Taco Expansion Tanks: A Technical Guide to the PAX, CA, CX, and CBX Se

Taco Comfort Solutions has been an industry leader in American-made hydronics, plumbing equipment, and pressure vessel technology. With a respectable product range, Taco continues to invest in R&D to introduce even more advanced equipment.

In any closed-loop environment, heating and cooling water can cause volumetric expansion, which, if unmanaged, can lead to pressure spikes, pressure fluctuations, and relief valve failure.

This article aims to cover the Taco’s expansion tank range - from the potable-grade PAX and CA series to the hydronic-focused CX and CBX models - which bring advanced captive-air technology and replaceable bladders options protecting system integrity while reducing the required equipment footprint by up to 80% when well selected.

Understanding the Physics of Thermal Expansion

To understand the purpose of an expansion tank in a water system, one must look at the physical properties of water. Water is virtually incompressible. In a closed HVAC or domestic hot-water system, as the fluid temperature rises, its specific volume increases, meaning it occupies more physical space.

If the system has no capacity to accommodate this additional volume, the loop pressure will increase exponentially. This is known as thermal expansion pressure. Without a properly sized water expansion tank to accommodate this extra volume in the line, the system’s Temperature and Pressure (T&P) relief valve would relieve pressure, resulting in water waste, floor damage, and reduced equipment life.

Water Thermal Expansion - Scheme
Image credit: Watts

What is the Purpose of an Expansion Tank in Water Systems?

An expansion tank is a hydraulic pressure vessel whose primary purpose is to protect heating systems from overpressure caused by water expansion as it gains or loses energy.

When water is heated, it expands; in a "closed" system, this extra volume has nowhere to go. The expansion tank's role is to safely absorb thermal expansion by pressurizing an internal bladder or diaphragm, preventing leaks, relieving valve "weeping," and preventing pipe failure.

Taco Expansion Tank Series: PAX, CA, CX, and CBX

When evaluating the available Taco options, it is important to understand the application to choose the right expansion tank. It depends primarily on whether the application is potable (domestic drinking water or applications requiring potable water) or non-potable (hydronic heating or chilled-water systems).

PAX Series (Potable – ASME)

The PAX series is mainly designed for commercial potable water.

Primary Application: Commercial potable water systems requiring ASME Section VIII compliance.
Key Feature: The heavy-duty butyl bladder is field-replaceable, ensuring long-term service life.

PAX Series - Expansion Tank (PAX25 - PAX10)

CA Series (Potable/Hydronic – ASME)

The CA series features a "Full Acceptance" bladder design that expands to fill the entire tank shell. (not only partially)

Primary Application: Large-scale systems.
Key Feature: Compatible with both potable and hydronic applications.

CX Series (Hydronic – Diaphragm)

The CX series utilizes a heavy-duty built-in diaphragm.

Primary Application: Small heating and cooling installations.
Constraint: The diaphragm style is non-replaceable and has limited acceptance, meaning the diaphragm doesn't cover the entire tank volume.

CBX Series (Hydronic – Bladder)

The CBX series is a bladder-type tank designed for non-potable hydronic heating and cooling systems.

Primary Application: High-capacity hydronic loops where a replaceable bladder is preferred.
Installation: Unlike the PAX or CA, the CBX is typically limited to vertical installation only.

Technical Selection Matrix

Below is a comparison table for quick selection based on type, application, ASME, and replaceability:

Series	Type	Application	ASME Rated	Replaceable?
PAX	Bladder	Potable Water	Yes	Yes (Bladder)
CA	Bladder	Potable/Hydronic	Yes	Yes (Bladder)
CX	Diaphragm	Hydronic	Yes	No (diaphragm)
CBX	Bladder	Hydronic	Yes	Yes (Bladder)

Taco Expansion Tanks - Quick Selection Table

The video below takes a deep dive into the Taco Expansion Tanks options, with model insights and valuable technical/commercial information:

Deep Dive into Taco’s Expansion Tanks
Video credit: Taco, Inc.

Engineering Selection & Sizing Logic

Proper sizing of the water expansion tank is the most critical step in the design process. A tank that is too small will not prevent the T&P valve from weeping; a tank that is too large is a waste of budget and can cause installation issues due to its size.

