Expansion Tank Vs Pressure Tank- Which One Do You Need?

What is the difference between an expansion tank and a pressure tank? Expansion tanks create a much-needed space for the water that expands following the law of thermodynamics. In contrast, the pressure tanks work to maintain constant pressure inside the tank, providing sufficient water supply at your home whenever you need it. And they also differ in price range, types, and overall sizes.

So, do you want to learn more about these differences in detail? Then you’re just a step away!

Pressure Tank Vs Expansion Tank – Quick Comparison

But before we head to the main comparison, here’s a preview of the aspects that separates them from each other.

Key PointsExpansion TankPressure Tank
MechanismWorks as a cushion to support the excess volume of hot water in the heater.Maintains the pressure inside the tank to keep the pump from starting again and again.
Best UseFor houses having more than two floorsFor houses that depend on a good source for water supply
Tank types2 types3 types
Tank SizeDepending on the water heater’s capacity and the pressure in the faucetDepends on the flow rate and run time or the HP of the motor
Price Range$40 to $200$200 to $500


The mechanism itself is the first aspect of our comparison between pressure vs expansion tank.

Let’s start with the expansion tank.

Expansion tanks absorb water overflow due to thermal expansion in the water heater. Therefore, whenever you heat the water, it increases in volume.

So, imagine that your water tank can contain 40 gallons of water. As you heat it up using a water heater, the volume becomes 42 gallons. Where would these extra 2 gallons of water go?

Yes, to the expansion tank that hangs off of the water heater. Previously, the municipal authority allowed the increased water level to flow back through the sewage pumps or drain. But this caused a huge waste of freshwater. That’s why to stop the mixing of pure water with waste, check valves were installed.

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The purpose of check valves is to stop the change of direction for the liquid inside the pipe. That means they can only move in one direction.

Hence, if your water system includes a check valve, you must consider an expansion tank. It has a bladder that contains overflown water and a compression chamber to reduce air volume. Once the faucet is open and water is used, bladder expansion tanks push the water back into the system.

Now let’s move on to the pressurization tank. The main purpose of this tank is to maintain the level of pressure. So the air can be compressed easily.

In the tank, as the water level rises, the air keeps reducing its volume and increases pressure. The pump will stop working as soon as the pressure point reaches 40 to 50 psi.

When you open the faucet, it causes a reduction of pressure inside the tank. As the pressure falls below 40 to 20 PSI, the pump kicks in and starts pumping again. In short, the pressure tank works like a container where the pressure is at an optimal level at all times.

Best Use

Buildings with more than 3 floors should install an expansion tank complementing the water heating system. Because as the amount of hot water increases, the overflow rate raises as well. So, you must attach a cushion that can support it. An expansion tank does just that.

But if the house is only one or two floors, you can skip the tank. This is because the volume of hot water used regularly is significantly low in such houses.

On the contrary, houses that pull water from a good source can get the most from a pressure tank. But even if your house receives water from a municipal line, you can increase the water supply rapidly through a pressure tank and pump.

Tank Types

You will notice two types of tank on the market for an expansion tank: bladder expansion tank and diaphragm expansion tank.

The first one has a bladder made of vinyl inside the tank. It separates the water from the air and maintains the pressure.

Once the faucet is open, and the pressure is low in the system, the stored water is then moved down. One benefit of this type is, the bladder is replaceable.

Another option is the diaphragm. A flat rubber diaphragm is mounted on the tank to separate the water in this tank. The procedure is the same as the former one. The only difference is that you can’t replace the diaphragm only.

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However, pressure tanks come in 3 different kinds.

No-barrier system: This is the old system where the water and air stay inside the tank without any barrier. There are chances that air will get absorbed into the water, causing the air pressure to reduce frequently. This type of tank gives you a lot of waterlogging problems.

Diaphragm system: The tank comes with an attached flat diaphragm that keeps the two components inside the tank away. As a result, the air pressure doesn’t reduce.

Bladder system: a bladder-like compartment is created inside the tank containing only water. This airtight system stretches when the water comes in and deflates after it exits the chamber.

Tank Size

Determining the size of an expansion tank and a pressure tank requires different approaches.

Did you find your expansion tank pressure too high? Not installing the right size can be the reason.

For calculating the expansion tank size, you need to find the capacity of the water heater or boiler. And then measure the pressure level in your water supply system using a gauge.

Water heater capacity in gallonsPressure in supply pipe (psi)Tank Size (gallons)
40 to 6040 to 502
40 to 6060 to 803.2
8050 to 603.2

To measure the size of your pressure tank, remember the following guideline.

With a pump having 20 GPM, your tank must have 20 to 40 gallons of storage. The minimum is 10 gallons for any pressure tank. And, the average pump nowadays requires 44 gallons of tank storage.

But you can’t go wrong with a larger pressure tank. Since a larger tank can contain more water for a longer period, it will require less starting. Frequent starting can heavily damage your pump’s efficiency in the long run.

This technique will work for both tanks perfectly.

Price Range

Price range is another dissimilarity between the two tanks. The expansion tank comes in a range of $40 to $200.And based on the capacity, the pressure tanks can cost around $250 to $500. That’s a big gap, right?

But the high price of a pressure tank can be justified. Because it tends to reserve water for the time of urgency and has a bigger shape in general, the cost of manufacturing is higher.

The price can be lower if you go with a galvanized one that doesn’t have a special air compartment. But that will eventually get you to spend on repairing, as the air gets absorbed into the water and creates a waterlogging problem.

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Frequently Asked Questions (FAQs)

What Is The Difference Between Expansion And Pressure Tank?

The main difference between an expansion tank and a pressure tank is their functionality. The expansion tank makes sure that your water heater system has enough space to release a thermal expansion. It protects your plumbing fixtures from breaking off due to excess pressure build-up in the system. On the other hand, you need a pressure tank so that your water pump turns on or off less frequently.

Will an Expansion Tank Increase Water Pressure?

Actually, no. The tank lets the excess water come in and compresses air to make room.
Unless the building is very tall, the pressure inside an expansion tank stays significantly low.

Do I Need An Expansion Tank If I Have A Pressure Tank?

People sometimes consider the pressure tank as an alternative to an expansion tank. But unfortunately, it’s not. If the water system in your house is a closed-loop, you must install an expansion tank for the safety of your plumbing fixtures. It doesn’t matter whether you have a pressure tank or not.

Can An Expansion Tank Reduce Water Pressure?

When you heat up the water in your pump, the volume increases significantly, and so does the pressure. This excess pressure can be very risky. But an expansion tank allows this extra volume of water to enter into its bladder. This tactic reduces the water pressure in the supply pipe and valves.

Wrap Up

You see, the question is not about the expansion tank or pressure tank anymore. If you have read the whole article, you would understand that neither of them can replace one another.

You need an expansion tank to protect your bathroom fittings and water supply pipes against excessive thermal pressure. And note that without an expansion tank for the boiler, the pressure in the pipes will rise until the system shuts itself off.

On the other hand, we have pressure tanks that level up the pressure in the tank and save the pump from turning on/off again and again. You can’t neglect either one if you want the water system of your building to last longer.

Hopefully, you enjoyed this detailed comparison!

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