What Is A Boiler Expansion Tank?

image of boiler expansion tank

Have you ever found yourself concerned about the state of your water heater? Perhaps you have noticed a loud noise when you shut off the shower. Maybe your HVAC expert has recommended replacing the boiler expansion tank, but you are unsure of its purpose or function. Don’t worry. The boiler expansion tank is crucial in your home’s water heating or hydronic heating system. In this article, we will provide essential insights into boiler expansion tanks. 

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What Is A Boiler Expansion Tank?

A boiler expansion tank is a compact tank positioned atop the main tank of your water boiler’. It serves as a space for heated water to expand. Within the expansion tank, there is a diaphragm that divides it into two compartments. One section allows expanding water to escape, while the other functions as an air cushion. They accommodate changes and regulate pressure resulting from heated water. 

While expansion tanks were originally single-section units, their design was enhanced with two sections to keep air from entering and causing corrosion. These expansion tanks play a crucial role in both heating and cooling processes. They are particularly effective in preventing issues related to water hammer or hydraulic shock. Continue reading to learn more. 

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What Is  A Water Hammer?

Water hammer, also known as hydraulic shock, is a frequent occurrence that can contribute to the degradation of your water heating system. Have you ever experienced a loud banging sound when shutting off a faucet or bathtub? This noise occurs when the water flow suddenly changes direction, resulting in shock waves. Water hammer can not only be irritating but also detrimental to your piping and heating systems. To prevent water hammer and safeguard your heating system, consider installing a boiler expansion tank. 

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Kinds Of Boiler Expansion Tanks

There are two types of boiler expansion tanks: the steel type and the bladder type. Let’s take a closer look.

Steel Type Expansion Tanks

image of copper pipes and expansion tank in boiler room

Steel type expansion tanks have a long history of use and are often found in older homes. They don’t have a diaphragm, so water and air come into direct contact with each other. To prevent corrosion, these tanks are constructed from steel. You may notice a small window, known as a sight glass, which aids in monitoring the water-to-air ratio in the tank. 

The appropriate water-to-air ratio varies depending on several factors, including the material of your convectors (usually iron or copper), your heating tank’s water capacity, and other considerations. Steel tanks can encounter issues related to the mingling of air and water, such as corrosion, tank flooding, or gasket malfunctions. Consult with an HVAC professional to get the correct size of steel-type expansion tanks.

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Bladder Type Expansion Tanks

Bladder tanks differ significantly from steel tanks due to the inclusion of a diaphragm that creates a barrier between the water and air within the expansion tank. This design enhances their durability but does not entirely eliminate the problem of air in the system. Even in a heating system with a bladder expansion tank, air can still separate from the water and must be removed.

To maintain the correct pressure within the system and determine the appropriate size of a bladder expansion tank for your boiler, consult a professional HVAC technician. They can provide guidance in making sure your heating system operates effectively.

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Is Your Expansion Tank Working Properly?

If you have an expansion tank but are uncertain about its proper functioning, check for common issues that can lead to malfunctions. In the case of steel expansion tanks, one prevalent problem is incorrect pressure within the chambers. This is often indicated by a pressure relief valve venting hot water. Checking the water level through the sight glass can confirm whether there is a pressure issue. 

For bladder tanks, examine the pressure on the airside of the diaphragm. If necessary, drain the water lines to allow the bladder to refill with air. Monitoring and addressing these pressure-related concerns can help ensure the expansion tank operates properly.

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Boiler expansion tanks help maintain the pressure within the heating systems, yet their purpose remains a mystery to many. These handy reservoirs work in tandem with heating systems to regulate pressure when heating water in the boiler tank. They help prevent problems like water hammer and overpressurization. By doing so, they contribute to the longevity of your heating system. For a complimentary installation quote, reach out to Hart Home Comfort today!

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Call Hart Home Comfort For All Of Your HVAC Requirements


Hart Home Comfort delivers top-notch heating and cooling solutions throughout Nassau County, Suffolk County, and Queens, New York. Our HVAC team is certified and highly skilled in tune-ups, repairs, installations, and replacements. With each technician boasting the expertise to service your HVAC system effectively, we ensure the best care for your home.

At Hart Home Comfort, we stand behind our commitment to provide the region’s most competitive heating and cooling service rates. Our maintenance and tune-up services not only enhance your comfort but also boost energy efficiency while curbing your heating and cooling expenses. Whether you need HVAC repairs or a replacement system, we can offer expert recommendations that align with your budget. We back all our services with a satisfaction guarantee. To book a service appointment or take advantage of our free, in-home estimates, contact Hart Home Comfort today.

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