Bladder Tanks vs. Diaphragm Tanks

Bladder tanks and diaphragm tanks both have the main function of separating system water from the air charge inside an expansion or pressure tank.

The main difference is how the separation occurs and what they use to do that: a bladder tank uses a replaceable balloon-like bladder, while a diaphragm tank uses a fixed rubber diaphragm permanently attached inside the tank.

Both designs help control system pressure, reduce waterlogging, create extra room in the water line to absorb liquid expansion due to temperature changes, and support more stable operation in hydronic, plumbing, and pressure systems.

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Bladder Tanks

A bladder tank uses a balloon-like bladder to adjust to pressure changes and absorb thermal expansion within a heating and cooling system. The bladder separates the expanded water from the air.

This way, the water does not come into contact with the tank, preventing corrosion by oxygen and rusting. It also increases pump life by reducing the number of pump cycles.

In addition, this system prevents water clogging by using an air wall between the tank’s interior and the bladder’s exterior.

Bladder tanks utilize space inside the tank to create pressurized air. That pressurized air then pushes water at a psi depending on the configuration and intended use. The more water a pressure tank loses, the less powerful the air pressure is.

The air pressure in a bladder pressure tank continues to thrive because of the vacuum created by the tank and balloon. The major advantage of bladder-type tanks is that the bladder is replaceable.

Diaphragm Tanks

Diaphragm tanks are water vessels that are subject to a high amount of pressure. The diaphragm keeps the water and air separate inside the tank. It prevents the system from becoming waterlogged and prevents air from being absorbed.

Diaphragm pressure tanks are the most efficient type of pressure tank and require the least amount of maintenance due to the use of a rubber lining inside that separates the air and the water. This rubber lining acts as the bladder would, but its connection is permanent to the tank. Therefore, it is not replaceable.

However, the lining doesn’t take up as much space as the balloon-like bladder, increasing efficiency.

Diaphragm tanks are divided into the upper water storage area and the lower air pressure area, with a rubber diaphragm separating the two areas. When the tank begins to store water, the weight of the water will squeeze the rubber toward the lower air pressure zone.

As a result, the pressure in the air pressure zone will become larger and larger (the air can be compressed). The pump stops pumping water when the pressure exceeds the high preset value. On the contrary, when there is water demand, the air in the air pressure zone is released, and the rubber diaphragm is compressed.

As a result, the water was squeezed toward the joint, causing it to flow out.

Pre-Charged Tanks

Pre-charged tanks can contain either a bladder or a diaphragm. When a tank is pre-charged to the minimum system operating pressure(P_MIN), it accepts only the expanded system water as the temperature increases. The highest temperature condition will compress the pre-charge to the maximum system operating pressure (P_MAX).

As the temperature decreases, the tank pressure pushes the expanded water back into the system. This design allows for a smaller tank since space for charging water to compress air at atmospheric pressure to P_MIN is no longer necessary, as it is when using a conventional tank.

Bladder Tank vs. Diaphragm Tank: Key Differences

Both types of tanks operate on the same principle, so the right choice depends on the system type, tank size, maintenance expectations, and whether replaceable internal components are feasible.

Below is a quick comparison table to help you understand when to use each:

Comparison Point	Bladder Tank	Diaphragm Tank
Internal design	Uses a balloon-like bladder	Uses a fixed rubber diaphragm
Water and air separation	Bladder separates water from air	The diaphragm separates water from air
Replaceability	Bladder is often replaceable	Diaphragm is typically not replaceable
Maintenance consideration	Allow bladder replacement instead of replacing the full tank	Usually lower-maintenance, but the tank need replacement if the diaphragm fails
Common use	Larger hydronic systems, pressure systems, and applications where serviceability is important	Hydronic systems, commercial systems, and applications where compact design and simple operation are preferred

In simple terms, bladder tanks are often selected when replaceability and serviceability are priorities. Diaphragm tanks are often selected when a compact, low-maintenance design is preferred. Pre-Charged Tank

How to Choose Between a Bladder Tank and a Diaphragm Tank

When choosing between a bladder tank and a diaphragm tank, the first step is to confirm the application.

Expansion tanks used in closed-loop hydronic heating and cooling systems are selected differently from pressure tanks used in domestic water, well, or drawdown applications.

