rubber bladder into the compressed air on the other side of the tank, further compressing the air inside the tank.
Do You Need a Water Heater Expansion Tank?
Not every home needs a water heater expansion tank. How do you know if you need one?
You need a water heater expansion tank if:
You have a traditional tank-style water heater
You’re on a closed-loop water supply system
Your city or water heater manufacturer requires it
You don’t need a water heater expansion tank if:
You’re on an open-loop water supply system
Without a water heater expansion tank to backup your water heater and handle the excess water and pressure from expansion, the water will either flow back into the municipal water supply or make its way into your plumbing.
This is determined by whether you have an open-loop or closed-loop water supply system.
Open-Loop Water Supply Systems
If your home is on an open-loop water supply system, the excess water (due to expansion from heat) has somewhere else to go – it simply drains back into the municipal water supply.
This does not cause any additional pressure or strain on the plumbing in the home, meaning you would not need a water heater expansion tank.
Closed-Loop Water Supply Systems
In a closed-loop water supply system, the water supply has a one-way valve (a backflow, check , or pressure-reducing valve) that prevents water from flowing back into the municipal supply.
In closed-loop water supply systems, the excess water has nowhere else to go, so it goes into the plumbing and increases the pressure.