Expansion Tanks & Air Vents

What Are Expansion Tanks and Air Vents?

In any closed-loop hydronic heating or cooling system, water is constantly being heated and cooled. Water is incompressible — as it heats up, it expands, and if that expanding volume has nowhere to go, pressure builds rapidly inside the system. It does not take much heat before the pressure exceeds the safe working pressure of the system components, and something may break, or a safety relief valve will open. Neither outcome is desirable. That is where expansion tanks come in.

An expansion tank is a crucial component that absorbs this excess pressure. It provides a reservoir for expanded water to move into as it heats up, helping maintain a stable pressure within the system and preventing damage to pipes, valves, and other components.

Expansion tanks typically contain a diaphragm or bladder that separates the water from the air, allowing for controlled expansion and contraction. Newer diaphragm expansion tanks have improved upon older models to effectively separate air from water, allowing for expansion without the risk of air reintroduction. Compression-style tanks, by contrast, have no bladder or diaphragm — they contain nothing but air, and when you fill the hydronic system, the water enters the tank and compresses that air.

The Role of Air Vents

When using a bladder or diaphragm tank in an air elimination system, there is an air vent on top of the air separator, and active automatic air vents are used throughout the system. Automatic air vents prevent the tank piping from becoming air-bound and are a critical partner component to any properly functioning expansion tank installation. Together, expansion tanks and air vents form a complete pressure and air management solution for boilers, radiant floor heating, chilled water systems, and more.

Key Considerations When Shopping

• Tank Type: Diaphragm and bladder tanks are ideal for air elimination systems; compression tanks work well where no automatic air venting is used.
• Sizing: Proper sizing and installation of expansion tanks are paramount to the performance and longevity of your hydronic heating system.
• Pre-Charge Pressure: Expansion tanks rely on an internal air cushion that compresses as the system water heats up and expands, keeping the system properly pressurized at all times, regardless of temperature. Verifying and adjusting the pre-charge to match your system's fill pressure is essential.
• Maintenance: Expansion tanks require periodic maintenance; for bladder-type tanks, this should include testing the pressure charge on an annual basis.

Top Brands in Expansion Tanks & Air Vents

Xylem Bell & Gossett is one of the most recognized names in hydronic system components, offering expansion, compression, and diaphragm tanks for proper system pressurization in hydronic heating and potable water plumbing systems. Their broad lineup and deep engineering heritage make them a go-to for commercial and residential installers alike.

Resideo by Honeywell brings decades of controls and HVAC expertise to the expansion tank category, offering reliable diaphragm-style tanks built to the exacting standards that the Honeywell heritage demands — trusted by plumbing and heating professionals across North America.

Caleffi is an Italian-engineered brand renowned for precision hydronic components. Their expansion tanks and automatic air vents are widely specified by engineers for their quality construction, compact design, and comprehensive performance across both heating and cooling applications.

SpiroTherm specializes in air and dirt separation technology for hydronic systems. Their products are engineered to work in concert with expansion tanks to deliver cleaner, more efficient closed-loop systems with minimal maintenance.

Amtrol is a long-established manufacturer with a strong reputation in expansion tanks for both hydronic heating and potable water applications, known for durable construction and a wide range of capacities to suit residential and light commercial installations.