What is an Expansion Tank, and Why Do You Need One? Posted on February 25, 2025 Hot water is one of the conveniences of modern life that we take for granted. Turn the tap, water comes out. Rotate the handle to the left, and the water gets heated. But behind this seemingly simple act is a whole plumbing system designed to deliver hot water at the flick of the tap. And one of those hidden conveniences of everyday life is the thermal expansion tank, responsible for allowing your plumbing system to store and deliver hot water safely and reliably. In this guide, we explore what a thermal expansion tank is, why it’s an essential part of plumbing, and how to find one that matches your water heater system. What is a Thermal Expansion Tank? A thermal expansion tank is a device designed to manage the increased pressure caused by the thermal expansion of water in a closed plumbing system. When water is heated, it expands. In a closed system with no room for this expansion, the pressure builds up. The built-up pressure can damage plumbing fixtures, appliances, and the water heater itself. A thermal expansion tank acts as a safety device, absorbing the extra water volume and preventing over-pressurization in the system. The concept of thermal expansion control dates back to the early 20th century when closed plumbing systems became more common in residential households. Early systems relied on rudimentary methods, such as pressure relief valves, which often led to water wastage. Before sealed expansion tanks, most homes relied on open tanks installed in the roof space to accommodate the water’s expansion. Such setups were inefficient, since the water was exposed to cold air, and there were concerns about potability due to the risk of contamination from the open air. By the 1950s, engineers in the USA developed expansion tanks with diaphragms and sealed air chambers which are more efficient and cut down on water wastage. The evolution of materials, such as corrosion-resistant metals and durable rubber, has further improved the efficiency and lifespan of modern expansion tanks. Today, thermal expansion tanks are a standard feature in most plumbing codes. In the United States, the American Society of Mechanical Engineers (ASME) regulates the construction and certification of thermal expansion tanks. In Europe, they are governed by EN 13831 according to Pressure Equipment Directive (PED) 97/23/EC. How a Thermal Expansion Tank Works Step 1: As water in the system heats up, it expands and increases in volume. Step 2: This expanded water flows into the thermal expansion tank. Step 3: The flexible bladder or diaphragm in the expansion tank compresses the air chamber. This creates space for the additional water volume. Step 4: Once the pressure normalizes, the expansion tank empties and the expanded water returns safely to the plumbing system. Parts of a Thermal Expansion Tank Outer Shell: The metal casing that houses the internal components and protects the tank from external damage. Bladder or Diaphragm: A flexible membrane separating the air and water chambers within the tank. Air Chamber: A pre-charged area containing pressurized air to accommodate the expansion of water. Water Chamber: The section that receives and holds the expanded water. Connection Fitting: A threaded fitting that connects the tank to the plumbing system. Where Are Thermal Expansion Tanks Required? Thermal expansion tanks are often required by building codes in systems where backflow preventers or check valves are installed. Since these devices create a closed system to prevent water from flowing back into the municipal supply, expansion tanks are necessary for the safety of the household or building. Common scenarios requiring expansion tanks include: Residential homes: Especially those with water heaters that lack built-in expansion relief mechanisms. Commercial properties: For large-scale water heating systems and closed-loop HVAC systems. Hydronic heating systems: To prevent pressure buildup in boilers. Municipal water supplies: When pressure-reducing valves (PRVs) or backflow preventers are in use. The Different Types of Thermal Expansion Tanks 1. Potable Water Expansion Tanks These are designed for residential and commercial water heating systems. Featured potable water expansion tank: Amtrol ST-5 Therm-X-Trol In-Line Thermal Expansion Tank for 40 – 50 Gallon Water Heaters The ST-5 is designed for 40-50 gallon water heaters. It has a tank volume of 2 gallons and is 8″ in diameter by 13″ high, being constructed of stainless steel with stainless steel connections. It has a maximum working pressure of 150 PSI with a factory pre-charge of 50 PSI and a maximum operating temperature of 200°F. The ST-5 includes a Fresh Water Turbulator – a patented diffuser that agitates the incoming water to keep it fresh and clean while its water-jet scrubbing action prevents sediment from collecting inside the tank. Price: $67.00 Amtrol ST-5 Therm-X-Trol In-Line Thermal Expansion Tank for 40 - 50 Gallon Water Heaters A Thermal Expansion Tank is a requirement, by law, when your hot water heater includes a back flow preventer at your water main or a check valve anywhere in you... View Product 2. Hydronic System Expansion Tanks These specialized tanks are used in closed-loop heating or cooling systems to accommodate the expansion of water or glycol solutions. Featured hydronic expansion tank: Amtrol EX-60 Extrol 6.7 Gallon In-Line Hydronic Heating Expansion Tank The EX-60 is designed for expansion overflow for specific types of hydronic heating systems. With a tank volume of 6.7 gallons, this version is for BTU outputs of 75,000 – 200,000. It includes a convenient InSight Indicator Cap that changes color when your expansion tank needs to be replaced. When it becomes discolored, it is time to replace your tank. Price: $109.00 Amtrol EX-60 Extrol 6.7 Gallon In-Line Hydronic Heating Expansion Tank Amtrol ExTrol Expansion Tanks for residential hydronic heating systems take on the expanded fluid in closed-loop systems. The purpose of an expansion tank is to... View Product 3. Inline Expansion Tanks More compact than conventional expansion tanks, these are designed so they can be installed directly into the water line or in systems with tankless water heaters. Featured inline expansion tank: Amtrol ST-1 Therm-X-Trol In-Line Thermal Expansion Tank This compact expansion tank is designed for point of use and tankless water heaters with up to 2 gallons of water volume. Constructed of stainless steel with stainless steel connections, it has a maximum working pressure of 150 PSI and a maximum operating temperature set point of 140°F. The Butyl diaphragm inside the steel tank keeps your potable water from contact with the steel, and is sent back into the system when the call for hot water comes again. Price: $31.00 Amtrol ST-1 Therm-X-Trol In-Line Thermal Expansion Tank for Point Of Use and Tankless Water Heaters A Thermal Expansion Tank is a requirement, by law, when your hot water heater includes a back flow preventer at your water main or a check valve anywhere in you... View Product 4. Horizontal Expansion Tanks These are larger tanks typically mounted on a stand or suspended from ceilings for high-volume systems. 5. Diaphragm Tanks These are equipped with a rubber diaphragm to separate the air and water chambers. Conclusion Thermal expansion tanks are a vital component of modern water heating systems. By managing the pressure fluctuations caused by thermal expansion, they protect your system from potential damage, ensuring safety and efficiency. Whether you need a new expansion tank for your new hot water system or it’s time to replace your existing tank, we carry a range of thermal expansion tanks to suit different needs, from hot water heaters to boiler room applications. Also check out our feature article on AMTROL, the leading brand when it comes to expansion tanks! Mickey Luongo Mickey is the resident heating and air conditioning expert with over 15 years of experience in the industry.