Important factors include:

System type: hydronic heating, chilled water, well water, or pressure boosting
Required tank volume
Acceptance volume or drawdown requirement
System operating pressure
Maximum working pressure
Fluid temperature
ASME requirements
Available installation space
Whether the internal bladder or diaphragm needs to be replaceable
Manufacturer compatibility and model availability

For many commercial hydronic systems, the decision is not simply “bladder or diaphragm.” The correct tank must also be sized properly for the system volume, temperature change, fill pressure, relief valve setting, and operating pressure range.

Bladder & Diaphragm Tanks Available from Different Manufacturers

Bell & Gossett Bladder Expansion Tanks

Bladder expansion tanks from Bell & Gossett contain a balloon-like bladder or rubber membrane within the shell that can be replaced when required. Pre-charged bladder expansion tanks from Bell & Gossett absorb thermal expansion forces in hydronic heating and cooling systems. Their rubber membranes isolate the air from the system fluid, preventing waterlogging, and the pre-charge minimizes the tank's size.

Bell & Gossett Expansion Tanks — Bell & Gossett Bladder Expansion Tanks

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Bell & Gossett Diaphragm Expansion Tanks

Bell & Gossett diaphragm expansion tanks contain a butyl rubber diaphragm attached to an internal polypropylene tank liner. The heavy-duty butyl diaphragm separates system water from the air in the tank, preventing waterlogging problems. Diaphragms cannot be replaced independently. The entire tank would need to be replaced if there was a problem with the diaphragm. However, the diaphragm is made of thick, high-quality rubber that will last for years.

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Taco Bladder Expansion Tanks

Taco offers several models of bladder-style expansion tanks, which utilize butyl rubber bladders to permanently separate the air charge from the system fluid in the expansion tank. Taco bladder expansion tanks are designed for hydronic systems, hydro-pneumatic, and draw-down applications.

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Taco Diaphragm Expansion Tanks

Taco diaphragm expansion tanks control the pressure in hydronic systems in commercial, institutional, and industrial applications. Diaphragm tanks use a diaphragm to separate air and water permanently. In a diaphragm tank, the air is held captive by using a diaphragm, with the expanded water being held on one side of the diaphragm and air on the other.

Taco Bladder Diaphragm Tanks — Taco Diaphragm Expansion Tanks

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Armstrong Bladder Expansion Tanks

Armstrong Type L bladder expansion tanks feature flow rates of 10 to 1056 US GPM. Expansion tanks from Armstrong eliminate air ingress and dampen pressure transients (water hammer) for reduced system corrosion and stress, ultimately extending service intervals and overall system life.

Armstrong Bladder Diaphragm Tanks — Armstrong Bladder Expansion Tanks

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Armstrong Diaphragm Expansion Tanks

Armstrong AET plain steel expansion tanks feature 15 to 525 US GPM flow, while AX diaphragm expansion tanks feature 8 to 211 US GPM flow. Expansion tanks from Armstrong use compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. They isolate the expanded water from the pressure-controlling air cushion, which is pre-charged at the factory and can be adjusted in the field to meet final system requirements.

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Frequently Asked Questions

How do I know if I need a bladder or diaphragm tank?

Start by identifying the application and system requirements. For hydronic heating and cooling systems, confirm system volume, fill pressure, relief valve setting, maximum operating temperature, and required acceptance volume. For pressure or drawdown applications, confirm the pump cut-in and cut-out pressure, required drawdown, and water usage pattern.

Is a bladder tank better than a diaphragm tank?

Not always. A bladder tank may be better when a replaceable internal bladder is important, especially in larger systems or applications where serviceability matters. A diaphragm tank may be better when a compact, simple, low-maintenance design is preferred. The right choice depends on the system, pressure requirements, tank size, and manufacturer recommendations.

Can a diaphragm tank bladder be replaced?

A diaphragm tank does not use a separate removable bladder. The diaphragm is usually permanently attached inside the tank. If the diaphragm fails, the entire tank typically needs to be replaced.

What causes an expansion/pressure tank to fail?

Common causes include loss of pre-charge pressure, bladder or diaphragm failure, corrosion, improper sizing, excessive system pressure, high temperature exposure, or incorrect installation. In closed-loop hydronic systems, improper fill pressure or a failed air separator can also contribute to pressure problems.

Expansion Tanks Available at Liberty Supply

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