What Size Expansion Tank Do I Need? Ultimate Guide

Last Updated on May 10, 2022

Warm water is a basic necessity in the areas with the cold environment, and a water extension tank is present almost in every household that has a warm water facility and a radiator. So, it makes sense to know more about a water heater expansion tank and its sizes. Many of us have a water heater expansion tank at home but are unfamiliar with it. A lot of people think that it’s a device that expands the capacity of the water heater so that it will produce more hot water, even though it is a fine guess, but in reality, a water extension tank has a lot more things than just expanding the capacity of a water heater.

So, in this article, I will try to answer all the questions like what size expansion tank do I need? Expansion tank sizing, the thumb rule of sizing, and expansion tank sizing formula for all those people who are concerned about their water heating systems.

A water heater expansion tank is a standard residential expansion tank that is similar in size and shape to a small propane tank. It absorbs water into a bladder and reduces excess pressure in the water heater. It has an air wall one on the top and another on the bottom for the pipeline connection. This safety device is also known as a thermal expansion tank.

If you have a water heating system at your home, then you must attach an expansion tank with it because going without it can be risky. Although there are a few water heaters that do not have an expansion tank, it is highly recommended to add one, as overflowing hot water causes pressure that is dangerous. It can result in negative consequences for the water heating system, a rise in water pressure can result in overflowing, leaks, and broken components.

The hot water can leak out through the T&P valve and may cause extensive water damage. Along with it, hot water pressure can shorten the service life of a water heater. So, for the proper functioning and safety of a water heating system, you must install a suitable hot water expansion tank.

These small tanks are located just above a water heater and play a crucial role in supplying hot water for household use. Almost everyone knows the importance of a water expansion tank, but still, many don’t know about the size of the expansion tank and ask questions like what size expansion tank do I need.

So here, we will go into more detail about choosing the right size of the expansion tank, and for selecting the correct size, there are two basic factors that must be kept in view.

1. First of all, you must know the water heating capacity of the tank, and this information can easily be found on the instruction booklet or on the factory label that is attached to the water heater.
2. The next step is to measure the water pressure in your home’s plumbing system with the help of a meter. Just attach the meter to the holes and turn on the water. It will be a good strategy to check water pressure for 24 hours and always remember to measure in PSI (Pounds per square inch), but another option is to contact the water company and ask them about the water pressure. Ideal water pressure ranges from 50 to 60 PSI, and if the pressure is more than 80 psi, then you can install a pressure-reducing valve.

Water expands when it is heated, and the heat transfer to the system. So, an expansion tank justifies its name, as they provide a place for the expanded fluid, but still, it’s important to determine the correct size of the tank. So, for expansion tank sizing rule of thumb is to oversize it rather than an undersized one because due to expansion, the amount of water will increase. Therefore it will be better to choose a thing that is one size larger than the size required.

There are four types of water expansion tanks, and the basic formula for measuring the size of a tank is this.

VtVsEw- EpPsTcPiTs-Ps ThPmaxTs- Ewt 1-PsTcPmaxTs+Et-0.02Vs

Here,

Vt = total tank volume

Vs = system volume

Ps = starting pressure of water

Pi = initial pressure

Pmax = maximum pressure

Ep = unit expansion ratio of piping due to temperature rise =3a(Th-Tc)

a = coefficient

Th = maximum average water temperature

Tc = minimum average water temperature

Ts = starting air temperature

Ewt = expansion ratio of water due to temperature rise

Et = expansion ratio of the tank due to temperature rise

Ew = unit expansion ratio due to temperature rise = (VhVc-1)

Vh = specific volume at maximum temperature

Vc = specific volume at a minimum temperature

1. Open Steel or vented tank

These open Steel tanks are located at the highest point of the system. Water temperature should not be above 100 degrees (212 Fahrenheit), and due to its open Steel tank, the air migrates into the system, causing corrosion.

1. Closed Steel tank

This tank is also known as a plain steel tank or compression tank. It is the same in style as a vented tank but has a covered duct. That means it can be located anywhere, it works amazing with higher temperatures, but still, air can come in and cause corrosion.

1. Diaphragm tank

This tank has a flexible membrane that works as an air-water barrier. It was designed to reduce the size of the tank and is usually attached to the side of the tank near the middle. So, if it ruptures, it must be replaced.

1. Bladder tank

This tank has a balloon-like bladder so it can tolerate the expanded water. It is usually suitable for the entire tank volume and is also known as a “full acceptance bladder.” It is replaceable and common for large commercial expansion tanks.

The capacity of the water heater	PSI (supply pressure)	Size of an expansion tank
40-60 gallon	40-50 psi	2-gallon
40-60 gallon	60-80 psi	3.2-gallon
80-gallon	40 psi	2-gallon
80-gallon	50-60 psi	3.2-gallon