About Expansion Tanks for Boilers & Heating Applications
What is an expansion tank and how does it work?
An expansion tank is a cylindrical vessel with two chambers separated by a rubber diaphragm. On one side of the diaphragm is the hot system water, on the other - pressurized air. As temperature of the water inside the heating system increases, so does its' volume. Excess water volume is pushed into and stored inside the tank (in a manner similar to an inflated baloon). As the system cools down, the water is slowly released from the tank back into the system due to the air back pressure on the other side of the diaphragm. This repeating process is part of a normal and safe operation of any boiler heating system, including gas, oil and electric boilers - both standard efficiency and high-efficiency (condensing).
Why are expansion tanks important?
If an expansion tank is missing or not working properly, excess pressure generated by expanding water will either cause a frequent discharge of the boiler pressure relief valve (PRV), or (if the PRV is faulty or missing) may cause a dangerous system overload with subsequent burst or rupture with release of scalding hot water and air. In summary, the expansion tank is the first line of defense against over-pressurization inside the boiler system, with PRV being the second one.
How to identify a faulty, undercharged or leaking expansion tank?
The first and easiest method is to gently touch (tap) the tank during normal operation - the tank should be hot on the top (where it connects to the system) and normal/room temperature of the bottom (pressurized air side). If the air side is hot, then either the diaphragm has burst or the air chamber has lost pressure and needs to be re-pressurized.
The second method is to use Amtrol's Insight feature, which is standard on all new Extrol series models we carry. This clear indicator on the bottom of the tank changes color if diaphragm is leaking or has burst and is a great and easy way to determine of the tanks needs to be changed.
The third method is to inspect the system for any signs of PRV discharge (the valve must always have pipe a connected which leads the valve to discharge towards the floor). Any puddles or rust spots may indicate that the valve has recently discharged, which is commonly caused by a malfunctioning expansion tank.
Difference between diaphragm and bladder type expansion tanks
Bladder and diaphragm expansion tanks operate in the same manner, but differ in construction.
Diaphragm expansion tanks such as ones made by Amtrol are constructed of a steel shell with built-in rubber diaphragm located about in the middle. The diaphragm separates the tank into (2) separate chambers - one with pressurized air, the other - with system water. As hot water expands and enters the tank, it presses against the diaphragm while pressurized air on the other side “cushions” the expansion, thus maintaining system pressure equilibrium and at the same time allowing for absorption of excess water.
Bladder type expansion tanks are comprised of a rubber “balloon” installed inside a pressurized steel shell. As hot water expands, it enters and expands the bladder, while pressurized air inside the tank chamber presses back against the bladder and acts as a cushion. As a result, system pressure is maintained within safe levels and excess water is absorbed.
Bladder type tanks are expected to have a longer lifespan than diaphragm ones since there’s no direct contact of water with steel shell and no corrosion. In practice however, corrosion is rarely an issue in closed-loop heating systems if a proper air eliminator device is installed.
Frequently Asked Questions
Q: Can expansion tank be installed horizontally?
A: It is not a good practice to install an expansion tank horizontally as a tank full of water is heavy (noticeably heavy for larger models) and would put uneven stress on the connection, potentially causing leaks and subsequent issues. Some manufacturers would not warranty the tank if it is installed this way. It is possible, however, to adequately support the tank in a horizontal position (using band iron or similar) to prevent its' movement and to relieve stress from the system connection point.
Q: Should expansion be installed above the pipe or below the pipe?
A: Below the pipe is the standard, most reliable and universally recommended method. It is also the most convenient for use with an air eliminator and boiler fill valve. The tank can be installed above the pipe, as long as its' level and supported (using support bands or similar) to prevent it from tipping over.
Q: What is the standard air pressure inside a boiler expansion tank and what to do if it's below the required level?
A: The standard pressure is 12 psi (15 psi on select models). The pressure can be checked easily with a standard bike tire gauge. Although it is not common for expansion tank to leak air, the air can be added using a standard tire pump as the valves on the tanks are standard Schrader type.
Q: Which products/accessories are recommended for new expansion tank installations?
A: In a new installation, expansion tank is almost always installed in conjunction with an air eliminator and boiler fill valve. Support band or wall mounting bracket offer convenient means for installing & securing the tank. Expansion tank service valve is very convenient for piping the fresh water supply and changing out the tank without draining the system.