The Expansion Formula

The following formula is used as a general rule for thermal expansion tank volume calculation:

Vt = Total Expansion Tank Volume
Vs = Total System Water Volume (gallons)
v1&v2 = Specific volume of water at initial and final temperatures
Pa - Atmospheric pressure (14.7 psia)
P1 - Pre-charge pressure (absolute - PSIG + 14.7 = PSIA)
P2 - Max operating pressure (absolute - PSIG + 14.7 = PSIA)

Sizing tip - Manual calculations can be useful for pre-sections or quick discussions. For accurate selections and technical validation, we highly recommend contacting the Liberty Supply team or using the Taco Expansion Tank Wizard Calculation Tool.

Installation: The "Point of No Pressure Change"

Where you install the tank is just as important as the tank volume calculation. In hydronic systems with closed loops, the expansion tank should typically be located on the pump's suction side.

Why the Pump Suction Side?

Because the expansion tank is the only part of the water loop with a compressible element, this connection point is known as the Point of No Pressure Change (PoNPC). Since the pump can't change pressure at this junction, it uses the tank as its system pressure anchor.

When the expansion tank is installed on the pump suction side, its internal fixed pressure is added to the system, ensuring positive pressure throughout the building and keeping air vents functioning correctly.

If the tank is installed on the discharge side, the pump must "pump toward" this fixed pressure point. To create differential pressure, the pump must subtract the system head, which can create a vacuum on the suction side. This condition sucks air into the system through air vents, causing gurgling noises, air binding, and pump cavitation. This is why expansion tanks must be installed on the pump's suction side; they ensure quiet, efficient operation.

General Hydraulic System - Schematic
Image credit: Plumbing and HVAC (CA)

Key Installation Steps

Set the Pre-charge:
Always check the water expansion tank pressure setting while the tank is empty. It should match the system's static fill pressure.
Support the Weight:
A full expansion tank is extremely heavy. Ensure the floor or wall brackets are rated for the "flooded weight."
Isolation & Drain:
Every tank should have a lockable isolation valve and a drain valve located between the tank and the isolation valve. This allows you to check the air charge without draining the whole building.

Annual Maintenance Checklist

Air Pre-charge Verification: The pre-charge must match the system static pressure to prevent the tank from becoming waterlogged.
The "Schrader" Test: Briefly depress the air valve. If water squirts out instead of air, the bladder or diaphragm has ruptured, and the unit (or bladder) must be replaced.
External Corrosion Inspection: Check the entire tank, including the shell and connections, for external corrosion and rust. Corrosion can lead to leaks and pressure loss.
Air Migration Check: Check the annual pressure loss. If the air pressure drops by more than 10% without a visible leak, the bladder may be losing integrity and may need to be replaced.
Sediment Scaling Flush: Open the tank drain valve to flush sediment from the water loop. It prevents debris from clogging the orifice.

Frequently Asked Questions

How long does a Taco expansion tank typically last?

In a properly maintained system with correct pre-charge, an expansion tank can last 10 to 15 years.

Can a Taco expansion tank be mounted horizontally?

Most Taco PAX, CA, and CX models can be mounted horizontally (please check with the Liberty Supply Team before ordering), provided they are supported by a properly sized saddle or bracket set capable of handling the fluid's weight. However, the CBX series is specifically designed for vertical installation.

What happens if I don't install an expansion tank?

Without an expansion tank in a closed-loop system, the expanding water (when heated) will increase the system pressure until it reaches the T&P Relief Valve setting. It will cause valve discharge to avoid pipe/equipment damage. Over time, this constant pressure cycling weakens pipe joints, damages the water heater's inner linings, and can lead to premature, costly system failure.

Taco Pumps and Parts Available in Liberty Supply

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Liberty Supply is a customer-centric organization. We focus on hydronic and steam systems for commercial and industrial facilities. As a B2B distributor within the "needs business," our foundational pillars are knowledge, communication, and speed. Relationships are built through hard work and consistent delivery of value. No exceptions. Our team maintains that with each transaction, we start back at zero.